MS in Asians is often characterised by selective and severe involvement of the optic nerve and spinal cord. Fifteen to forty percent of Japanese people with MS have this opticospinal type, while others have features similar to those of MS in Caucasians (conventional type).
Opticospinal MS is characterised by frequent relapses, severe disability, few brain lesions being visible on magnetic resonance imaging (MRI) and long extensive lesions on spinal cord MRI. Further, in approximately 90 percent of people with MS, there are a series of distinct bands found in the immunoglobulin of the cerebral spinal fluid, known as oligoclonal IgG bands. In the non-conventional type of MS there is an absence of these bands.
Moreover, in Caucasians, familial clustering of MS is well known, but familial occurrence of MS in Japanese people is rare. A nationwide survey of MS in Japan, undertaken in 2004, found familial cases of MS constitute less than one percent. The underlying mechanisms causing these differences are unknown but appear to be partly based on differences in immunogenetic backgrounds.
In people of European descent, susceptibility to MS is associated with a specific haplotype, or a set of closely linked alleles (genes or DNA polymorphisms) inherited as a unit (HLADRB1* 1501-DQA1*0102-DQB1*0602). One allele of this haplotype (HLA-DRB1*1501) has also been recently identified in African-Americans as being associated with susceptibility to MS. Among groups who have higher proportions of opticospinal MS - such as Japanese, Chinese, Southeast Asians, and Gulf Arabs - the same haplotype has not been associated with MS susceptibility. However, when MS is clinically classified into conventional MS and opticospinal MS, conventional MS in Japanese is associated with the same allele identified in African-Americans, whereas opticospinal MS is associated with a different allele (HLA-DPB1*0501).
Even in conventional MS, oligoclonal IgG bandpositive MS is associated with that seen in African- Americans, while oligoclonal IgG band-negative MS is associated with an allele that has been identified in people with MS on the island of Sardinia, Italy and in Turkey (HLA-DRB1*04). These observations suggest that differences in clinical features and immune responses in MS are in part related to polymorphisms, or a common variation or mutation in DNA, in HLA class II genes, a type of gene that codes for a protein.
Genome-wide studies on MS susceptibility genes have not been undertaken in Asians. However, such studies might help identify novel genes involved in certain forms of the disease including the opticospinal MS common in Japanese people.
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