Mutation analysis of PEX7 in 60 probands with rhizomelic chondrodysplasia punctata and functional correlations of genotype with phenotype.
PEX7 encodes the cytosolic receptor for the set of peroxisomal matrix enzymes targeted to the organelle by the peroxisome targeting signal 2 (PTS2). Mutations in PEX7 cause rhizomelic chondrodysplasia punctata (RCDP), a distinct peroxisome biogenesis disorder. In previous work we described three novel PEX7 mutant alleles, including one, L292X, with a high frequency due to a founder effect. We have now extended our analysis to 60 RCDP probands and identified a total of 24 PEX7 alleles, accounting for 95% of the mutant PEX7 genes in our sample. Of these, 50% are L292X, 13% are IVS9+1G>C, and the remainder are mostly private. IVS9+1G>C occurs on at least three different haplotypes and thus appears to result from recurrent mutation. The phenotypic spectrum of RCDP is broader than commonly recognized and includes minimally affected individuals at the mild end of the spectrum. To relate PEX7 genotype and phenotype, we evaluated the consequence of the disease mutation on PEX7 RNA by Northern analysis and RT/PCR. We evaluated the function of the encoded Pex7 protein (Pex7p) by expressing selected alleles in fibroblasts from RCDP patients and assaying their ability to restore import of a PTS2 marker protein. We find that residual activity of mutant Pex7p and reduced amounts of normal Pex7p are associated with milder and variant phenotypes.