Interactive effect of HLA and Gm and genetic heterogeneity tested in 79 rheumatoid arthritis families

The hypothesis that there is an interactive effect between HLA and Gm genes in rheumatoid arthritis (RA) was tested in a sample of 79 RA families. An analysis, of the joint segregation of these two markers in RA sibpairs, confirmed the previously described effect of HLA in RA but did not provide evidence of an independent effect of Gm alone or in interaction with HLA. The additional hypothesis that the previously described correlation between RA and autoimmune thyroid disease is due to genetic factors, was also investigated in relation to HLA and Gm. Therefore, we examined the segregation of HLA and Gm in RA sibships depending on the presence or the absence of autoimmune thyroid disorder. This analysis showed significant heterogeneity in HLA segregation (P = 0.02) with an HLA effect restricted only to sibships without thyroid disease. There was no evidence that thyroid disease influenced Gm segregation (P = 0.19). The effect of thyroid disease on HLA segregation suggests that the familial association between RA and autoimmune thyroid disease is at least partially due to genetic factors.