This genotype codes for Met/Val heterozygote at codon 129 in PrP, the Prion Protein. This is strongly protective against development of sporadic, and confers resistance against vCJD.
A total of 4228 cases and 4324 controls in 16 case-control studies were included in the meta-analysis. The results indicated that the variant V allele carriers (VV+MV) had a 13% decreased risk of AD, when compared with the homozygote MM [PMID 23399523]
A 2006 study compared 593 Germans with sporadic CJD (sCJD) to 748 healthy controls. The researchers found an unusual pattern — people with one copy of each variant at rs1799990, a SNP in the gene PRNP, were less likely to have sCJD compared to those who had two identical copies of either the A or G type. People with the AG genotype had 4.6 times lower odds of sCJD compared to those with AA; a G at both copies of the SNP reduced a subject's odds of having sCJD 1.7 times.[PMID 17047093]
All reported cases of vCJD, (linked to ingestion of 'mad cow disease' contaminated meat) have been 129 Met homozygous [PMID 22505363]