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.

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.