High incidence of propionic acidemia in greenland is due to a prevalent mutation, 1540insCCC, in the gene for the beta-subunit of propionyl CoA carboxylase

Abstract

Propionyl CoA carboxylase (PCC) is a mitochondrial, biotin-dependent enzyme involved in the catabolism of amino acids, odd-chain fatty acids, and other metabolites. PCC consists of two subunits, alpha and beta, encoded by the PCCA and PCCB genes, respectively. Inherited PCC deficiency due to mutations in either gene results in propionic acidemia (PA), an autosomal recessive disease. Surprisingly, PA is highly prevalent among Inuits in Greenland. We have analyzed reverse transcriptase-PCR products of the beta-subunit mRNA, to characterize the responsible mutation(s). A 3-bp insertion, 1540insCCC, was found in homozygous form in three patients and in compound heterozygous form in one patient. The resulting PCC has no measurable activity, and the mutant beta-subunit appears to be very unstable. To test the hypothesis that a common mutation is responsible for PA in the Greenlandic Inuit population, 310 anonymous DNA samples of Inuit origin were screened for 1540insCCC. We found a carrier frequency of 5%, which is very high compared with those of most other autosomal recessive diseases. Analysis of alleles of a very closely linked marker, D3S2453, revealed a high degree of linkage disequilibrium between one specific allele and 1540insCCC, suggesting that this mutation may be a founder mutation.

Figure 1

Western blot analysis of PCC α- and β-subunits. A,α- and β-subunits expressed in fibroblasts. Lane 1, Purified recombinant human PCC (100 ng). Lane 2, Control fibroblast cell line 2047 (80 μg). Lane 3, Cell line 3173, from the patient homozygous for 1540insCCC (80 μg). B, α- and β-subunits expressed in E. coli. Lane 1, Purified recombinant human PCC (100 ng). Lanes 2 and 3, Equal amounts of crude bacterial extracts (30 μg), expressing either wild-type PCC (lane 2) or 1540insCCC (513insP) (lane 3) protein. Lane 4, Bacterial extract from DH5α cells without pPCCAB plasmid (30 μg).