Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency: clinical course and description of causal mutations in two patients

Hereditary deficiency of mitochondrial HMG-CoA synthase (mHS, OMIM 600234) is a poorly defined, treatable, probably underdiagnosed condition that can cause episodes of severe hypoketotic hypoglycemia. We present clinical follow-up and molecular analysis of the two known mHS-deficient patients. The diagnosis of mHS deficiency is challenging because the symptoms and metabolite pattern are not specific. Moreover, enzyme analysis is technically difficult and requires sampling of an expressing organ such as liver. The patients, now aged 16 and 6 y, have normal development and have had no further decompensations since diagnosis. Patient 1 is homozygous for a phenylalanine-to-leucine substitution at codon 174 (F174L). Interestingly, although the F174 residue is conserved in vertebrate mHS and cytoplasmic HS isozymes, a Leu residue is predicted in the corresponding position of HS-like sequences from Caenorhabditis elegans, Arabidopsis thaliana, and Brassica juncea. Bacterial expression of human F174L-mHS produces a low level of mHS polypeptide with no detectable activity. Similarly, in purified cytoplasmic HS, which in contrast to purified human mHS is stable and can be studied in detail, the corresponding F-->L substitution causes a 10,000-fold decrease in V(max) and a 5-fold reduction in thermal stability. Patient 2 is a genetic compound of a premature termination mutation, R424X, and an as-yet uncharacterized mutant allele that is distinguishable by intragenic single nucleotide polymorphisms that we describe. Molecular studies of mHS are useful in patients with a suggestive clinical presentation.

Mitochondrial disease in superoxide dismutase 2 mutant mice

Oxidative stress has been implicated in many diseases. The chief source of reactive oxygen species within the cell is the mitochondrion. We have characterized a variety of the biochemical and metabolic effects of inactivation of the mouse gene for the mitochondrial superoxide dismutase (CD1-Sod2(tm1Cje)). The Sod2 mutant mice exhibit a tissue-specific inhibition of the respiratory chain enzymes NADH-dehydrogenase (complex I) and succinate dehydrogenase (complex II), inactivation of the tricarboxylic acid cycle enzyme aconitase, development of a urine organic aciduria in conjunction with a partial defect in 3-hydroxy-3-methylglutaryl-CoA lyase, and accumulation of oxidative DNA damage. These results indicate that the increase in mitochondrial reactive oxygen species can result in biochemical aberrations with features reminiscent of mitochondrial myopathy, Friedreich ataxia, and 3-hydroxy-3-methylglutaryl-CoA lyase deficiency.

HMG CoA lyase deficiency: identification of five causal point mutations in codons 41 and 42, including a frequent Saudi Arabian mutation, R41Q

The hereditary deficiency of 3-hydroxy-3-methylglutaryl (HMG) CoA lyase (HL; OMIM 246450 [http://www3.ncbi.nlm.nih. gov:80/htbin-post/Omim/dispmim?246450]) results in episodes of hypoketotic hypoglycemia and coma and is reported to be frequent and clinically severe in Saudi Arabia. We found genetic diversity among nine Saudi HL-deficient probands: six were homozygous for the missense mutation R41Q, and two were homozygous for the frameshift mutation F305fs(-2). In 32 non-Saudi HL-deficient probands, we found three R41Q alleles and also discovered four other deleterious point mutations in codons 41 and 42: R41X, D42E, D42G, and D42H. In purified mutant recombinant HL, all four missense mutations in codons 41 and 42 cause a marked decrease in HL activity. We developed a screening procedure for HL missense mutations that yields residual activity at levels comparable to those obtained using purified HL peptides. Codons 41 and 42 are important for normal HL catalysis and account for a disproportionate 21 (26%) of 82 of mutant alleles in our group of HL-deficient probands.

3-Hydroxy-3-methylglutaryl coenzyme A lyase (HL): cloning and characterization of a mouse liver HL cDNA and subchromosomal mapping of the human and mouse HL genes

3-Hydroxy-3-methylglutaryl coenzyme A lyase (HL) is a homodimeric mitochondrial matrix enzyme that catalyzes the last step of ketogenesis. Using a human HL cDNA as a probe, we isolated a 1.4-kb mouse HL cDNA (HLM) from a mouse liver library and extended the sequence in the 5' direction, using RACE PCR to include the complete coding sequence. The nucleotide sequence of the mouse HL coding region is 85.7% identical to human HL, and 52.6% to Ps. mevalonii HL. Peptide identities of 87.4% and 54.3% respectively were observed. Southern analysis of 29 strains of laboratory mice and of Mus spretus revealed a total of about 25 kb of hybridizing fragments and three polymorphic fragments in both EcoRI and Hin-dIII digestions. The mouse HL locus (Hmgcl) was localized on Chromosome (Chr) 4: Pmv-19-12.6 +/- 3.6 cM-Hmgcl-7.3 +/- 2.3 cM-Xmv-8-1.5 +/- 1.0 cM-Gpd-1. The human HL locus (HMGCL) was mapped to distal Chr 1p by analysis of a human-hamster hybrid cell panel and by in situ hybridization.