This genotype codes for Val/Val homozygous at codon 129 in PrP, the Prion Protein. This is slightly protective against development of sporadic CJD, and confers resistance against vCJD.
Studies in Papua New Guinea showed that this genotype prevents transmission of kuru [PMID 19923577], a form of Prion disease transmitted by cannibalism. So eat as many brains as you want!
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]