Earlier work led by Liu showed that the CD24 protein is required for the development of an MS-like disease in mice, experimental autoimmune encephalomyelitis. Mice missing the protein did not develop the disease.
For the present study, Liu and colleagues investigated whether CD24 was associated with the human disease by studying the two versions of the gene represented by the SNP.
"Other investigators had discovered the SNP, but no one had linked the CD24 gene to MS or any other disease," Liu says.
SNPs are usually harmless differences, or polymorphisms, between genes in different people. But in this case, the single nucleotide change has important consequences: It causes the replacement of one amino acid, alanine, with another amino acid, valine, in the CD24 protein.
The gene encoding the alanine-version of the protein is known as CD24a; the gene for the valine-version is known as CD24v. Because everyone has two copies of the CD24 gene---one from each parent---people might have two copies of the "a" gene (CD24a/a), two copies of the "v" gene (CD24v/v) or one of each (CD24a/v).
Liu and his colleagues first collected blood samples from 242 MS patients treated at the Ohio State University Multiple Sclerosis Center. They then analyzed the white blood cells in the samples to determine if the person carried CD24a/a, CD24v/v or CD24a/v genes.
They repeated the analysis on white cells collected from 207 people in the general population for comparison.
The investigators found that 13 percent of people with MS carried the CD24v/v genes, compared to 6 percent of the general population. The differences represented about a two-fold increase in risk for MS.