Results

The a and p cardiac MHC genes are only 4.5 kb apart (Saez et al., 1987; Matsuoka et al., 1989). Each gene consists of 40 exons and encompasses approximately 30 kb of DNA. A simple and rapid method for identifying mutations within a large gene (s), like the cardiac MHC genes, has not been devised. One could determine the nucleotide sequence of the putative mutated gene and compare this to the nucleotide sequence of the normal gene. The entire nucleotide sequence of the human p cardiac MHC gene has been determined (Liew et al., 1990; Jaenicke et al., 1990), but comparable data on the a cardiac MHC gene are currently unavailable. This direct approach would therefore require nucleotide sequence analysis of 30 kb of genomic DNA from a normal a cardiac MHC gene and 80 kb of DNA containing the a and p cardiac MHC genes from an affected individual. If, however, a mutation altered a restriction enzyme recognition site, one could identify this by Southern blot analyses of DNA from affected individuals. Given the size of the cardiac MHC genes, we elected to begin our analyses with the latter approach.

To determine if the cardiac MHC genes of affected individuals in family A contained a mutation similar to that detected in affected individuals of family B (Tanigawa et al., 1990), the restriction enzyme maps of the a and p cardiac MHC genes from affected and unaffected individuals were compared (see Figure 2). Southern blot analyses were performed with several restriction enzymes that recognize 8 bp sites. These blots were hybridized to probes derived from the a and p cardiac MHC genes (see Figure 2). All restriction fragments identified in the DNAs of affected individuals were identical in size to those found in the DNAof unaffected family members, suggesting that the cardiac MHC genes in the# K-la allele were not grossly altered.