Clinical manifestations

Presentation and course

The symptoms and age of onset of isolated beta-methylcrotonyl-CoA carboxylase deficiency are variable. More than a dozen children with the disorder have been reported (18; 59; 20; 10; 06; 27; 35; 40; 64; 53; 05; 44; 66; 41; 45; 78; 80; 34). The first patient reported was the only one not confirmed enzymatically to have isolated beta-methylcrotonyl-CoA carboxylase deficiency (18; 59). The onset of symptoms has ranged from the neonatal period to 4.5 years of age. One 4-year-old affected sibling was asymptomatic when her symptomatic younger brother was diagnosed (10). Most patients have had feeding difficulties, vomiting, dehydration, respiratory problems (apnea), hypotonia, lethargy, and seizures. The seizures have been characterized as infantile, myoclonic, tonic-clonic, generalized or involuntary movements, and back-arching. Status epilepticus can be the major symptom of a child with isolated beta-methylcrotonyl-CoA carboxylase deficiency (17). The occurrence of symptoms may be episodic. Several children have become comatose and have died of pneumonia or cardiorespiratory collapse (59; 20; 05). Several patients have developed mental retardation, even with seemingly appropriate therapy. One child had developmental delay but caught up with treatment (40). Two children were described as having a Reye syndrome-like presentation with hypoglycemia (06; 40). Although this disorder is usually considered to present with mild symptoms, severe hypoglycemia can occur (48). The first child described with the disorder had muscle atrophy, tongue fibrillations, and absent deep tendon reflexes, suggestive of spinal muscular atrophy (Werdnig-Hoffmann disease) (18; 59), although none of the others were reported with these findings. This patient also had urine that smelled like that of a male cat. One child had an asymmetrical head and a short neck (20). Several enzyme-deficient children have remained asymptomatic (10; 64). A 15-year-old girl was diagnosed with the disorder after she was initially thought to have cerebral palsy (45). The girl had symptoms since birth, including growth and mental retardation, seizures, spastic quadriplegia with opisthotonic dystonia, and marked brain atrophy involving the white and gray matter. An adult with the deficiency presented with severe muscle pain and weakness (11).

Bannwart and colleagues suggested classifying isolated beta-methylcrotonyl-CoA carboxylase deficiency into three categories according to the clinical features (05). The first, the neonatal or early-infantile form, is characterized by feeding difficulties, vomiting, hypotonia, and seizures that result in neurologic damage; it has a poor prognosis despite protein restriction. A child with the neonatal form of the disorder exhibited severe necrotizing encephalopathy that failed to respond to protein restriction and pharmacological doses of biotin (09). The second, the late-infantile form (onset at 1 to 2 years of age), may present with seizures, hypoglycemia, hyperammonemia, seizures and hepatomegaly, or a Reye syndrome-like presentation. Most of these patients have responded well to a protein- or leucine-restricted diet. The third, the juvenile form (onset after 2 years of age), may present with mild symptoms, such as vomiting and dehydration, with improvement on a protein-restricted diet. The only patients included in this third group were Vietnamese siblings who were initially on a high-carbohydrate, protein-poor diet until they came to the United States and were exposed to a high-protein diet and became symptomatic (10). It seems premature to use this classification scheme to predict prognosis. The variability in expression seen in isolated beta-methylcrotonyl-CoA carboxylase deficiency is no different from that seen in most other inherited metabolic disorders. Genotype and phenotype correlations will await further biochemical and molecular elucidation of enzyme defects.

A 3-month-old infant who presented with failure to thrive, hypotonia, and respiratory difficulties was diagnosed with 25% of mean normal enzyme activity in the patient’s leukocyte extracts (partial enzyme deficiency) but only 3.8% of normal activity in fibroblast extracts (70). The incorporation of radiolabeled isovalerate in the patient’s fibroblasts was, however, higher than that observed in fibroblasts from individuals with profound enzyme deficiency. The clinical features of this child were as severe as those of children with profound deficiency. The authors suggest that enzyme activity may be different in different tissues and that the severity of the disorder may depend on the enzyme activity in the brain.

The phenotype in this disorder is variable, even within the same family (19). A 16-month-old mildly developmentally delayed child exhibited seizures and hemiparesis and was found to have acute hemilateral focal edema and gliosis consistent with a “metabolic stroke” (58). The child was treated with protein restriction and carnitine but still had developmental delay and hemiparesis after 5 years. Another child presented with seizures and cerebral ischemia (49). A child who had failure to thrive, hypotonia, and respiratory difficulties died at 6.5 months of age, with partial isolated beta-methylcrotonyl-CoA carboxylase deficiency (25% of normal activity in peripheral blood lymphocytes and about 4% in cultured fibroblast extracts) (70). This child had moderate brain atrophy, and it was speculated that the severe clinical presentation was due to deficiency of enzyme activity in the brain. A 7-week-old male with the enzyme deficiency exhibited dilated cardiomyopathy that did not improve with carnitine supplementation, whereas his enzyme-deficient brother and father had mild symptoms but did not have cardiomyopathy (67). A female exhibited mild Reye syndrome-like features, hair loss, developmental delay, attention deficit, and symptoms of multiple sclerosis at 13 years of age (14). She initially responded well to steroid therapy and mitoxantrone therapy but not to beta-1a treatment.

A 9-month-old boy with severe psychomotor delay was diagnosed with mild beta-methylcrotonyl-CoA carboxylase deficiency (heterozygous for a missense mutation) (23). He initially exhibited infantile spasms at 3 weeks of age. Clinical symptoms and biochemical abnormalities improved markedly after biotin therapy. Protein restriction was subsequently instituted. However, the developmental deficits and some seizure activity have continued.

An adult male, who was followed for epilepsy for 5 years, was found to have beta-methylcrotonyl-CoA carboxylase deficiency after his nephews were diagnosed with the disorder; one nephew presented with atonic seizures and the other with language delay (19). These individuals exemplify the wide phenotypic variation seen in the disorder.

Male and female siblings with symptomatic beta-methylcrotonyl-CoA carboxylase deficiency were found to have bilateral thalamus and periventricular white matter calcifications (56).

A 1-year-old child with two variants of the MCCC2 gene presented with recurrent viral infections, nonspecific gastrointestinal symptoms of vomiting, hematochezia, and gaseous distention of the abdomen in the absence of metabolic abnormalities (32).

Prognosis and complications

Although few patients with isolated beta-methylcrotonyl-CoA carboxylase deficiency have been described, there are patients with mild symptoms, and some have developed normally when rapidly diagnosed and treated with carnitine and a protein-restricted, high-carbohydrate diet. The oldest patient is now about 9 years old (10). Patients should be carefully monitored because symptoms may be mild and episodic; several patients have developed sudden, severe metabolic compromise.

Clinical vignette

A 10-month-old girl had recent onset of difficulty feeding, vomiting, hypotonia, and episodes of "twitching." Previously, she had been healthy, except for some mild developmental delay. Her growth had been normal, at the 10th percentile. Laboratory studies indicated mild metabolic acidosis with an arterial blood gas pH of 7.30, a bicarbonate concentration of 14 mEq/L, and a base deficit of 20. Plasma ammonia concentration was normal. Plasma amino acid analysis was normal, but urinary organic acids revealed elevated concentrations of beta-hydroxyisovalerate and beta-methylcrotonylglycine. There was no elevation of lactate, methylcitrate, or hydroxypropionate. The child was tentatively diagnosed with isolated beta-methylcrotonyl-CoA carboxylase deficiency. The child was treated initially with intravenous fluids, a trial of oral biotin, and a protein-restricted diet. She improved markedly over several days. Beta-methylcrotonyl-CoA carboxylase activity was undetectable in extracts of her cultured fibroblasts, whereas propionyl CoA carboxylase and pyruvate carboxylase activities were normal, thereby confirming that she had isolated beta-methylcrotonyl-CoA carboxylase deficiency. The child was continued on a protein-restricted diet and did well, with only a single episode of vomiting, hypotonia, and metabolic compromise when she had a severe bout of gastroenteritis.

With the advent of newborn screening, there has been an unexpectedly large number of children identified with isolated beta-methylcrotonyl-CoA carboxylase deficiency. Thirty-five children whose concentration of blood C5OH metabolites correlated with their residual enzyme activity had variable clinical outcomes (03). A considerable number of these children developed metabolic and neurologic symptoms. In another study of 88 children with the disorder, although over half were asymptomatic, many of them exhibited metabolic abnormalities (43). The results of newborn screening in California have also indicated that many children confirmed to have the enzyme deficiency have mild clinical phenotypes (38). Genotype and biochemical phenotype did not predict clinical outcome.

When neonates are found with elevated C5-OH metabolites on newborn screening and only a single mutation in the carboxylase genes is identified, their mothers should be tested to exclude the possibility that they have isolated beta-methylcrotonyl-CoA carboxylase deficiency (30; 37).

Biological basis

Etiology and pathogenesis

The primary defect in this disorder is an isolated deficiency in the activity of beta-methylcrotonyl-CoA carboxylase in peripheral blood leukocytes or skin fibroblasts, whereas the activities of the other two mitochondrial carboxylases, propionyl-CoA carboxylase and pyruvate carboxylase, are normal. In the leucine degradation pathway, beta-methylcrotonyl-CoA carboxylase converts beta-methylcrotonyl-CoA to beta-methylglutaconyl-CoA. The accumulating beta-methylcrotonyl-CoA is subsequently converted to beta-hydroxyisovalerate and beta-methylcrotonylglycine, which are excreted in the urine.

Isolated beta-methylcrotonyl-CoA carboxylase deficiency is inherited as an autosomal recessive trait. Males and females have been affected, and the disorder has occurred in siblings. Consanguinity has been reported in several families.

The genes encoding the two subunits of human beta-methylcrotonyl-CoA carboxylase have been characterized by several groups (24; 08; 25; 31; 47; 79; 76). The gene for the biotin-containing alpha subunit is located on chromosome 3q26-28 (25; 31; 47). The gene encoding the non-biotin-containing subunit is located on chromosome 5q12-13 (25; 31). Mutations, including missense, nonsense, and splicing defects, have been identified in each of the subunits (24; 25; 31; 16; 65; 12; 42). These results failed to indicate a correlation between the mutation and the severity of the clinical symptoms. This was interpreted to suggest that epigenetic factors affect the phenotype.

Two children, one with severe neurologic symptoms, exhibited increased urinary excretion of beta-methylcrotonylglycine and beta-hydroxyisovaleric acid and were determined to have partial beta-methylcrotonyl-CoA carboxylase deficiency. Both exhibited biotin-responsiveness and were heterozygous for a missense mutation in the alpha subunit of the enzyme (R385S). This interesting mutation represents a dominant negative allele that results in clinical and biochemical abnormalities in heterozygous individuals and responds to biotin therapy (07).

Mutation analysis has been reported in more than 100 children with isolated beta-methylcrotonyl-CoA carboxylase deficiency (46; 21; 75). About two thirds of these were found in asymptomatic children identified on newborn screening. A point mutation in the carboxylase gene was shown to disrupt an exon splice enhancer that causes exon skipping in addition to utilization of a cryptic exon (60). There was no clear genotype-phenotype correlation in the symptomatic children with the deficiency (13). However, having only a single mutation in either of the subunits of the enzyme may be the cause of positive newborn screening for the disorder (43). The authors suggest that other factors besides genotype are responsible for the variability in the phenotype.

Isolated beta-methylcrotonyl-CoA carboxylase deficiency may present with mild metabolic acidosis, with or without ketosis. Elevated concentrations of beta-hydroxyisovaleric acid and beta-methylcrotonylglycine in urine were observed essentially in all patients. One patient excreted elevated concentrations of urinary 2-oxoglutaric acid (20). Several patients developed hypoglycemia, whereas others had mild hyperammonemia. Serum lactate was not elevated in most patients. Some patients had elevated concentrations of branched-chain amino acids in plasma. When tested, patients have excreted large concentrations of acyl-carnitines in the urine, resulting in carnitine deficiency similar to that seen in other organic acidemias. One patient had elevated serum liver enzyme activities. A liver biopsy of one of the patients with Reye syndrome-like presentation revealed fatty infiltration (40). A 14-month-old child was reported with leukodystrophy and abnormal purine metabolites in the CSF (15). EEG studies were reported in only one patient and were normal. A CT scan and an MRI of another patient examined were normal. An infant with rapid onset of neurologic symptoms who failed therapy had disseminated encephalomalacia, cystic changes, ventricular dilatation, and cerebral atrophy on cerebral ultrasonography and cranial CT findings (09). A 7-year-old symptomatic female initially had a normal CT of the brain; however, a subsequent MRI of the brain showed periventricular white matter changes as well as other abnormalities of the medulla, pons, and cerebellum (01). These reports indicate the variability of expression of this disorder.

None of the patients treated with biotin were responsive, which is probably because the defective enzyme is structurally abnormal and does not involve abnormal biotin binding.

A report of the Inborn Errors of Metabolism Collaborative found that newborn screening identified the vast majority of individuals with the disorder, but there was a small group of affected individuals who were identified symptomatically (22). There was no correlation between the elevation of C5OH concentrations and the presentation of symptoms or developmental delays. As yet, there is still no predictive marker for identifying which enzyme-deficient children are at risk for biochemical or developmental abnormalities.

Epidemiology

With the advent of newborn screening by tandem mass spectrometry, the incidence of beta-methylcrotonyl-CoA carboxylase deficiency screening has been estimated to be 1 in 64,000 newborns in North Carolina (36). Newborn screening has revealed that beta-methylcrotonyl-CoA carboxylase deficiency is one of the most common inherited metabolic disorders, with an incidence of about 1 in 36,000 births (02). However, there is still controversy about newborn screening for the disorder (71). Based on newborn screening, beta-methylcrotonyl-CoA carboxylase deficiency was found to be very common in regions of China; however, the question about the disorder being benign has questioned the necessity for screening (68; 69).

Prevention

Prenatal diagnosis for this disorder has been reported in one pregnancy by determining beta-methylcrotonyl-CoA carboxylase activity in amniocytes. The enzyme activity was undetectable in this pregnancy (05). In addition, beta-hydroxyisovaleric acid and beta-methylcrotonylglycine concentrations were elevated 8-fold to 20-fold in the amniotic fluid using a stable isotope dilution assay, and the pregnancy was terminated at 21 weeks gestation (33). Unfortunately, the diagnosis could not be confirmed on fetal tissues. It may also be possible to perform prenatal diagnosis by measuring beta-methylcrotonyl-CoA carboxylase activity in chorionic villi biopsy samples. There is now the potential for prenatal diagnosis by mutation analysis of DNA from amniocytes or chorionic villi biopsy samples.

Differential diagnosis

Nonspecific clinical symptoms, including vomiting, hypotonia, and seizures, are often characteristic of treatable disorders such as sepsis, gastrointestinal obstruction, and cardiorespiratory problems. After exclusion of these conditions, or when these findings are accompanied by metabolic ketoacidosis, the presence of an inborn error of metabolism should be considered.

Isolated beta-methylcrotonyl-CoA carboxylase deficiency must be differentiated from the other isolated carboxylase deficiencies as well as from the biotin-responsive multiple carboxylase deficiencies (73). Organic acid analysis of the urine is the most useful method. Elevated concentrations of urinary beta-hydroxyisovalerate and beta-methylcrotonylglycine are consistently observed in isolated beta-methylcrotonyl-CoA carboxylase deficiency. These metabolites are also found in biotin holocarboxylase synthetase deficiency, biotinidase deficiency, and acquired biotin deficiency but are not usually seen in propionyl-CoA carboxylase and pyruvate carboxylase deficiencies. Elevated concentrations of urinary lactate, methylcitrate, and beta-hydroxypropionate, in addition to beta-hydroxyisovalerate and beta-methylcrotonylglycine are indicative of multiple carboxylase deficiency. Isolated beta-methylcrotonyl-CoA carboxylase deficiency can be confirmed by demonstrating deficient enzyme activity, whereas the activities of the other mitochondrial carboxylases (propionyl-CoA carboxylase and pyruvate carboxylase) are normal in peripheral blood leukocytes or in cultured fibroblasts.

Several children were described with an infantile mitochondrial DNA depletion syndrome that is characterized by elevated urinary organic acids. Decreased activities of beta-methylcrotonyl-CoA carboxylase and propionyl-CoA carboxylase were found in their fibroblasts. The depleted mtDNA is assumed to result in aberrations in normal mitochondrial function, altering carboxylase activities (77).

One study showed that nonspecific phenotypes attributed to beta-methylcrotonyl-CoA carboxylase deficiency are associated with consanguinity and are likely not due to mutations in the beta-methylcrotonyl-CoA carboxylase enzyme but result from rare homozygous mutations in other disease genes (57).

Diagnostic workup

Elevated concentrations of beta-hydroxyisovaleric acid and beta-methylcrotonylglycine on urinary organic acid analysis suggest a diagnosis of isolated beta-methylcrotonyl-CoA carboxylase deficiency but may also occur in biotin holocarboxylase synthetase deficiency, biotinidase deficiency, and acquired biotin deficiency (73). Isolated beta-methylcrotonyl-CoA carboxylase deficiency has been diagnosed by measuring elevated concentrations of beta-hydroxyisovalerylcarnitine in plasma and urine (54; 66). However, several children who were ultimately shown to have beta-methylcrotonyl-CoA carboxylase deficiency initially had no or only trace quantities of urinary 3-methlycrotonylglycine (74). The disorder is definitively diagnosed by finding deficient beta-methylcrotonyl-CoA carboxylase activity in the peripheral blood leukocytes and in cultured skin fibroblasts, whereas the activities of propionyl-CoA carboxylase and pyruvate carboxylase are normal. The disorder was diagnosed in four Amish and Mennonite mothers by finding elevated concentrations of abnormal acylcarnitine metabolites in blood spots obtained from their newborns who did not have the enzyme deficiency (26). Newborn screening using tandem mass spectrometry of blood spots has identified infants with elevated 3-hydroxyisovalerylcarnitine. About one half of the identified infants had elevated urinary concentrations of 3-hydroxyisovaleric acid and 3-methylcrotonylglycine. In some of these children, the abnormal organic acids were shown to be of maternal origin. About 40% of the remaining children were shown to have isolated beta-methylcrotonyl-CoA carboxylase deficiency by enzyme analysis in their fibroblasts and lymphoblasts (36).

Based on the results of newborn screening for beta-methylcrotonyl-CoA carboxylase deficiency, the disorder is more common than originally thought (51). This is likely due to finding many asymptomatic enzyme-deficient mothers of infants found to have elevated metabolites of the disorder by newborn screening.

Management

Isolated beta-methylcrotonyl-CoA carboxylase deficiency should be treated quickly and aggressively. Affected individuals should be adequately hydrated, especially if there is evidence of dehydration. Large amounts of parenteral fluids help to facilitate the excretion of abnormal organic acid metabolites. Adequate nutrition is also essential. Acutely, protein is usually restricted because leucine is the source of organic acids. Care must be taken to reintroduce protein, especially with formulas that are not low in leucine. If protein is restricted for too long, the child may still degrade endogenous protein. Because inadequate caloric intake can result in tissue breakdown and endogenous protein degradation, and because hypoglycemia has been described in several patients, it is imperative to supply sufficient calories in the form of parenteral glucose or oral polysaccharides. Severe acidosis may require bicarbonate supplementation in addition to hydration. Intervening infections must be diagnosed and treated aggressively.

Although beta-methylcrotonyl-CoA carboxylase is a biotin-dependent enzyme, pharmacological doses of biotin have not resulted in true vitamin responsiveness or increased enzyme activity in any of the patients. A trial of biotin is, however, important to differentiate isolated beta-methylcrotonyl-CoA carboxylase deficiency from the biotin-responsive multiple carboxylase deficiencies, especially because therapy may be life-saving in the latter disorders and because it takes time before these disorders can be excluded by enzymatic testing.

Oral carnitine (100 mg/kg per day) is useful in resupplying the body with the carnitine lost on acylcarnitine excretion. In addition, glycine supplementation appears helpful by conjugating with beta-methylcrotonyl-CoA and increasing the excretion of beta-methylcrotonylglycine (55). A study indicates that some individuals with isolated beta-methylcrotonyl-CoA carboxylase deficiency benefit from carnitine supplementation (63).

During periods of metabolic stability, therapy consists of protein restriction or restricting the dietary intake of leucine. During infancy, this can be accomplished by the use of special commercial formulas. Because of the rarity of isolated beta-methylcrotonyl-CoA carboxylase deficiency, a formula devoid of only leucine is not currently available. Therefore, a formula such as that used to treat maple syrup urine disease should be supplemented with valine and isoleucine. These formulas are then supplemented with normal formulas to provide adequate leucine for anabolic growth and must be adjusted regularly, usually with the assistance of a dietitian who is trained in metabolic disorders. Dietary leucine restriction is expected to be life-long, and the patient will develop an increased dependency on foods that are low in leucine. Affected children should benefit from supplementation with carnitine. Care must be taken in using anticonvulsive medications such as carbamazepine in these children because it reduces the function of biotin-dependent enzymes, including any residual activity of beta-methylcrotonyl-CoA carboxylase (50). There should be routine consultation with metabolic specialists and dietitians as well as periodic developmental assessment and psychosocial support of the patient and family.

With the advent of newborn screening and the high incidence of the disorder, many children are being identified with beta-methylcrotonyl-CoA carboxylase deficiency. Because of the variability of clinical features of the disorder, a panel of metabolic experts was convened to determine evidence-based guidelines for the diagnosis and management of the disorder (02). A group of healthcare providers was surveyed about the management of variants with beta-methylcrotonyl-CoA carboxylase deficiency (04). Although agreement was reached on many issues, the panel did not reach a consensus on a variety of other issues and recommended additional systematic evaluation of outcomes. The long-term clinical outcomes of individuals with isolated beta-methylcrotonyl-CoA carboxylase deficiency identified by newborn screening have been reported (39).

Special considerations

Pregnancy

There are no reports of pregnant individuals with isolated beta-methylcrotonyl-CoA carboxylase deficiency.

Anesthesia

There has been a report of the successful administration of anesthesia to a patient with beta-methylcrotonyl-CoA carboxylase deficiency (52).