Three polymorphic loci APOC2, CKMM and p134C were used to haplotype 15 Finnish dystrophia myotonica (DM) families representing about one third of all DM patients in this isolated population. Compound APOC2 and CKMM haplotypes reveal linkage disequilibrium: 90% of DM chromosomes co-occur with the haplotypes that occur in 31% of normal chromosomes only. The same disequilibrium is present when only polymorphisms occurring at the APOC2 locus are used. Surprisingly, no statistically significant linkage disequilibrium was discovered at the CKMM locus alone. Of the meiotic events, 84% were informative when both APO2 and CKMM loci were used. When studied selectively, 60% of meiotic events were informative at the APOC2 locus, whereas CKMM alone resulted in 65% meiotic informativeness. The distal marker p134C was found to have an unfortunately low information content in our population.

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Three DNA probes (APOC2, PSC11, and LDR152) detecting RFLP polymorphisms were used to test the usefulness of the RFLP approach in myotonic dystrophy (MD) families from the isolated Finnish population. The informativeness of these polymorphisms did not differ from that reported in more mixed populations: in the 13 families of the study most of the 79 meiotic events studied were informative. One known recombinant is included in the study. The highest lod score obtained in the multilocus linkage analysis was z = 5.941 at recombination fraction theta = 0.02. The RFLP results significantly facilitated genetic counseling in problematic cases among the families studied. Although evidence could be found for linkage disequilibrium of the RFLP haplotypes formed in Finnish MD patients, our results do not exclude the possible existence of more than one ancient MD mutation in this population.

The chromosomal locations of the human and mouse syndecan genes have been shown to associate with the corresponding locations of members of the myc gene family, proto-oncogenes that code for transcription factors. Here, we demonstrate the precise localization and order of the human syndecan-1 and N-myc genes using fluorescence in situ hybridization techniques that provide different levels of resolution. By visualizing the actual centromere-telomere orientation of these two genes, the human syndecan-1 gene is localized to 2p23-24, just centromeric to the N-myc gene at 2p24.1.