“Mosaicism is common in a single person,” sez criminal defense lawyers in variation of “the other dude did it” defense.

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Let’s say you leave your DNA at the scene of a crime. The prosecutor puts up the  expert on DNA analysis: it’s a one in a bazillion chance that it wasn’t you that did the crime.

Should the expert be disqualified (and busted down to being a fact witness)  for failing to consider that, given the commonplace occurrence of mosaicism within a single person, those statistics aren’t valid?

Piotrowski et al. , in new publication about mosaicism within a single person arising out of somatic cell growth,

. . . Our results indicate that humans are commonly affected by somatic mosaicism for stochastic CNVs, which occur in a substantial fraction of cells. The majority of described CNVs were previously shown to be polymorphic between unrelated subjects, suggesting that some CNVs previously reported as germline might represent somatic events, since in most studies of this kind, only one tissue is typically examined and analysis of parents for the studied subjects is notroutinely performed. . .

PUBMED abstract reprinted after the jump.

Hum Mutat. 2008 Sep;29(9):1118-24.

Somatic mosaicism for copy number variation in differentiated human tissues.

Piotrowski A, Bruder CE, Andersson R, de Ståhl TD, Menzel U, Sandgren J, Poplawski A, von Tell D, Crasto C, Bogdan A, Bartoszewski R, Bebok Z,Krzyzanowski M, Jankowski Z, Partridge EC, Komorowski J, Dumanski JP.

Department of Genetics, University of Alabama at Birmingham, Birmingham, Alabama 35294-0024, USA.

Two major types of genetic variation are known: single nucleotide polymorphisms (SNPs), and a more recently discovered structural variation, involving changes incopy number (CNVs) of kilobase- to megabase-sized chromosomal segments. It is  unknown whether CNVs arise in somatic cells, but it is, however, generally assumed that normal cells are genetically identical. We tested 34 tissue samplesfrom three subjects and, having analyzed for each tissue < or =10(-6) of all cells expected in an adult human, we observed at least six CNVs, affecting asingle organ or one or more tissues of the same subject. The CNVs ranged from 82 to 176 kb, often encompassing known genes, potentially affecting gene function. Our results indicate that humans are commonly affected by somatic mosaicism for stochastic CNVs, which occur in a substantial fraction of cells. The majority of described CNVs were previously shown to be polymorphic between unrelated subjects, suggesting that some CNVs previously reported as germline might represent somatic events, since in most studies of this kind, only one tissue is typically examined and analysis of parents for the studied subjects is notroutinely performed. A considerable number of human phenotypes are a consequence of a somatic process. Thus, our conclusions will be important for the delineation of genetic factors behind these phenotypes. Consequently, biobanks should consider sampling multiple tissues to better address mosaicism in the studies of somatic disorders. PMID: 18570184 [PubMed – in process]
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