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Somatic mosaicism in health human tissues Somatic mosaicism arises a result of genomic alterations of different sizes ranging from a single nucleotide to chromosome gains or loss within somatic cells. These alterations within somatic cells begin at an early stage; pre-implantation or conception and continue during aging, giving rise to phenotypic heterogeneity within cells and can lead to the development of diseases such as cancer. But these mutations can also be found in normal tissues. Several mechanisms are involved in the generation of genetic alterations that can lead to somatic mosaicism. These mechanisms can be exogenous or endogenous. Exogenous mutagenic factors such as UV radiation, nicotine usage cause DNA damage leading to cancer development. On the other hand, endogenous mechanisms and mutagenic factors give rise to both healthy and diseased tissues. These factors can be tissue specific or even particular to a certain stage of development and with each round of mitotic cell division, the number of somatic mutations in a tissue increases. Thereby making the rate of somatic mutations vary from one tissue to another and with age. During embryonic development, Long Interspersed Nuclear Elements-1(LINE-1) retrotransposons give rise to copy number alterations. Though recent studies show the L1 transposon mediated mosaicism in neuronal progenitor cells and adult human brain tissues. The diversity of immunoglobulin and t-cell receptor generated by VDJ recombination resulting from Large copy number variations due to erroneous non allelic homologous recombination and nonhomologous end joining. Genetic alterations involving gains or loss of entire chromosomes predominantly occur during anaphase stage of cell division. But these are uncommon in somatic cells because they are usually selected against due to their deleterious consequences. In some cells, the somatically acquired alterations can be reversed back to wild type alleles by Reversion mosaicism. Reversion mosaicism can be due to endogenous mechanism such as homologous recombination, codon substitution, second-site suppressor mutations, DNA slippage and mobile elements. And in some drug screening approaches chemical agents that promote or suppress expansions o the trinucleotide repeats can cause reversion mosaicism.

Novel array based techniques for screening genome wide copy number variants and loss of heterozygosity in single cells showed that chromosome aneuploidies, uniparental disomies, segmental deletions, duplications and amplifications frequently occur during embryogenesis. And not all somatic mutations are propagated to the adult individual. This is due to a phenomenon, Cell competition by which mutated cells with lower fitness are outcompeted and eliminated by neighboring healthy cells during early development. Somatic variations during embryonic development can be represented by homozygous twins since they carry different copy number profiles and epigenetic marks that keep on increasing with age. Aneuploidy which is found in both developing and adult brain tissue is much less common in other cell types but the reason as to why is still unknown. Early research on somatic mutations in aging showed that deletions, inversion and translocations of genetic material are common in aging mice. And aging genomes tend to contain visible chromosomal changes, mitotic recombination, whole gene deletions, intragenic deletions and point mutations. The key mutagenic factor in aging is reactive oxygen species. Other factors include the loss of methylation and increasing gene expression heterogeneity correlating to genomic abnormalities. It is uncertain if transcription based DNA repair takes part in the maintaining of somatic mutations in aging tissues. During aging, the length of telomere repeats tend to reduce in size thereby limiting the replicative capacity of cells and this telomere shortening varies between different individuals.

Mitochondrial DNA also accumulates somatic mutations and, in some cases, this is found in a subpopulation leading to mitochondrial heteroplasmy. Mt DNA has a higher substitution rate compared to genomic DNA because a single cell can have various mitochondrial genomes. It should be noted that many Mt DNA alterations are associated with diseases.

Somatic alterations have the potential to alter genetic function and usage. The expression of genes bearing somatically acquired mutations allows selection to operate on the cells carrying advantageous mutations while those with a lower fitness are subsequently eliminated by competing neighboring cells.