Chapter 15. Disorders of Organic Acid Metabolism

Chapter 15

TABLE 15-1



Major Causes of Metabolic Acidosis

in the Neonatal Period



Disorders of Propionate-Methylmalonate Metabolism

Propionic acidemia

Methylmalonic acidemia

Disorders of Pyruvate and Mitochondrial Energy Metabolism

Pyruvate dehydrogenase deficiency

Pyruvate carboxylase deficiency

Defects of the electron transport chain (complexes I, IV, V)

Disorders of Branched-Chain Amino Acid–Ketoacid

Metabolism

Maple syrup urine disease

Isovaleric acidemia

beta-Methylcrotonyl–CoA carboxylase deficiency

beta-Ketothiolase deficiency

Hydroxymethylglutaryl–CoA lyase deficiency

Mevalonic aciduria

Disorders of Fatty Acid Metabolism

Medium-chain acyl–CoA dehydrogenase deficiency

Other Organic Acid Disorders

Multiple carboxylase deficiency

Glutaric acidemia, type II

Glutathione synthetase deficiency (5-oxoprolinuria)

Sulfite oxidase deficiency (molybdenum cofactor deficiency)

Disorders of Carbohydrate Metabolism

Galactosemia

Glycogen storage disease, type I (von Gierke glucose6-phosphatase deficiency)

Fructose-1,6-diphosphatase deficiency

Phosphoenolpyruvate carboxykinase deficiency

Renal Tubular Acidosis



excessive propionate to lactate by a normally minor metabolic pathway (see Fig. 15-2), inhibition of pyruvate

dehydrogenase8 with resulting increased conversion of

pyruvate to lactate, and accumulation of ketone bodies

by poorly understood mechanisms.

Hyperammonemia

The hyperammonemia that is a nearly consistent feature of the neonatal varieties of propionate and



TABLE 15-2



Disorders of Organic Acid Metabolism



687



Common Features of Disorders of

Propionate and Methylmalonate

Metabolism



Clinical Features

Vomiting

Tachypnea

Stupor, coma

Seizures

Metabolic Features

Acidosis

Propionic acidemia with or without methylmalonic

acidemia (aciduria)

Hyperglycinemia

Hyperammonemia

Other Features

Neutropenia, anemia, thrombocytopenia

Neuropathological Features

Myelin disturbance

Basal ganglia injury (caudate, putamen in propionic

acidemia; globus pallidus in methylmalonic acidemia)

Cerebral cortical atrophy (later)



methylmalonate disturbances appears to result from

two closely related mechanisms.8-13 Both relate to an

accumulation of the CoA esters of the acids proximal to

the enzymatic blocks (particularly propionyl-CoA,

tiglyl-CoA [a metabolite of isoleucine], and methylmalonyl-CoA) and to the effects of these derivatives

on the activity of carbamyl phosphate synthetase, the

first step in the Krebs-Henseleit urea cycle (see Chapter

14). Thus, these CoA esters have been shown to have a

direct inhibitory effect on carbamyl phosphate synthetase and an indirect inhibitory effect at this step by

inhibition of the synthesis of N-acetylglutamate, the

important activator of carbamyl phosphate synthetase

(see Fig. 14-12). Hyperammonemia and acidosis have

major deleterious effects on the brain (see Chapter 14)

and are thought to be major determinants of the acute

neurological dysfunction and brain injury that result in

the neonatal period.



Figure 15-1 Simplified scheme for differential diagnosis of neonatal lactic acidosis. Note the critical initial role of determinations of urine

organic acids and blood lactate and pyruvate levels and ratio. L/P ratio, lactate-to-pyruvate ratio; PC, pyruvate carboxylase (deficiency); PDHC,

pyruvate dehydrogenase complex (deficiency). *Organic acid disorders: propionic acidemia, methylmalonic acidemia, isovaleric acidemia, multiple

carboxylase deficiency, fatty acid oxidation defects, among others. *Glycolytic defects: glucose-6-phosphatase, fructose-1,6-diphosphatase, phosphoenolpyruvate carboxykinase deficiencies. *Citric acid cycle defects: fumarase and succinate dehydrogenase deficiencies. *Electron transport

disorders: see mitochondrial disorders in Chapter 16.



688



UNIT V



METABOLIC ENCEPHALOPATHIES

L-Isoleucine



␣-Methylacetoacetyl-CoA

2 3



1

D-Methylmalonyl-CoA



Propionyl-CoA

Biotin



Lactate



Succinyl-CoA



L-Methylmalonyl-CoA



Adenosylcobalamin

Methyl-branched

6

fatty acids

(Cobalamin)R



Odd-numbered

fatty acids

Homocystine



Homocysteine

5

Me-cobalamin



Methionine



Cobalamin

4

B12(cobalamin)



Figure 15-2 Metabolism of propionate and methylmalonate and sites of defects in their metabolism. Major pathways are shown by solid arrows,

and alternate minor pathways by broken arrows. Sites of defects are numbered and include (1) propionyl–coenzyme A (CoA) carboxylase, (2) and (3)

methylmalonyl-CoA mutase (two different structural defects), (4) cobalamin binding, internalization, lysosomal release and cytosolic reduction, (5)

mitochondrial cobalamin reductase, and (6) mitochondrial adenosyltransferase. See text for details. (Adapted from Fenton WA, Gravel RA,

Rosenblatt DS: Disorders of propionate and methylmalonate metabolism. In Schriver CR, Beaudet AL, Sly WS, et al, editors: The Metabolic and

Molecular Bases of Inherited Disease, 8th ed, New York: 2001, McGraw-Hill.)



Hyperglycinemia

A striking aspect of propionate and methylmalonate

metabolism is hyperglycinemia. This condition is unlike

the nonketotic hyperglycinemia described in Chapter 14

because of the association of ketoacidosis (i.e., ketotic

versus nonketotic hyperglycinemia) and because the

glycine abnormality is a secondary and not a primary

metabolic phenomenon. Analogous to the cause of the

hyperammonemia in these disorders of the propionate

and methylmalonate pathway, the cause of the hyperglycinemia appears related to an inhibition of glycine

cleavage by the accumulation of branched-chain alphaketoacids and, more specifically, their CoA derivatives.

A disturbance of glycine cleavage was demonstrated

indirectly and directly in studies of patients with ketotic

hyperglycinemia caused by deficiencies of propionyl-CoA

carboxylase and methylmalonyl-CoA mutase, as well as

of isovaleryl-CoA dehydrogenase and beta-ketothiolase

(the latter two disorders of branched-chain amino acid

metabolism are discussed later).3,4,14-18 Analyses of the

individual protein components of the glycine cleavage

system of patients with propionic acidemia and methylmalonic acidemia have shown that the H-protein, one

of the four proteins of the system, is the component

initially inactivated.19

That the impairment of glycine metabolism involves the

inhibition of the glycine cleavage system by CoA derivatives of accumulated metabolites is suggested by data

based on studies of rat liver.20 In vitro studies of the

solubilized hepatic glycine cleavage system show

marked inhibition by CoA derivatives found in the



catabolic pathway for isoleucine. Such derivatives

would be expected to accumulate in the disorders

of the propionate and methylmalonate pathway (see

Fig. 15-2). Coupled with the data referable to the genesis of the hyperammonemia, these observations

suggest that the CoA derivatives of the accumulated organic

acids are responsible for the major, critical, secondary

metabolic effects that accompany the primary enzymatic disorders.

Myelin Disturbance and Fatty Acid

Abnormalities

In the disorders associated with the accumulation of

propionic and methylmalonic acids, a disturbance of

myelin, detectable by neuropathological examination

(see ‘‘Neuropathology’’), appears to be important in

the genesis of the neurological sequelae.21-24

Vacuolation of myelin appears in the first months of

life and is followed by an apparent disturbance of

myelin formation.21,24 The magnetic resonance imaging (MRI) correlate of the myelin disturbance, present

in many reported cases (see later), is, acutely, diffusely

swollen, T2-hyperintense cerebral white matter, followed later by white matter atrophy. The genesis of

the myelin disturbance is not clear but may be related

to changes in the fatty acid composition of oligodendroglial membranes. Distinct changes exist in the composition of fatty acids in the brain of patients with

disorders resulting in the accumulation of propionate

or methylmalonate,25-29 and these changes can be

reproduced in cultured rat glial cells.30,31 The major

alterations are increases in the amounts of odd-numbered



Chapter 15

TABLE 15-3



689



Disorders of Organic Acid Metabolism



Fatty Acid Composition of Brain Lipids with Disorder of Propionate-Methylmalonate Metabolism

ODD-NUMBERED FATTY ACIDS



Brain Lipid Class



METHYL-BRANCHED FATTY ACIDS



Control (%)



Control (%)



Patient (%)*



Trace

7.5%

18.9%

21.7%



Choline phospholipid

Sphingomyelin

Cerebroside

Sulfatide



Patient (%)*

9.8%

18.2%

29.0%

31.1%



—

—

—

—



2.1%

—

—

—



*Child with methylmalonic aciduria.

Data from Ramsey RB, Scott T, Banik NL: Fatty acid composition of myelin isolated from the brain of patient with cellular deficiency of co-enzyme

forms of vitamin B12, J Neurol Sci 34:221-232, 1977.



and methyl-branched fatty acids (see later). These increases

have been demonstrated in phospholipids (i.e., components of all cellular membranes) as well as in myelin

lipids (e.g., cerebrosides and sulfatides; Table 15-3).28

Because the fatty acid composition of membrane lipids

is important not only for structural integrity but also for

the function of a variety of membrane proteins (e.g.,

enzymes, transport carriers, surface receptors),32 these

alterations may have major implications for the genesis

of the neurological dysfunction and the disturbance of

myelination.



Disturbances of Fatty Acid Synthesis

The fatty acid abnormalities described in the previous

section are caused presumably by disturbances of fatty

acid synthesis. The nature of the disturbances observed

in disorders of propionate and methylmalonate metabolism is depicted in Figure 15-3. Thus, under normal

circumstances, de novo synthesis of fatty acids in brain

is catalyzed by the multienzyme complex fatty acid synthetase.33,34 The first two carbons (i.e., the primer) of

the resulting even-numbered fatty acids (primarily

the 16-carbon acid, palmitic acid) are derived from



Normal



O

C H3– C –SCoA

Acetyl-CoA



Fatty acid synthetase



O

-O–C

+



O



C H2– C –SCoA

Malonyl-CoA



CO2 from each malonyl-CoA utilized

O



C H3 C H2( C H2)n C –SCoA

FATTY ACID-CoA (EVEN-NUMBERED)

Propionyl-CoA accumulation



O

C H3– C H2– C –CoA



O

-O –C

+



O



C H2– C –SCoA



Propionyl-CoA



Fatty acid synthetase



Methylmalonyl-CoA accumulation



Malonyl-CoA



CO2



O



O

-O–C



C H3– C –SCoA +

Acetyl-CoA



C H3

Methylmalonyl-CoA



Fatty acid synthetase



CO2

O



O

C H3 C H2 C H2( C H2)n C –SCoA



C H 3 C H2



C –SCoA



CH

C H3



FATTY ACID-CoA (ODD-NUMBERED)



O



C H– C –SCoA



n



FATTY ACID-CoA (METHYL-BRANCHED)



Figure 15-3 Disturbances of fatty acid synthesis in disorders of propionate and methylmalonate metabolism. Under normal conditions, the

enzyme complex, fatty acid synthetase, catalyzes the addition of two-carbon fragments from malonyl–coenzyme A (CoA) to the single primer

molecule of acetyl-CoA to form even-numbered fatty acids. With propionyl-CoA accumulation, this three-carbon compound replaces acetyl-CoA as

primer, and therefore with the addition of the two-carbon fragments from malonyl-CoA, odd-numbered fatty acids result. With methylmalonyl-CoA

accumulation, this branched compound replaces malonyl-CoA, and therefore methyl-branched fatty acids result.



690



UNIT V



METABOLIC ENCEPHALOPATHIES



acetyl-CoA, whereas the remaining carbons for chain

elongation are derived from the two-carbon units

obtained from malonyl-CoA (see Fig. 15-3). When propionyl-CoA is present in excessive amounts, it can replace

acetyl-CoA with a three-carbon fragment as primer, and,

thus, an odd-numbered fatty acid results after the addition of the two-carbon units from malonyl-CoA (see

Fig. 15-3). When methylmalonyl-CoA is present in excessive

amounts, it can replace malonyl-CoA, and, thus, a methylbranched unit is derived from malonyl-CoA, resulting

in methyl-branched fatty acids (see Fig. 15-3). These

unusual fatty acids are incorporated into cellular membranes, including myelin, as discussed in the previous

section.

Propionic Acidemia and Propionyl–

Coenzyme A Carboxylase Deficiency

Propionic acidemia is caused by a defect in the first step

of the pathway from propionyl-CoA to succinyl-CoA,

a step catalyzed by the enzyme propionyl-CoA

carboxylase.

Clinical Features

Onset is in the first days of life, with a dramatic clinical

syndrome consisting primarily of vomiting, stupor,

tachypnea, and seizures (see Table 15-2).7,8,35-37 The

usual time of onset is the second to fourth days of

life.38,39 Infants whose condition is not diagnosed and

treated properly rapidly lapse into coma and die.

Indeed, in earlier studies, approximately 75% of

patients died in early infancy.1 However, even with

‘‘early diagnosis and vigorous treatment,’’ median survival in a later series was only 3 years.38 More recent

improvements in management have resulted in

improved survival rates.39 In one series of six infants,

all survived the neonatal period.36 Lethal cerebellar

hemorrhage, occurring in association with thrombocytopenia and hyperosmolar bicarbonate therapy, has

occasionally been observed in the neonatal period.40

Survivors of the neonatal period are prone to episodic

attacks of vomiting and stupor, with severe ketoacidosis, often precipitated by infection, and to subsequent

retardation of neurological development. Of 11 infants

reported in one series, no survivor had an intelligence

quotient (IQ) higher than 60.38 Similar cognitive

impairment was documented in a later series of six

infants.36 In a recent series of 38 infants, 95% had ‘‘cognitive and neurologic’’ deficits.39 Chorea or dystonia has

been observed in 20% to 40% of surviving children, and

this extrapyramidal involvement is common in this disorder (see later discussion of neuropathology).38,41

The genetic data for this disorder indicate autosomal

recessive inheritance. This conclusion is based, in part,

on the pattern of familial occurrence, partial disturbance of enzymatic activity in parents, and complementation testing of cells in culture.7,8,13

Metabolic Features

Major Findings. The constellation of ketoacidosis,

propionic acidemia, hyperglycinemia (and hyperglycinuria), hyperammonemia, neutropenia, anemia, and



thrombopenia is characteristic and composes the

‘‘ketotic hyperglycinemia’’ syndrome.8 However, hyperglycinemia with propionic acidemia and propionyl-CoA

carboxylase deficiency has occurred in the neonatal

period without consistent ketonuria.42 This finding is

important because patients with disorders of propionate and methylmalonate metabolism should be managed differently from those with the more common

nonketotic hyperglycinemia described in Chapter 14.

Enzymatic Defect. The enzymatic defect involves propionyl-CoA carboxylase.8,43 Structural alterations of

the two nonidentical subunits (alpha and beta) of the

carboxylase molecules account for the enzymatic

defect.8 The enzyme contains four copies each of the

alpha and beta subunits, with the gene for the alpha

subunit encoded on chromosome 13 and the gene for

the beta subunit encoded on chromosome 3.8 Because

this enzyme requires biotin for activity, the possibility

of a defect in activation or binding of biotin to the carboxylase apoprotein as the basis of the disturbed activity in certain patients must be considered. The initial

observation of a beneficial response of one patient to

large amounts of biotin suggested that such an additional defect may occur.44 The delineation of impaired

activity of propionyl-CoA carboxylase (as well as of

other carboxylases) in two disorders of biotin metabolism, holocarboxylase synthetase deficiency and biotinidase deficiency, corroborated this suggestion (see later

discussion). However, only one of these disorders

(holocarboxylase synthetase deficiency) consistently

causes prominent clinical phenomena in the newborn,

as discussed later.

Pathogenesis of Metabolic Features. The genesis of

the various metabolic consequences of this disorder is

now understood to a considerable degree. The origins

of the hyperglycinemia and the hyperammonemia relate to

the secondary effects of the CoA derivatives of certain

of the accumulated metabolites on the pathways of glycine cleavage and ammonia detoxification by the urea

cycle (see earlier discussion). The acidosis must relate to

several factors (i.e., accumulation of the propionic acid

proximal to the primary enzymatic block, of lactate produced by the alternate pathway of propionate degradation, and of the various acids that accumulate proximal

to propionic acid as a consequence of continuing degradation of branched-chain and other amino acids).

Increased numbers of odd-numbered fatty acids have

been observed in the tissues of infants with propionic

acidemia.29,45,46 The genesis of the odd-numbered fatty

acids relates to the utilization of propionyl-CoA as a

primer for the fatty acid synthetase reaction, as

described previously (see Fig. 15-3).

Neuropathology

A well-studied neonatal case of propionic acidemia

involved a 1-month-old patient.21 The dominant neuropathological findings involved myelin and consisted

of marked vacuolation, with a less striking diminution of

the amount of myelin. Similar pathological findings

have been described in other affected cases.22,24



Chapter 15



Disorders of Organic Acid Metabolism



691



A



B



C

Figure 15-4 Myelin disturbance in propionic acidemia in a 26-day-old infant who exhibited lethargy, poor feeding, tachypnea, profound metabolic acidosis in the first week of life, and generalized seizures in the third week. A, Vacuolation of myelinated fibers traversing the globus

pallidus. B, Vacuolation of the medial longitudinal fasciculus just rostral to the trochlear nucleus. C, Demyelination and endoneurial fibrosis of a

mixed spinal nerve of the lumbosacral plexus. (From Shuman RM, Leech RW, Scott CR: The neuropathology of the nonketotic and ketotic

hyperglycinemias: Three cases, Neurology 28:139-146, 1978.)



The disturbance of myelin is similar to that noted in

nonketotic hyperglycinemia and other aminoacidopathies (see Chapter 14). Vacuolation appears to be the

early change, occurring principally in systems actively

myelinating at the time of the illness (e.g., medial lemniscus, superior cerebellar peduncle, posterior columns, and peripheral nerve in the 1-month-old

patient of Shuman and co-workers21) (Fig. 15-4). The

impaired myelination appears to occur subsequent to

the vacuolation.21 Vacuolation has been observed in

oligodendrocytes in areas just before myelination.24

The cause of this defect in myelination in ketotic

hyperglycinemia may relate to the disturbance of fatty

acid synthesis and the resultant altered fatty acid

composition of myelin (see earlier discussion). Thus,

the odd-numbered fatty acids may alter the stability

of the oligodendroglial-myelin membrane, thereby

impairing oligodendroglial differentiation and rendering the newly formed myelin unstable. Vacuolation and

the subsequent deficit of myelin would result. Other

possibilities, such as disturbance of synthesis of

myelin proteins because of the amino acid imbalance

(e.g., the elevated glycine levels), must be considered as

well.21

An interesting additional feature of the neuropathology of propionic acidemia is the prominence of



involvement of the basal ganglia in patients who survive

for several or more years.47-49 Thus, neuronal loss

and gliosis are prominent, and, in one case, the addition of aberrant myelin bundles caused a ‘‘marbled’’

appearance, reminiscent of status marmoratus of perinatal asphyxia (see Chapter 8). In contrast to methylmalonic acidemia (see later), caudate and putamen,

rather than globus pallidus, are preferentially

involved.50 The importance of excitotoxicity in the

basal ganglia neuronal injury and the potential role of

glycine in the genesis of excitotoxic neuronal injury

(see discussion of nonketotic hyperglycinemia in

Chapter 4) are of interest in this context. The involvement of basal ganglia in older infants and children has

been documented repeatedly by brain imaging.37,51

Finally, this derangement of basal ganglia may underlie

the relative frequency of extrapyramidal movement disorders observed subsequently in infants with propionic

acidemia.38,47 Cerebral cortical atrophy is noted in survivors of several years or more.50,52

As with several other metabolic disorders in which

the enzymatic defect is present in brain (see later), agenesis or hypoplasia of the corpus callosum may result

(Fig. 15-5). Indeed, the presence of callosal abnormalities, without an obvious syndromic or other cause,

should raise the possibility of a metabolic disorder.



692



UNIT V



METABOLIC ENCEPHALOPATHIES



isolated patients.8 Oral antibiotic therapy to reduce

propionate production by bacteria in the gastrointestinal tract may also be useful later.8

Biotin. Because some biochemical benefit was

observed in one infant treated with biotin, large doses

of this vitamin are worthy of a trial in affected

patients.44,45,54 Biotin responsiveness should be

assessed by observation of changes in metabolite

levels in blood and urine and in enzyme activity in

white blood cells. The effect of biotin in vitro on the

enzyme in cultured fibroblasts may be useful in determining the likelihood of a beneficial response in vivo.13

Marked biotin responsiveness is characteristic of multiple carboxylase deficiency (see later discussion).



Figure 15-5 Propionic acidemia, magnetic resonance imaging

(MRI) scan. An infant with severe lactic acidosis was scanned on

the sixth day of life. This T1-weighted MRI scan shows absence of

the corpus callosum. (Courtesy of Dr. Omar Khwaja.)



Gene Therapy. Liver transplantation early in infancy

may be of value in the management of neonatal-onset

propionic acidemia.7 Approximately 20 patients so

treated have been reported.55,56 Initial mortality rates

after transplantation exceeded 50%, and thus the

number of infants followed sufficiently long after transplant is small. However, the most recent survival rates

have improved, and prevention of severe acidotic episodes has been noted.56 A beneficial effect on neurological development remains to be defined.

Methylmalonic Acidemias



Management

Antenatal Diagnosis. Antenatal diagnosis has been

accomplished by measuring propionyl-CoA carboxylase activity in cultured amniotic fluid cells or chorionic

villus samples, by analyzing metabolites in amniotic

fluid, and by molecular genetic testing of DNA

extracted from fetal cells.7,8,15,53 Thus, the possibility

of preventing the disorder is real.

Early Detection. Early diagnosis, particularly in distinguishing this disorder from other causes of severe

metabolic acidosis in the neonatal period (see Table

15-1), is critical. Identification of the accompanying

metabolic features is particularly valuable in this

regard. Organic acid analysis of urine by tandem mass

spectrometry is especially useful. Definitive diagnosis is

established by measurement of propionyl-CoA carboxylase activity in leukocytes or cultured fibroblasts.

Acute and Long-Term Therapy. Acute episodes

should be treated by withdrawing all protein and

administering sodium bicarbonate parenterally.

Hyperammonemia may be severe enough to require

specific measures for ammonia removal, as described

in Chapter 14. Subsequently, a low-protein diet

(restricted especially in isoleucine, valine, methionine,

and threonine) is administered.8 The use of gastrostomy feeding to guarantee nutritional intake has been

valuable.36 Supplementation with L-carnitine may be

indicated, because the excretion of carnitine as propionyl carnitine may lead to decreased plasma levels of

free carnitine, and supplementation with carnitine has

produced beneficial clinical and metabolic responses in



Methylmalonic acidemias constitute the single most

frequent group of organic disorders.6,8 The accumulation of large quantities of methylmalonic acid in blood

and urine is associated with at least five discrete metabolic defects (see Fig. 15-2): (1 and 2) defects of methylmalonyl-CoA mutase (two different defects of the

mutase apoenzyme, one resulting in complete deficiency and the other in partial deficiency of the

mutase), (3 and 4) defects in the synthesis of adenosylcobalamin, and (5) defective synthesis of both

adenosylcobalamin and methylcobalamin (Table

15-4).7,8,45,57-59 The latter three defects of vitamin

B12 metabolism result in diminished activity of



TABLE 15-4



Methylmalonic Acidemias:

Biochemical and Metabolic Features*

METABOLIC ACCUMULATION



Defective Enzyme

Methylmalonic acid

mutase

Mitochondrial cobalamin

reductase (cblA)

Mitochondrial cobalamin

adenosyltransferase

(cblB)

Abnormal lysosomal or

cytosolic cobalamin

metabolism

(cblC, cblD, cblF)

*See text for references.



Methylmalonic

Acid

Homocysteine

+



À



+



À



+



À



+



+



Chapter 15



methylmalonyl-CoA mutase, for which adenosylcobalamin is a coenzyme. Additionally, the last of these

defects also results in diminished activity of the methyltransferase required for methylation of homocysteine;

the formation of the methyltransferase requires methylcobalamin. In one series of 45 carefully studied

patients with methylmalonic acidemia (without homocystinuria), 15 had complete mutase deficiency, 5 had

partial mutase deficiency, 14 had deficient mitochondrial cobalamin reductase, and 11 had deficient cobalamin adenosyltransferase (the latter two defects

resulting in defective synthesis of adenosylcobalamin).45 These disorders are discussed collectively.

Clinical Features

The clinical features are similar to those noted for disorders of propionate metabolism (i.e., vomiting, stupor,

tachypnea and seizures; see Table 15-2). Onset of these

features in the neonatal period depends on the nature

of the enzymatic defect (Table 15-5). Neonatal onset is

most likely with complete mutase deficiency, and

nearly all neonates with this severe enzymatic lesion

present in the first 7 days of life. Fewer than half of

all patients with the other three metabolic defects present in the first 7 days. The outcome also is related to

the type of metabolic defect (see Table 15-5). The gravity of outcome correlates approximately with the frequency of neonatal onset. Thus, infants with

complete mutase deficiency nearly invariably die or

exhibit subsequent neurological impairment. In earlier

series, mortality rates for such patients were approximately 60%, although in more recent series, approximately 30% of infants have died.6,8 In a series of 20

infants, the range of subsequent IQ was 65 to 84, with

a median of 75.60 One infant with severe mutase

deficiency detected at 3 weeks of age by neonatal

screening was reported to be normal at the age of 5

years after treatment with a low-protein diet.61 Patients

with methylmalonic acidemias who survive are subject

to episodic decompensation, especially with minor

TABLE 15-5



Time of Onset and Outcome in

Methylmalonic Acidemias According

to Type of Metabolic Defect

METABOLIC DEFECT



Onset or Outcome

Age at Onset

0–7 days

8–30 days

> 30 days

Outcome

Dead

Impaired

Well



mut



mut–



cblA



cblB



80%

7%

13%



40%

20%

40%



42%

—

58%



33%

22%

55%



60%

40%

—



40%

20%

40%



8%

23%

69%



30%

40%

30%



cblA, deficiency of mitochondrial cobalamin reductase; cblB, deficiency

of cobalamin adenosyltransferase; mut, complete mutase deficiency; mut–, partial mutase deficiency.

Data from Rosenberg LE, Fenton WA: Disorders of propionate and

methylmalonate metabolism. In Scriver CR, Beaudet AL, Sly WS,

et al, editors: The Metabolic Basis of Inherited Disease, 6th ed,

New York: 1989, McGraw-Hill.



Disorders of Organic Acid Metabolism



693



intercurrent infections. Brain imaging reveals the

abnormalities of myelin as noted earlier for propionic

acidemia. Involvement of basal ganglia, similarly, is

very common, but in the case of methylmalonic acidemia, it involves globus pallidus rather than the caudate/

putamen as in propionic acidemia.50,51,62-65

The smaller number of infants, approximately 35,

reported with a defect in cobalamin metabolism characterized by impaired synthesis of both methylcobalamin and adenosylcobalamin (see Table 15-4) (see the

next section, ‘‘Metabolic Features’’) and onset in the

first month of life also had a generally unfavorable neurological outcome (not shown in Table 15-5).59,66-70

The clinical and neuroradiological features were similar, albeit milder, than those observed in patients with

the mutase deficiencies, and the metabolic features

included homocystinuria as well as methylmalonic acidemia. At least 80% subsequently exhibited mental

retardation, and completely normal intellectual functioning was very unusual. Available genetic data indicate that these disorders all exhibit autosomal recessive

inheritance.8

Metabolic Features

Major Findings. The constellation of severe ketoacidosis, methylmalonic acidemia, hyperglycinemia,

hyperammonemia, neutropenia, and thrombopenia is

characteristic. Approximately 40% of neonatal patients

have also exhibited significant hypoglycemia with their

attacks of ketoacidosis.

As noted earlier, approximately 35 infants were

observed with a genetic defect that resulted in impaired

synthesis of both methylcobalamin and adenosylcobalamin and the additional metabolic feature of homocysteinemia/homocystinuria.8,59,66-68,70 However, unlike

the classic homocystinuria resulting from cystathionine

synthase deficiency (which is associated with elevated

levels of methionine and depressed levels of cystathionine), this type is associated with hypomethioninemia and

cystathioninuria (the product of homocysteine and

serine) (see Fig. 15-2).

Enzymatic Defects. The enzymatic defects in methylmalonic acidemias involve the methylmalonyl-CoA

mutase apoenzyme (two major defects) and the metabolism of vitamin B12 (three major defects), as noted in

the introduction to this section (see Table 15-4). The

defects have been demonstrated primarily in liver and

in cultured fibroblasts.7,8,15,45,58,59,71,72

The two major defects of the mutase apoenzyme

result, as noted earlier, in either complete or partial

deficiency of enzyme activity. In most reported examples of complete deficiency of mutase activity, little or

no immunoreactive enzyme protein was present.8,58 In

the cases with partial deficiency of activity, a presumably altered enzyme with defective catalytic function

was present, because the amount of immunologically

reactive protein varied from 20% to 100% of control

values.8,58

The three major sites of the defects in vitamin B12

metabolism are shown in Figure 15-2. Under normal

circumstances, vitamin B12, bound to a carrier protein,



694



UNIT V



METABOLIC ENCEPHALOPATHIES



is internalized by the cell through endocytosis; the

endosome is taken up by the lysosome, proteases of

which degrade the carrier protein, and the cobalamin

is released into the cytosol, where reduction and methylation take place. A portion of the cobalamin released

into the cytosol enters the mitochondrion for reduction

and adenosylation.45,73 The defect that results in

impaired synthesis of both methylcobalamin and adenosylcobalamin involves an event after binding and

internalization (i.e., after cellular uptake).8 The defects

of vitamin B12 metabolism have been defined through

studies of cultured fibroblasts from affected

patients.8,45,74-76

Pathogenesis of Metabolic Features. The causes of

the various metabolic consequences of the methylmalonic acidemias are similar in many ways to those

described for other disorders in the propionate and

methylmalonate pathway, especially regarding the

hyperglycinemia and the hyperammonemia. The ketoacidosis

is not as readily accounted for because it is more severe

than would be expected from the accumulation of

methylmalonic acid. Methylmalonyl-CoA is an inhibitor of pyruvate carboxylase, and its product, succinylCoA, is involved in gluconeogenesis by conversion to

pyruvate (see earlier discussion). Together, these effects

could lead to an impairment of gluconeogenesis to

account for the hypoglycemia in nearly one half of the

neonatal cases and, secondarily, to increased catabolism of lipid, with resultant ketosis and acidosis.77

The accumulation of odd-numbered and methylbranched fatty acids in neural and other tissues of

relates,

respectively,

to

affected

patients26,28



A



substitution of propionyl-CoA for acetyl-CoA as

primer for the fatty acid synthetase reaction and to

the substitution of methylmalonyl-CoA for malonylCoA for chain elongation in the same reaction (see

Fig. 15-3). The genesis of the defects of sulfur amino

acid metabolism in the disorder with impaired synthesis of both methylcobalamin and adenosylcobalamin

relates to a disturbance of the methylation of homocysteine to form methionine; the enzyme for this reaction,

methionine synthase, requires methylcobalamin (see

Fig. 15-2). The consequences of the disturbance of

homocysteine methylation, as noted earlier, are homocystinuria, hypomethioninemia, and cystathioninuria, the

last resulting because some of the accumulated homocysteine is converted to cystathionine.

Neuropathology

The neuropathological features of the methylmalonic

acidemias suggest a derangement of myelination.27,78,79

An abnormality of myelin with features similar to those

described for propionic acidemia has been observed.27

Particular involvement of spinal nerve roots rather than

central myelin was noted in one premature infant

studied.78 Whether the myelin defect relates to

the abnormal accumulation of odd-numbered and

methyl-branched fatty acids in glial membranes, as discussed earlier, remains to be established.

A carefully studied infant of 36 weeks of gestation

(death at 4 days of age) exhibited selective death of

immature neurons (i.e., residual neuronal cells in germinal matrix), migrating neuroblasts, and neurons of

the external granule cell layer of cerebellum (Fig. 15-6).

The cytological characteristics, marked karyorrhexis,



B



Figure 15-6 Selective death of immature neurons in a 4-day-old infant with methylmalonic acidemia. A, Photomicrograph of the cerebellar

cortex shows karyorrhectic immature neuroblasts in the external (top) and internal granule cell layers (bottom). Purkinje cells (arrows) are relatively

well preserved. B, At higher magnification, karyorrhexis in the external granule cell layer is prominent. (From Sum JM, Twiss JL, Horoupian DS:

Selective death of immature neurons in methylmalonic acidemia of the neonate: A case report, Acta Neuropathol (Berl) 85:217-221, 1993.)



Chapter 15



Disorders of Organic Acid Metabolism



695



were compatible with apoptotic cell death (see

Chapter 2) and suggested that the toxic effect of the

metabolites of methylmalonic acidemia particularly

involved provocation of apoptosis of immature neuronal cells. Involvement of the external granule cell layer

of cerebellum may be related etiologically to the

occasional occurrence of cerebellar hemorrhage with

methylmalonic acidemia.40

As noted earlier, as with propionic acidemia, evidence for basal ganglia lesions has been obtained by

brain imaging later in infancy and childhood. In

methylmalonic acidemias, the globus pallidus is preferentially affected. This finding is consistent with the

occurrence of dystonia and extrapyramidal features on

follow-up in approximately 20% to 25% of cases of

methylmalonic acidemia of neonatal onset.60



follow-up, whereas of the 10 who did not respond to

vitamin B12, none was normal.6



Management

Antenatal Diagnosis. Antenatal detection of the

methylmalonic acidemias has been accomplished primarily by detecting elevated methylmalonate content in

the amniotic fluid and maternal urine and by enzymatic

assay of cultured amniotic fluid cells (mutase activity

and adenosylcobalamin synthesis).8,45,57,80-83 The possibility of prenatal therapy with cobalamin supplements

was shown initially by demonstrating a decrease in

maternal excretion of methylmalonic acid after administration of such supplements to the mother of an

affected fetus.57,82 A subsequent case, treated similarly

in utero and postnatally, had normal growth and development in early infancy.84 However, because at least

60% of neonatal cases are not cobalamin responsive,6,8,60 this approach may not be highly useful for

the majority of affected fetuses.

In the rare infants with the combined defect

resulting in both methylmalonic acidemia and

homocystinuria, large doses of hydroxycobalamin also

are important.85 Betaine, another methyl donor, also

may be beneficial. Follow-up data are too sparse

to assess effects on neurological development. It

is likely that both prenatal and postnatal therapy will

be critical.70,85



Disorders of pyruvate and mitochondrial energy

metabolism have been the topic of active research in

the past two decades and constitute uncommon but

serious neonatal neurological disorders. Together

with disorders of propionate and methylmalonate

metabolism and of branched-chain ketoacid metabolism, these disorders are important examples of organic

acid disturbances. In large part because of the difficulties in studying the complex enzyme systems involved,

the elucidation of abnormalities of pyruvate and mitochondrial energy metabolism has been relatively recent.

Disorders of pyruvate and mitochondrial energy

metabolism may lead to striking metabolic acidosis

with lactic acidemia in the neonatal period. Disorders

related to pyruvate metabolism may involve either the

Krebs citric acid cycle or the electron transport

system. Disorders of the citric acid cycle with neonatal

onset include deficiencies of alpha-ketoglutarate decarboxylation (dihydrolipoyl dehydrogenase deficiency),

succinate dehydrogenase, or fumarase.87 However,

because only a few well-studied neonatal cases have

been documented, these disorders are not discussed

in detail. Fumarase deficiency with fumaric acidemia

is the most common of these conditions with a neonatal presentation. Reports delineate a rare neonatal or

early infantile syndrome of hypotonia, seizures, dysmorphic facial features, frontal bossing, microcephaly,

neonatal polycythemia, diffuse polymicrogyria, dysgenetic corpus callosum, hypomyelination, and ventriculomegaly.87-89 Disorders of the electron transport chain are

more common. In addition to the metabolic abnormalities, the prominent features of these disorders

include manifestations of encephalopathy, myopathy,

or both, and thus they are discussed in Chapters 16

and 19. In this chapter, I focus on disorders of pyruvate

metabolism, because the metabolic manifestations,

particularly lactic acidemia, tend to dominate the neonatal clinical presentation.



Early Detection and Acute and Long-Term

Therapy. Early detection in the neonatal period and

the importance of acute and long-term therapy are

essentially as described for propionic acidemia.

Therapy consists of a low-protein diet or a diet low in

the amino acid precursors of methylmalonate, supplemented with cobalamin (see next section) and L-carnitine.8 The possible role of antibiotic therapy to reduce

production of methylmalonate by bacteria in the gastrointestinal tract may also be relevant in this

condition.8,58

Vitamin B12. Because some patients with isolated

methylmalonic acidemias respond to vitamin B12 (see

earlier discussion), a trial of this vitamin as hydroxycobalamin in high doses is indicated in such

patients.8,45,54,58,86 In a series of 21 infants with

neonatal-onset methylmalonic acidemia, of the

11 who responded to vitamin B12, 3 were normal on



Gene Therapy. As with propionic acidemia, liver

transplantation has been used in infants with methylmalonic acidemia.55,56 Initial results in transplanted

infants were not clearly beneficial. The situation is

complicated in methylmalonic acidemia because the

defective enzyme is active not only in liver but also in

kidney and brain. Later renal failure is a recognized

complication of the disease. Whether early liver transplantation or combined liver-kidney transplantation is

optimal is currently under study.

DISORDERS OF PYRUVATE AND

MITOCHONDRIAL ENERGY METABOLISM



Normal Metabolic Aspects

Pyruvate occupies a central position in intermediary

metabolism (Fig. 15-7).87,90,91 It is formed primarily

from glucose through the process of glycolysis, in

brain as in other tissues. The major metabolic fates of



696



UNIT V



METABOLIC ENCEPHALOPATHIES



Glucose



T



R



Alanine



PYRUVATE

C



Lactate



D

Acetylcholine



Oxaloacetate



Acetyl-CoA

Fatty acids

Cholesterol



Aspartate



Citric

acid

cycle



CO2



ATP



ADP



Figure 15-7 Major metabolic fates of pyruvate. See text for details. ADP, adenosine diphosphate; ATP, adenosine triphosphate; C, carboxylation;

CoA, coenzyme A; D, decarboxylation; R, reduction; T, transamination.



pyruvate are shown in Figure 15-7 and are summarized

in Table 15-6.

Transamination results in the formation of alanine,

used, in part, for protein synthesis. The reverse reaction is particularly important in liver for gluconeogenesis from alanine.

Reduction to lactate is catalyzed by lactate dehydrogenase. Lactate can be used for gluconeogenesis

through the reversal of this reaction.

Pyruvate carboxylation, catalyzed by the biotindependent enzyme pyruvate carboxylase, results in

the formation of oxaloacetate. This step is critical in

gluconeogenesis in liver and in several other tissues

(but not to any significant extent in brain).90

Oxaloacetate also is an important intermediate in the

citric acid cycle, and this reaction plays a role in priming the cycle. Oxaloacetate transamination results in

the formation of aspartate, an excitatory neurotransmitter in brain, a precursor for protein synthesis, and

an important component of the urea cycle.

Pyruvate decarboxylation, catalyzed by the thiaminedependent pyruvate dehydrogenase complex, is an

TABLE 15-6



Major Metabolic Fates of Pyruvate



Transamination

Gluconeogenesis

Alanine synthesis

Reduction

Lactate formation

Carboxylation

Gluconeogenesis

Aspartate synthesis

‘‘Prime’’ citric acid cycle

Decarboxylation

Energy production

Lipid synthesis

Acetylcholine synthesis



exceedingly important reaction in all tissues, including

brain, in view of the nature of its product, acetyl-CoA.

The reaction thereby plays a major role in citric acid

cycle function, in adenosine triphosphate synthesis,

and in the syntheses of acetylcholine and lipids (i.e.,

fatty acids and cholesterol).

Biochemical Aspects of Disordered

Metabolism

Enzymatic Defects and Essential

Consequences

Of the four major fates of pyruvate, impairment of two,

decarboxylation and carboxylation, has been described

(see later discussion). Defects in the pyruvate dehydrogenase complex and in pyruvate carboxylase have been

the enzymatic defects for these disorders. Both defects

cause accumulation of pyruvate, lactate, and alanine

proximal to the enzymatic block, but obviously the

consequences distal to the enzymatic blocks differ to

some degree.

Relation to Acute Neurological Dysfunction

and to Neuropathology

The mechanisms for brain injury in defects of the

pyruvate dehydrogenase complex or of pyruvate carboxylase include acute and more long-lasting effects.

The common metabolic feature of these disorders,

lactic acidosis, may be very important in causing the

acute neurological dysfunction. Moreover, an irreversible structural deficit is a likely consequence when the

acidosis is severe and prolonged.

A second metabolic feature, important in the genesis

of the acute neurological dysfunction associated with

disturbances in pyruvate metabolism, is impairment of

the synthesis of factors important in neurotransmission.

Thus, a deficiency of pyruvate dehydrogenase complex

activity may be expected to lead to a diminution in the



Chapter 15



synthesis of acetylcholine (an established and important neurotransmitter).90,91 A deficiency of pyruvate

carboxylase activity, with resulting decreased synthesis

of oxaloacetate and function of the citric acid cycle,

could lead to a disturbance of the excitatory amino

acid transmitters, aspartate (a transamination product

of oxaloacetate) and glutamate (a transamination product of the citric acid cycle intermediate alphaketoglutarate).

A third metabolic feature, probably important in the

genesis of both acute and chronic effects, is impairment

of energy production.90 This impairment would be

expected with a disturbance of acetyl-CoA synthesis

by pyruvate dehydrogenase complex deficiency, but

the disturbance of oxaloacetate synthesis by pyruvate

carboxylase deficiency may also have a similar

consequence.

A fourth metabolic feature, which may be of particular importance in the genesis of the long-term

irreversible structural deficits, is a disturbance in the

synthesis of brain lipids and proteins. Disturbed pyruvate

dehydrogenase complex activity would be expected

to lead to impairment in the synthesis of fatty acids

and cholesterol, critical constituents of neural

membranes, including myelin, by impairment of

acetyl-CoA formation. Experimental support for this

notion is available.92 Deficits of pyruvate dehydrogenase complex and of pyruvate carboxylase also would

lead to an alteration in levels of certain amino

acids (e.g., alanine and aspartate) and perhaps thereby

to a secondary disturbance of protein synthesis. The

relative roles of these several factors remain to be

defined in the inborn errors of pyruvate decarboxylation and carboxylation.

Pyruvate Dehydrogenase Complex

Deficiency

Clinical Features

In general, pyruvate dehydrogenase complex deficiency

is associated with three major categories of clinical

phenotype, divisible according to age of onset: (1) neonatal types, (2) infantile form (onset, 3 to 6 months),

and (3) later-onset benign forms with episodic

ataxia.87,90,91,93-96 The infantile form is characterized

particularly by hypotonia, cranial nerve signs (especially

ophthalmoplegia), ataxia, delayed development, ventilatory disturbance, and other features, and it is most

often (%85% of cases) associated with the neuropathological features of Leigh syndrome (see Chapter 16).

The later-onset forms may be punctuated by transient

episodes of ataxia or paraparesis, but, overall, development is normal or only mildly disturbed.

The clinical features of the neonatal forms of pyruvate

dehydrogenase complex deficiency consist of two basic syndromes: (1) marked lactic acidosis, often with dysmorphic craniofacial features and brain anomalies; and (2)

Leigh syndrome. Newborns with Leigh syndrome

overlap with those with the infantile forms of pyruvate

dehydrogenase complex deficiency with slightly later

onset of Leigh syndrome, noted in the previous



TABLE 15-7



Disorders of Organic Acid Metabolism



697



Common Features of Pyruvate

Dehydrogenase Complex Deficiency

with Neonatal Onset



Clinical Features

Stupor, coma

Tachypnea

Seizures

Hypotonia

Dysmorphic facial features

Metabolic Features

Acidosis

Lactic and pyruvic acidemia

Hyperalaninemia

Neuropathological Features

Cerebral cortical and white matter atrophy

Subcortical white matter cysts

Cerebral gyral abnormalities (polymicrogyria, pachygyria)

Impaired myelination

Agenesis of the corpus callosum

Dysgenetic brainstem and cerebellum

Subacute necrotizing encephalopathy (Leigh syndrome)*

*See Chapter 16.



paragraph and discussed in Chapter 16. In this group,

the onset of symptoms generally is in the first week of

life and is often within the first 24 hours.90,93-109

The major clinical features of the more common of the

two neonatal forms (i.e., marked lactic acidosis) include

stupor, tachypnea, hypotonia, and seizures (Table

15-7). The infant’s course may be fulminating, with

coma evolving in hours to a day or so. Most of the

infants with this severe presentation have died in the

first year of life. Newborns with pyruvate dehydrogenase complex deficiency may exhibit prominent craniofacial dysmorphic features (Fig. 15-8) and signs of

cerebral dysgenesis.87,89-91,94,96,110-114 The features of

dysgenesis include partial or total agenesis of the

corpus

callosum,

ventricular

dilation,

gyral



Figure 15-8 Pyruvate dehydrogenase deficiency: craniofacial dysmorphism. This affected girl shows frontal bossing, an upturned nose,

a thin upper lip, and low-set ears. (From De Meirleir L, Lissens W,

Wayenberg JL, Michotte A, et al: Pyruvate dehydrogenase deficiency:

Clinical and biochemical diagnosis, Pediatr Neurol 9:216-220, 1993.)