Numerous associations between HLA alleles and various diseases have been described. In multiple sclerosis (MS) patients from Northern Europe, there is an increase in the frequency of HLA-DR2, giving a relative risk of about 4 (Svejgaard et al. 1983). This suggests that DR2 haplotypes carry some genetic factors of susceptibility to MS.
Several studies suggest that DNA restriction fragment length polymorphism will provide new markers, improving our understanding of ItLA and disease associations (Owerbach et al. 1983, Cohen et al. 1985). Indeed, it has previously been shown that haplotypes carrying the same DR specificit~es can be associated with different DQo DNA restriction fragments (Cohen et al. 1984b).DR2 haplotypes were associated with two different sets of DQp fragments, named clusters. Fragments within a cluster are strongly correlated and they are generally determined by various restriction fragments. Two allelic DQp clusters were found in DR1, DR2, DRw6, or DR blank haplotypes. The first of these two, DQR1, characterized all DR1 haplotypes tested and also some DR2 or DRw6 haplotypes; the second, DQR2,6, was found in the remaining DR2 or DRw6 haplotypes. The DR2 haplotypes were then subdivided into two subsets: a DQRl-positive subset and a DQR2,6-positive subset. It was therefore interesting to analyze YIQR1 and DQR2,6 fragments in DR2 MS patients. This a_nalysis was performed using the restriction enzyme Eco RV, which determines a DQR1 fragment of 2.6 kb and a DQR2,6 fragment of 5.2 kb. These two fragments are assigned to th e DQp genes since they are detected only with a DQp probe and not with a DRp probe (Cohen et al. 1984b) nor a DPp probe (not shown) under the conditions used.
Genomic DNA was extracted from peripheral blood leukocytes (Cohen et al. 1983) of 44 unrelated patients with MS and 44 healthy individuals matched with the patients for HLA-DR antigens. Patients and controls were Caucasian Europeans. DNA was digested with the restriction endonuclease Eco RV (Biolabs). The restriction fragments were separated in 0.7% agarose gel by electrophoresis, transferred to hybridization membranes (Southern 1975), and then hybridized with