Comparison of protein expression levels and proteomically-inferred genotypes using human hair from different body sites.

The microanatomy of human hair differs as a function of the site of origin on the body. This was a major consideration when anatomical features of hair were used as a means of comparison and human identification. Recent advances have demonstrated that proteomics of the hair shaft can be used to develop profiles of protein abundance and genetically variant peptides, the latter in turn being used to infer genotypes of SNP alleles. Because the profile of proteins would be expected to change as hair anatomy changes, it is an open question if the profile of genetically variant peptides will also change. While some sample to sample variation is expected, a potential drawback of using genetically variant peptides to infer an individual genotype is that the proteomic profile might change as a function of body site origin as well as an individual's genotype. The hypothesis in this study is that the profile of hair shaft genetically variant peptides depends more on an individual's genotype than on the site of hair shaft origin. To test this an analysis of both protein expression levels and genetically variant peptides was conducted on 4 body sites (scalp, axillary, beard and pubic hair) from 5 individuals with 4 biological replicates. Levels of protein expression were estimated using label-free quantification on resulting proteomic mass spectrometry datasets. The same datasets were then also analyzed for the presence of genetically variant peptides. This study demonstrates that the protein profiles of hair shafts varied as a function of somatic origin. By contrast the profile of genetically variant peptides, and resulting inferred genotype of SNP alleles, were more dependent on the individual. In this study random match probabilities ranged up to 1 in 196. Individual identification based on genetically variant peptides therefore can be obtained from human hair without regard to the site of origin. If the site of hair shaft origin was legally relevant then microscopic analysis is still necessary. This study demonstrates the utility of proteomic analysis for extracting forensic information from hair shaft evidence.