Analysis of an IFN- gamma gene (IFNG) Polymorphism in Multiple Sclerosis in Europe: Effect of Population Structure on Association with Disease

An intronic dinucleotide polymorphism in the IFN- gamma gene (IFNG) was used as a marker for testing association with multiple sclerosis (MS). Disease association was analyzed in case-control sets sampled from four geographically separate European populations (Germany, Northern Italy, Sardinia, and Sweden). Only in the Swedish was a weak disease association of the IFNG allele pattern found, mainly due to a higher frequency of IFNG allele I1 in MS patients. No evidence for association was found in the German or Northern Italian populations. These results contrast with the situation in Sardinia. We have recently reported transmission disequilibrium of IFNG allele I2 in Sardinian MS siblings not carrying the predisposing DRB1*03 or *04 alleles ( Ann. Neurol. 44, 841-842, 1998). Further analysis now shows that I2 is significantly more often transmitted to DRB1*03 - /*04 - males, than to DRB1*03 - /*04 - females. The odds ratio (OR) for IFNG -associated susceptibility to MS in the total Sardinian DRB1*03 - /*04 - group was 1.88 for I2 heterozygotes but amounted to 8.235 for I2 homozygotes, suggestive of a recessive mode of inheritance. Score test-based statistics pointed to an I2 allele dosage effect acting in susceptibility. Comparison of the IFNG allele frequencies in seven European populations (Northern Finnish, Southern Finnish, Swedish, Danish, German, Italian, and Sardinian) revealed a highly different distribution pattern. We introduced latitude as a score variable in order to test for trend in binomial proportions. This test statistic showed that for both most common alleles, I1 and I2 (compiled allele frequency about 85%), a significant opposite northto-south trend is seen throughout Europe. This effect is primarily due to the extreme values found in the outlier populations of Finland and Sardinia. Our findings are discussed with respect to recent literature pertinent to the role of the IFNG chromosome region in autoimmune diseases.