Similar to other autoimmune disease such as rheumatoid arthritis, genes and environment/life style factors are likely to contribute to susceptibility to myositis.
Genetic risk factors
More studies have a focus on genetic risk factors and both MHC and non-MHC genes are associated with myositis. HLADRB1* 0301 and DQA1*0501 are major risk factors for both polymyositis and dermatomyositis [ 15. Shamim et al., 2002]. Within our consortium, based on the UK Adult Onset Myositis Immunogenetic Collaboration (AOMIC), for the first time genetic differences were demonstrated between polymyositis and dermatomyositis. Work at the Centre for Integrated Genomic Medical Research (CIGMR) showed that HLA-DRB1*07 proved a risk factor for dermatomyositis but was protective for polymyositis [16. Chinoy et al., 2006]. Alleles of the 8.1 ancestral haplotype (AH) (HLA-B*08-DRB1*03-DQA1*05-DQB1*02, 8.1 AH) show even stronger associations for certain IIM serotypes, particularly anti-histidyl-tRNA synthetase (anti-Jo-1) antibody. Similarly, the DRB1*07-DQA1*02- DQB1*02 haplotype is associated with dermatomyositis, but it is even more strongly associated with possession of the dermatomyositis-specific anti-Mi-2 Ab. Interestingly, in children the genetic/ serology associations are equally strong but the relative frequency of autoantibodies is different and certain clinical features are more prevalent (e.g. calcinosis) and can be associated with specific antibodies [17. Wedderburn et al., 2007].
Environmental risk factors
The best established environmental risk factor in inflammatory myopathies is UV light. There is an observed correlation between the occurrence of dermatomyositis and particularly the subset with anti-Mi-2 autoantibodies and UV-light exposure [14. Hengstman et al., 2000]. The association between UV-light exposure and this subtype of myositis could suggest that UV light is an exogenous modifier that can influence the clinical phenotype in two closely related diseases, poly- and dermatomyositis.
There are also case reports on associations to viral and parasitic infections, although large epidemiological studies to confirm these associations are lacking.