A very high prevalence (approximately 1/475 in 1985) of myotonic dystrophy (Steinert disease) is observed in the Saguenay region, which is located in the north-east part of the Province of Quebec. For various reasons, however, the literature on the subject generally associates a high degree of selective disadvantage with this gene, which seems to contradict the Saguenay data. Using a computerized regional population register, we have reconstituted patients' genealogies and family biographies. We have thus been able to study the origin of the gene and to compare the demographic behavior of patients and controls. On the whole, patients seem to be very little disadvantaged compared to controls, in terms of reproduction as well as of geographical and occupational mobility.

The geographical distribution relative to place of residence of patients with myotonic dystrophy (MD) and admitted to a Quebec hospital during a five year period (1980-1984) is presented and discussed. The sample consists of 72 males and 68 females of varying ages over 10 years. Analysis of the data shows a North Shore distribution of patients in a cline from Saguenay-Lac-St-Jean, through Quebec City and to Montreal. However, a low prevalence is apparent on the South Shore, east of Québec City, for which an historical and genealogical explanation are discussed. This geographic distribution favours the hypothesis of genetic homogeneity for the MD gene in the Quebec population. A stronger second argument comes from genealogical studies of 10 families sampled from the Chicoutimi Muscular Dystrophy Clinic. Genealogical paths traced to ancestors who founded Charlevoix for these 10 families go back to a cluster of 25 founders, one of whom must have been the carrier of the MD gene. The probative third argument for genetic homogeneity comes from the allellic distribution of the apolipoprotein E (ApoE) gene in the Québec City, Saguenay and in families with MD. The ApoE locus is on chromosome 19 and closely linked to MD. In MD-affected individuals there is a linkage disequilibrium for the e4 allelle while non-MD members of these families show allellic frequencies not differing significantly from the control population. This suggests that the gene that is carried by all MD patients (at least in Northeast Québec) came through a “founder effect” and was carried in -cis on the chromosome 19 of the first carrier. Consequences of this genie homogeneity in relation to studies of penetrance and phenotypic variation in clinical expressivity are discussed.

Plasma lipid, lipoprotein levels and apolipoprotein apo E phenotypes were determined in 70 patients with myotonic dystrophy (MyD) and 81 controls. Marked differences were noticed in the apo E phenotype frequencies between the two groups. Plasma triglycerides and VLDL cholesterol were higher in MyD than controls, but only the latter was related to differences in the apo E phenotypes between two groups. Accordingly, the ratio of VLDL cholesterol/plasma triglycerides was increased significantly in MyD, suggesting accumulation of intermediary density particles due to lower affinity of E2 containing lipoproteins for lipoprotein cell receptors. The LDL cholesterol concentration was lower in MyD than controls and was related to differences in the apo E phenotype frequencies between the two groups. These results indicate increased removal of LDL particles in the apo E2 phenotypes, perhaps due to upregulation of LDL (B, E) receptor activity. Plasma cholesterol and HDL cholesterol concentrations were similar in both groups. Another feature of the study was lower levels of plasma cholesterol, triglycerides, VLDL and LDL cholesterol in the homozygous E4:E4 phenotype. These results suggest increased clearance rate of both VLDL and LDL particles and support the concept that apo E4-containing lipoproteins have higher in vivo affinity for ape E and/or B, E receptors.

The genes for myotonic dystrophy (MD) and for apolipoprotein E (ApoE) belong to a chromosome 19 synthenic group of markers. A familial linkage analysis between MD and ApoE was performed using the J Ott LIPED program (IBM PC/XT, April 1984) to estimate the genetic distance between these 2 genes. Of a total of 136 individuals in 11 MD families, 81 were confirmed to be affected by the disease and 41 were asymptomatic. ApoE phenotypes were determined in 115 of these 122 individuals. No recombinant was observed out of 74 meioses which were informatives for both MD and the ApoE isoproteins. A global maximal lod score Z of 19.00 was obtained at the recombination fraction Θ = Θ. The upper 0 value at the confidence interval corresponding to the peak lod score (Z max) - 1 was 0.03. This suggests that the loci for MD and ApoE are at a distance of 0 to 0.03 Morgan. Since ApoE and apolipoprotein C2 (ApoC2) have been shown by others to be about 40 kb apart, our data are therefore consistent with the distance estimate of 0.02 Morgan reported between MD and ApoC2. The D19S19 (LDR152) polymorphic DNA sequence is also tightly linked to MD on chromosome 19. The segregation of ApoE isoproteins and of ApoC2 and D19S19 DNA polymorphism was utilized for evaluating the probability for individuals at risk of inheriting the disease gene in MD families. Data are presented on 3 families to emphasize the usefulness of genetic markers to estimate the MD gene carrier status of asymptomatic individuals and also for those presenting a partial syndrome. The limitations of such approach are also discussed.