The origins of hypertrophic cardiomyopathy-causing mutations in two South African subpopulations: a unique profile of both independent and founder events

Abstract

Hypertrophic cardiomyopathy (HCM) is an autosomal dominantly inherited disease of the cardiac sarcomere, caused by numerous mutations in genes encoding protein components of this structure. Mutation carriers are at risk of sudden cardiac death, mostly as adolescents or young adults. The reproductive disadvantage incurred may explain both the global occurrence of diverse independent HCM-associated mutations and the rare reports of founder effects within populations. We have investigated whether this holds true for two South African subpopulations, one of mixed ancestry and one of northern-European descent. Previously, we had detected three novel mutations-Ala797Thr in the beta-myosin heavy-chain gene (betaMHC), Arg92Trp in the cardiac troponin T gene (cTnT), and Arg645His in the myosin-binding protein C gene (MyBPC)-and two documented betaMHC mutations (Arg403Trp and Arg249Gln). Here we report three additional novel mutations-Gln499Lys in betaMHC and Val896Met and Deltac756 in MyBPC-and the documented betaMHC Arg719Gln mutation. Seven of the nine HCM-causing mutations arose independently; no conclusions can be drawn for the remaining two. However, the betaMHC Arg403Trp and Ala797Thr and cTnT Arg92Trp mutations were detected in another one, eight, and four probands, respectively, and haplotype analysis in families carrying these recurring mutations inferred their origin from three common ancestors. The milder phenotype of the betaMHC mutations may account for the presence of these founder effects, whereas population dynamics alone may have overridden the reproductive disadvantage incurred by the more lethal, cTnT Arg92Trp mutation.

Figure 1

A, Genetic map of the region flanking the βMHC locus on chromosome 14q11-12. The order of genetic markers, as well as the genetic distances between them, used in haplotype construction in families harboring the βMHC Arg403Trp and the Ala797Thr mutations are shown, according to GeneMap '99. MYH7 = βMHC; MYH6 = αMHC. B, Genetic map of the region flanking the cTnT locus on chromosome 1q3. The order of genetic markers, as well as the genetic distances between them, used in haplotype construction in families harboring the cTnT Arg92Trp mutation are shown, as mapped by Watkins et al. (1993a).

Figure 2

Identification of the Glu499Lys mutation in exon 15 of βMHC. A, Partial sequence of the coding strand of exon 15 of βMHC in an individual heterozygous for the Glu499Lys mutation, showing the g→a transition, which results in the amino acid substitution of Lys for Glu at codon 499. B, Genotyping the Glu499Lys mutation by ASREA. Products of MnlI digestion of the 243-bp PCR-amplified product of exon 15 were separated on a 12% polyacrylamide gel. Internal MnlI sites generated 87-, 73-, 37-, 16-, and 15-bp fragments in unaffected individuals (lane 2). Loss of one of these sites in an HCM-affected heterozygous individual resulted in an additional, undigested 102-bp fragment (lane 1). Lane 3, Lambda PstI molecular-size marker. The 16- and 15-bp fragments were not resolved on this gel.

Figure 3

Identification of the Val896Met mutation in exon 27 of MyBPC. A, Partial sequence of the noncoding strand of exon 27 in an individual heterozygous for the Val896Met mutation, showing the c→t transition (g→a, coding strand) that results in the amino acid substitution of Met for Val at codon 896. B, Genotyping of the Val896Met mutation, by ASREA. Products of NlaIII digestion of the 310-bp PCR-amplification product of exon 27 were separated on a 12% polyacrylamide gel. An internal NlaIII site generated fragments of 233 and 77 bp in an unaffected individual (lane 2). The creation of an NlaIII site in an HCM-affected heterozygous individual resulted in additional, 141- and 92-bp fragments (lane 1).

Figure 4

Identification of the cytosine deletion in codon 796 in exon 24 of MyBPC. A, Partial sequence of the noncoding strand of exon 24 of MyBPC in an unaffected individual (i) and a heterozygous HCM-affected individual (ii), showing the deletion of a g (c, coding strand) at nucleotide position 2298 (first position of codon 796), which results in a shift in reading frame. B, PCR-SSCP analysis of amplified products of exon 24 of MyBPC, separated on a 10% polyacrylamide gel containing 5% glycerol. The arrows indicate the mobility shifts caused by the Δc mutation at nucleotide 2298 in a heterozygous HCM-affected individual (lane 2). The single-stranded conformers of an unaffected individual are seen in lane 1.

Figure 5

Haplotypes across the β and αMHC loci associated with the βMHC Arg403Trp mutation. The disease-associated haplotypes in a representative subset of South African Ped106 and the French family 730, as well as the assumed haplotype of individual SB1140, bearing the βMHC Arg403Trp mutation are boxed. In the married-in individuals, the haplotypes were inferred on the basis of those of their children. The marker loci used in haplotype construction, covering a distance ⩾8.8 cM, are indicated to the left of the subsets, in centromeric-telomeric order, from top to bottom. The frequencies of the alleles of the markers genotyped in the control mixed-ancestry subpopulation are given in the legend to figure 6. Note that in the Ped106 subset shown there were no clinically HCM-affected mutation carriers; however, in the extended pedigree there were numerous affected individuals (Posen et al. 1995). MYH7 = βMHC; MYH6 = αMHC; R = arginine; W = tryptophan.

Figure 6

Haplotypes across the β and αMHC loci associated with the βMHC Ala797Thr mutation. The disease-associated haplotype in representative subsets of families Ped101, Ped104, Ped110, Ped124, Ped131, and Ped138, as well as the assumed haplotype of individuals SB902, SB983, and SB995, bearing the Ala797Thr mutation are boxed. The marker loci used in haplotype construction, covering a distance ⩾8.8 cM, are indicated to the left of the subsets, in centromeric-telomeric order, from top to bottom. The frequencies (white/mixed ancestry) of the alleles of the markers genotyped in the control subpopulations, listed in the order of marker-allele number, are as follows: D14S50—1 (.00/.02), 2 (.01/.00), 3 (.05/.09), 4 (.28/.26), 5 (.21/.18), 6 (.11/.11), 7 (.20/.24), 8 (.13/.08), 9 (.01/.01), and 10 (.00/.01); MYH7 5′UTR—1 (.01/.06), 2 (.03/.05), 3 (.15/.08), 4 (.38/.21), 5 (.32/.28), 6 (.11/.23), and 7 (.00/.09); MYH6—1 (.01/.00), 2 (.01/.00), 3 (.01/.00), 4 (.03/.04), 5 (.01/.00), 6 (.01/.06), 7 (.04/.01), 8 (.07/.04), 9 (.13/.13), 10 (.15/.17), 11, (.32/.21), 12 (.07/.15), 13 (.05/.06), 14 (.04/.07), 15 (.04/.04), and 16 (.01/.02); and D14S64—1 (.02/.01), 2 (.12/.08), 3 (.12/.15), 4 (.29/.06), 5 (.32/.11), 6 (.10/.36), 7 (.02/.14), 8 (.01/.07), and 10 (.00/.02). The allele frequencies at D14S283 and D14S264 were not determined in the control subpopulation groups. MYH7 = βMHC; MYH6 = αMHC; A = alanine; T = threonine.

Figure 7

Haplotypes across the cTnT locus associated with the Arg92Trp mutation. The disease-associated haplotype in representative subsets of families Ped100, Ped109, Ped137, Ped139, and Ped142, as well as the assumed haplotype of individual SB385, bearing the Arg92Trp mutation are boxed. In the married-in individual, the genotype at each marker locus is shown; a haplotype was not constructed, since the phase was unknown. The marker loci used in haplotype construction, covering a distance of 11.2 cM, are indicated to the left of the subsets, in centromeric-telomeric order, from top to bottom. The frequencies of the alleles at the markers genotyped in the control mixed-ancestry subpopulation were as follows, listed in the order of marker-allele number: F13B—1 (.28), 2 (.23), 3 (.14), 4 (.08), and 5 (.27); TnTex9—1 (.70) and 2 (.30); and D1S53—1 (.05), 2 (.09), 4 (.02), 5 (.02), 7 (.05), 8 (.02), 9 (.11), 10 (.12), 11 (.09), 12 (.06), 13 (.09), 14 (.05), 15 (.14), 16 (.02), and 17 (.07). Note that, although, in the Ped100 subset shown, there were no HCM-affected mutation carriers, in the extended pedigree there were numerous affected individuals (Moolman et al. 1997). R = arginine; W = tryptophan.