UPDATE: It's been brought to my attention that other groups have previously completed partial and complete methylomes of human cell lines. To name two examples, Ryan Lister's group from the Salk Institute and Josesh Costello from the University of California, San Francisco have completed methylomes in 2009. I want to thank my readers for catching this error. Please keep correcting me!
For the first time, Chinese researchers have identified every methylated site in a human genome. This collection of methylation sites, known as a ‘methylome’ is already yielding valuable information about gene regulation and inheritance.
A person’s genome determines exactly which genes he or she will have. Many genes come in different alleles, or flavors, which explain some of the variability between people. However, it’s not just enough to have a gene, that gene must be expressed. It turns out that gene expression is largely controlled by non-genetic or epigenetic factors, a principal one of these being DNA methylation. At certain places along the DNA strand, predominantly on the cytosine in cytosine guanine pairs, a methyl group is added. This addition has consequences in gene expression and regulation.
Because the scientists studied the methylome of a man whose genome had previously been sequenced, they were able to compare the methylation of the genes he’d inherited from his mother with those inherited from his father. They found that many alleles were differentially methylated between the two parents, and that this affected which version was manifested in the offspring.
This methylome was sequenced in only one man, and in only one type of cells (peripheral blood mononuclear cells). Methylation undoubtedly varies in different tissue types, and definitely varies among different individuals (since even the two parents of the man under study showed different methylation rates). Nonetheless, this is an important first step in understanding how epigenetic factors make us who we are.