Friday, August 9, 2013

Focus on: Epigenomics

DNA molecule that is methylated on both strands on the
center cytosine

by Christoph Bock

The human Roadmap Epigenomics Project is an NIH funded project which seeks to perform global analysis of changes in gene activity and expression that are not dependent on gene sequence. Although each cell in the human body contains the same genomic information; encoding about 25,000 genes; each cell type expresses these genes differently. Epigenetics is believed to be the major force behind the differences between cell types due to different cell types having a different pattern of methylation. Methylation is, chemically speaking, the addition of a methyl group that marks the DNA any time a cytosine resides next to a guanine in the DNA sequence (CpG's). The result of this marking is differential packaging of DNA and distinct DNA sequences being available for expression in each cell type.

A recent paper entitled 'Charting a dynamic DNA methylation landscape of the human genome' is a major milestone in epigenomics as it shines light on when, where and how many CpG's participate in genomic regulation. The group mapped most of the 28-million CpG's found among the 3-billion nucleotides that make up human DNA in 30 different cell and tissue types. The majority of the sites are unchanged but the ones that are changed are in locations that are likely important for gene expression. Now that they have identified these differentially methylated regions (DMR's) they believe they will be able to perform more directed methylation analysis on these regions. This means they will not have to perform complete sequencing on all of the more than 200 different human cell types but can instead concentrate on these DMR's and still discover the differences between the cell types.

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