Identification of four new mutations in the short-chain acyl-CoA dehydrogenase (SCAD) gene in two patients: one of the variant alleles, 511C-->T, is present at an unexpectedly high frequency in the general population, as was the case for 625G-->A, together conferring susceptibility to ethylmalonic aciduria

We have shown previously that a variant allele of the short-chain acyl- CoAdehydrogenase ( SCAD ) gene, 625G-->A, is present in homozygous form in7% of control individuals and in 60% of 135 patients with elevated urinaryexcretion of ethylmalonic acid (EMA). We have now characterized threedisease-causing mutations (confirmed by lack of enzyme activity afterexpression in COS-7 cells) and a new susceptibility variant in the SCADgene of two patients with SCAD deficiency, and investigated their frequencyin patients with elevated EMA excretion. The first SCAD-deficient patientwas a compound heterozygote for two mutations, 274G-->T and 529T-->C.These mutations were not present in 98 normal control alleles, but the529T-->C mutation was found in one allele among 133 patients withelevated EMA excretion. The second patient carried a 1147C-->T mutationand the 625G- ->A polymorphism in one allele, and a single pointmutation, 511C-->T, in the other. The 1147C-->T mutation was notpresent in 98 normal alleles, but was detected in three alleles of 133patients with elevated EMA excretion, consistently as a 625A-1147T allele.On the other hand, the 511C-->T mutation was present in 13 of 130 and 15of 67 625G alleles, respectively, of normal controls and patients withelevated EMA excretion, and was never associated with the 625A variantallele. This over-representation of the haplotype 511T-625G among thecommon 625G alleles in patients compared with controls was significant ( P< 0.02), suggesting that the allele 511T-625G-like 511C-625A- conferssusceptibility to ethylmalonic aciduria. Expression of the variant R147WSCAD protein, encoded by the 511T-625G allele, in COS-7 cells showed 45%activity at 37 degrees C in comparison with the wild- type protein,comparable levels of activity at 26 degrees C, and 13% activity whenincubated at 41 degrees C. This temperature profile is different from thatobserved for the variant G185S SCAD protein, encoded by the 511C-625Aallele, where higher than normal activity was found at 26 and 37 degrees C,and 58% activity was present at 41 degrees C. These results corroborate thenotion that the 511C-625A variant allele is one of the possible underlyingcauses of ethylmalonic aciduria, and suggest that the 511C-->T mutationrepresents a second susceptibility variation in the SCAD gene. We concludethat ethylmalonic aciduria, a commonly detected biochemical phenotype, is acomplex multifactorial/polygenic condition where, in addition to theemerging role of SCAD susceptibility alleles, other genetic andenvironmental factors are involved.