Multiple crm^- mutations in familial hypercholesterolemia. Evidence for 13 alleles, including four deletions

The low density lipoprotein (LDL) receptors in fibroblasts from 132 subjects with the clinical syndrome of homozygous familial hypercholesterolemia were analyzed by immunoprecipitation with an anti-LDL receptor monoclonal antibody. 16 of the 132 cell strains (12%) synthesized no immunodetectable LDL receptor protein, indicating the presence of two mutant genes that failed to produce cross-reacting material (crm^- mutations). DNA and mRNA from 15 of the 16 crm^- patients, representing 30 crm^- genes, were available for further study. Haplotype analysis based on 10 restriction fragment length polymorphisms (RFLPs) suggested that the 30 crm^- genes represent 13 mutant alleles. Four of the alleles produced no mRNA. Three of these four mRNA^- alleles had large deletions ranging from 6 to 20 kb that eliminated the promoter region of the gene. The fourth mRNA^- allele did not contain any deletion or alteration in the promoter sequence; the reason for the mRNA^- phenotype was not apparent. Nine alleles were positive for mRNAs, of which three encoded mRNAs of abnormal size. One of the abnormal mRNAs was produced by a gene harboring a deletion, and another was produced by a gene with a complex rearrangement. The third abnormal-sized mRNA (3.1 kb larger than normal) was produced by an allele that had no detectable alterations as judged by Southern blotting. The other six mRNA⁺ alleles appeared normal by Southern blotting and produced normal-sized mRNA but no receptor protein. The current studies demonstrate that mRNA analysis coupled with haplotype determination by Southern blot analysis can be used to classify crm^- mutations at a genetic locus where multiple alleles exist.