Study of HLA antigens in ten multiple-case rheumatoid arthritis families.

We studied the inheritance of HLA haplotypes in 10 families with more than one member affected with adult onset rheumatoid arthritis (RA). The frequency of DR4 was 81% among these patients. Nine families had DR4 bearing haplotypes and homozygosity for DR4 existed in 4 families. In 6 of these families DR4 positive haplotypes were shared among affected members. All but one of the affected sibs shared at least one haplotype with their index case. Ten percent of the unaffected relatives had rheumatoid factor (RF). HLA-DR4 or associated genes appeared to confer susceptibility for RF production and development of RA. However, these haplotypes were inherited also by many sibs who did not develop any manifestations of disease.