Tumour necrosis factor beta gene polymorphisms in myasthenia gravis.

Genetic analyses indicate that genes within the major histocompatibility complex (MHC) can be involved in susceptibility to autoimmune disease. To investigate the role of the tumour necrosis factor beta (TNFB) gene in myasthenia gravis (MG) susceptibility, we analysed an NcoI polymorphism within the TNFB gene in 63 MG patients and 93 healthy individuals. When patients were subdivided according to thymic pathology, we found differences between MG patients with thymic hyperplasia and thymoma versus controls. In MG patients with thymic hyperplasia we found a positive association with the TNFB*1 allele [Relative risk (RR): 2.6; P < 0.001] and phenotype (RR: 1.8; P < 0.005) and a negative association with the TNFB*2/2 genotype (RR: 0.2; P < 0.001) when compared to the controls. On the other hand, in MG patients with thymoma we found a positive association with the TNFB*2/2 genotype (RR: 5.6; P < 0.01) and a negative association with the TNFB*1 allele (RR: 0.3; P < 0.05) and *1/2 genotype (RR: 0.2; P < 0.01). These data suggest that the two different forms of MG can have different pathogenesis and that the TNFB gene could influence susceptibility to MG.