No description available.

Human skin fibroblasts derived from patients with all 7 known inborn errors of vitamin B$ sb{12}$ metabolism have been studied for functional integrity of methylmalonyl CoA mutase and methionine synthase. Cocultivation of cblC and cblF fibroblasts in the absence of polyethylene glycol resulted in a twofold increase over the expected in both ($ sp{14}$C) propionate and ($ sp{14}$C) methyltetrahydrofolate incorporation into acid-precipitable material, suggesting that metabolic cooperation between cells occurs. CblD fibroblasts, which are biochemically similar to cblC cells (Goodman et al, 1970; Willard et al, 1977), do not cooperate metabolically when mixed with cblF cells. Partial correction in phenotype was seen in mixtures of cblD and cblG cells, but not cblC and cblG cells. These observations lend further support for the division of cblC and cblD disease into two discrete complementation classes. Cocultivation of cblF fibroblasts with both cblE and cblG cells also resulted in partial correction in phenotype. / ($ sp{14}$C) Propionate incorporation in both cblC and cblF cells exposed to conditioned medium from control cells was increased more than twofold. ($ sp{14}$C) methyltetrahydrofolate incorporation in cblC cells exposed to conditioned medium from cblF cells was increased twofold. This suggests the presence of a diffusible factor correcting the defect in the mutant cell lines.

Methylmalonic aciduria (MMA) is an autosomal recessive metabolic disorder with an incidence of 1 in 48,000, which may be due to a defect in the mitochondrial homodimeric enzyme methylmalonyl CoA mutase (mut MMA). mut MMA is subdivided into $mut sp circ$ and $mut sp-$ subclasses on the basis of complementation analysis; $mut sp circ$ cell lines have very low incorporation of ($ sp{14}$C) from propionate into acid precipitable material while incorporation in $mut sp-$ cells is increased when cells are incubated in cobalamin. Intragenic complementation was first observed with WG 1130, a $mut sp circ$ fibroblast line with a homozygous R93H mutation, that is capable of complementing MCM activity when fused with some $mut sp circ$ and some $mut sp-$ cells (1). Extensive intragenic complementation in mut MMA was subsequently observed. Fibroblasts cultured from thirteen unrelated patients (6 $mut sp-$, 7 $mut sp circ$) were fused in all possible pairwise combination and MCM activity was assayed in the heterokaryons by measuring the incorporation of ($ sp{14}$C) from propionate into acid precipitable material. Intragenic complementation, indicated by stimulation of ($ sp{14}$C) -propionate incorporation following cell fusion with polyethylene glycol, was observed in fusions involving twelve of the thirteen strains. Of these thirteen strains, mutations have been identified in six; four have a homozygous mutation (WG 1130 (R93H), WG 1511 (H678R), WG 1610 (G717V), WG 1609 (G630E)), and two cell lines are compound heterozygous (WG 1681 (G623R and G703R), WG 1607 (W105R and A377E)); the remainders are yet to be determined. These intragenic complementations will provide information for grouping the mutations in defined domains in order to correlate structure and function of MCM.

Methylmalonic aciduria is an autosomal recessive metabolic disorder, which may be due to a defect in the methylmalonyl CoA mutase (MCM) apoenzyme. The mut$ sp circ$ mutation is characterized by undetectable enzyme activity in cell extracts, and by the low incorporation of ($ sp{14}$C) propionate in the presence of hydroxocobalamin in culture. A mut$ sp circ$ fibroblast cell line, WG 1681, from an African-American male infant was shown to complement another mut$ sp circ$ cell line, WG 1130. Subsequent cloning and sequencing of cDNA from WG 1681 identified two previously described homozygous polymorphisms: H532R and V671I(1). In addition, compound heterozygosity was observed for two novel changes at highly conserved sites: G623R and G703R. Hybridization of allele specific oligonucleotides to PCR amplified MCM exons from WG 1681 and family members identified a clinically normal mother, sister and half-brother as carriers of the G703R change in cis with both polymorphisms. The putative father was not identified as a carrier of the G623R change. transfection of each change, singly and in cis with both polymorphisms, into GM1673 cells demonstrated a lack of stimulation of ($ sp{14}$C) propionate uptake in the absence and presence of OH-Cbl, in comparison to controls. Co-transfection of each separate mutation with the previously identified R93H mutation of WG 1130 (2) stimulated propionate uptake. These results indicate that G623R and G703R are novel mutations responsible for deficient MCM activity and the mut$ sp circ$ phenotype in WG 1681, and both mutations are independently capable of complementing the R93H mutation of WG 1130.

No description available.

No description available.

Several of the inborn errors of vitamin B$ sb{12}$ (cobalamin, Cbl) metabolism (cblC, cblD, cblE, cblF, cblG) are associated with homocystinuria and hypomethioninemia due to a functional deficiency of the cytoplasmic enzyme methionine synthase which requires methylcobalamin (MeCbl) as a cofactor. We compared the growth of cultured fibroblasts from controls with those from patients with a selective deficiency of MeCbl (cblE and cblG) and with those from patients with a defect in both MeCbl and adenosylcobalamin (AdoCbl) (cblC, cblD and cblF). Cells were grown in methionine and folic acid free media and in fully supplemented medium. Control cells were able to grow in the deficient medium supplied with homocysteine, cobalamin and folate, while mutant cells were not, due to their inability to synthesize methionine from its immediate metabolic precursor, homocysteine. This differential growth is useful for screening for genetic defects of methionine biosynthesis. Moreover, by correcting methionine auxotrophy in these cells, it may be possible to isolate genes which code for the products that are deficient in these disorders.

Methionine synthase (MS) is a vitamin B12(cobalamin;cbl) dependent enzyme that catalyses the methylation of homocysteine to methionine. It uses methyl-cbl as coenzyme and in ethyl tetrahydrofolate as the methyl donor. Methionine sythase reductase (MSR) maintains MS in it active state using S-adenosyl methionine as the methyl donor. Functional MS deficiency may occur as a result of a defect in either enzyme. Patients with this disorder have been classified into two complemetation groups according to which protein is defective: cblG patients are deficient in MS and cblE patients in MSR. A subset of cblG, known as cblG variant, is unique in showing barely detectable MS activity and failure of cbl incorporation into MS in patient fibroblasts. I report the mutations responsible for three cblG variant patients, two of them siblings, and connect their phenotype to lack of protein expression. I also report the cloning of the MSR cDNA, aided by confirming the identity of the cDNA through the discovery of two deleterious mutations in three cblE patients. These findings contribute to the overall understanding of functional MS deficiency.

No description available.

No description available.