Enhanced resolution of the SstI polymorphic variants of the C2 locus: description of a new size class

Investigation of the SstI polymorphism at the 5' region of the gene encoding the serum complement component, C2, in unrelated individuals has been limited by inadequate resolution of the different size variants using conventional agarose gel electrophoresis. We have overcome this problem by the simple modification of electrophoretic conditions using both low current and temperature. With the technique described in this report it has been possible to assign the hetero- and homozygosities of the SstI polymorphism in unrelated individuals without the need for family studies. Using this approach we have investigated a population of 60 healthy, unrelated Caucasians. Five different variants of the SstI polymorphism, the 2.75-, 2.7-, 2.65-, 2.55-, and 2.4-kb fragments, were observed. The 2.7-kb SstI fragment is the most common variant, followed by the 2.65- and the less common 2.4-kb SstI fragments. These fragments occurred at frequencies of 53.3, 35.8, and 7.5%, respectively. We report here a new rare variant of the 2.75- and another rare variant of the 2.55-kb SstI fragments; both occurred at a frequency of 1.7%. The latter variant was previously described as the 2.6-kb SstI fragment but is reclassified in this report as 2.55-kb, based on the results of better resolution on electrophoresis. All the larger-size classes of the SstI polymorphic variants, the 2.75-, 2.7-, and 2.65-kb fragments, are associated with the 4.5-kb TaqI fragment, while the smaller-size classes of 2.55- and 2.4-kb fragments are associated with the 6.6-kb TaqI fragment at the 3' end of the C2 gene.