Alpha-Methylacetoacetic Aciduria

A number sign (#) is used with this entry because alpha-methylacetoacetic aciduria is caused by homozygous or compound heterozygous mutation in the acetyl-CoA acetyltransferase-1 gene (ACAT1; 607809) on chromosome 11q22.

Description

Alpha-methylacetoacetic aciduria, also known as 3-ketothiolase deficiency, is an inborn error of isoleucine catabolism characterized by urinary excretion of 2-methyl-3-hydroxybutyric acid, 2-methylacetoacetic acid, tiglylglycine, and 2-butanone.

Clinical Features

Daum et al. (1971) first described this disorder of the sixth step in the catabolism of isoleucine, that for the conversion of alpha-methylacetoacetate to propionate in a Dutch and a Chilean family. As in many of the other inborn errors of branched-chain amino acid catabolism, the presenting clinical feature was recurrent severe metabolic acidosis. Both parents and a sib had increased amounts of alpha-methyl-beta-hydroxybutyric acid in the urine, and this was increased by administration of isoleucine. The proband also showed excessive alpha-methylacetoacetate in the urine. A follow-up of the probands by Fukao et al. (1993) showed normal somatic and neuropsychologic development with no recurrence of acute metabolic decompensation.

Hillman and Keating (1974) described a female patient with the 'ketotic hyperglycinemia syndrome' (see 606054 and 251000) and normal propionate and methylmalonate metabolism but markedly impaired catabolism of isoleucine. Studies of her urine and cultured fibroblasts suggested a defect in the beta-ketothiolase reaction which cleaves alpha-methylacetoacetyl CoA to propionyl-CoA and acetyl-CoA. The authors suggested that this was another potentially treatable condition of young infants with vomiting and acidosis. Their patient was more severely affected than those reported by Daum et al. (1973).

Sewell et al. (1998) described a pregnancy in a 25-year-old woman in whom the diagnosis of MAT deficiency had been made at the age of 8 years. She presented at 32 weeks of gestation, having taken no medication other than oral iodine and iron supplements. Physical examination and routine laboratory tests were normal. Urinary organic acid analysis demonstrated a constantly elevated excretion of 2-methyl-3-hydroxybutyrate. She was begun on carnitine supplementation. She delivered a clinically normal infant by cesarean section because of cephalopelvic disproportion. At the time of report, the baby was 2 years old and had developed normally.

Mrazova et al. (2005) reported a patient with T2 deficiency confirmed by genetic analysis. He presented at age 21 months with loss of consciousness after a short period of lethargy and vomiting following all-day fasting. Laboratory studies showed severe metabolic acidosis, hypoglycemia, ketonuria, hyperuricerima, and abnormal liver function tests. Organic acid profiling and enzymatic analysis indicated acetoacetyl-CoA thiolase deficiency. At age 4.5 years, he showed normal development after following a dietary regime with limited protein and lipid intake and frequent feeding.

Inheritance

Alpha-methylacetoacetic aciduria is an autosomal recessive disorder. The family reported by Daum et al. (1973) was consanguineous. The parents of the patient reported by Hillman and Keating (1974) were not related (Hillman, 1974).

Pathogenesis

The metabolic block involves beta-ketothiolase (Gompertz et al., 1974), the mitochondrial short-chain-length-specific thiolase (T2). Goodman (1980) reviewed the inherited organic acidemias, with a description of gas chromatography-mass spectrometry in their detection and study.

In the patient reported by Hiyama et al. (1986), Yamaguchi et al. (1988) found deficiency of mitochondrial acetoacetyl-CoA thiolase; the protein appeared to be absent. Genetic complementation analysis of 7 cell lines with this deficiency suggested the existence of 3 distinct complementation groups (Sovik et al., 1992).

Diagnosis

Hiyama et al. (1986) suggested that the deficiency of beta-ketothiolase can be demonstrated in leukocytes, thus obviating the need for skin biopsy.

Molecular Genetics

In a German boy with 3-ketothiolase deficiency, born of nonconsanguineous parents, Fukao et al. (1991) found compound heterozygosity for 2 mutations in the ACAT1 gene: an A347T (607609.0001) mutation inherited from the mother, and a mutation inherited from the father that abolished expression of the gene. This was apparently the first definition of a mutant ACAT allele. The patient showed normal development until his first ketoacidotic attack at the age of 6 months, following which severe retardation developed. The diagnosis of 3-ketothiolase deficiency was made by urinary organic acid analysis during the attack.

In patients from the original Dutch and Chilean families with 3-ketothiolase deficiency described by Daum et al. (1973), Fukao et al. (1993) identified homozygosity for a splice site and a missense mutation (607809.0006 and 607809.0007), respectively, in the ACAT1 gene.

In a pregnant woman with 3-ketothiolase deficiency, Sewell et al. (1998) identified 2 mutations in the ACAT1 gene (607908.0010 and 607809.0011); her child inherited only 1 mutation. Both of her husband's alleles were normal.

Sakurai et al. (2007) identified 7 novel and 2 previously reported mutations in 6 mitochondrial acetoacetyl-CoA thiolase-deficient patients.

Population Genetics

Monastiri et al. (1999) suggested that beta-ketothiolase deficiency is unusually frequent in Tunisia.