We all carry two copies of each non-sex chromosome, and thus, two copies of most genes. For simplicity's sake, it's easy to think that half of the genes we express come from mom and half come from dad. But the reality is a bit more complex. While one or the other copy of any given gene might be equally active in a particular cell, there are certain specific genes that don't behave this way. For some genes that are 'imprinted,' one particular copy of the gene is more active than the other. Less than one percent of the protein coding genes we express are thought to be imprinted; and 130 had been identified in the mouse genome.
Now scientists have found a lot more of these imprinted genes - 71 in total - in the mouse genome. The findings, which have been reported in Nature Communications, can help us understand the epigenome.
"Imprinting affects an important family of genes, with different implications for health and disease, so the seventy-plus new ones add an important piece of the jigsaw," said study leader Professor Tony Perry of the Department of Biology & Biochemistry at the University of Bath.
Epigenetics are factors that can affect gene expression without changing the DNA sequence. Methyl groups that attach to DNA, for example, are one of the most common epigenetic tags we know of. This work has linked DNA methylation to the activation and deactivation of imprinted genes. Methyl groups are already thought to reduce gene activity.
This study also suggested that histones, which help organize the genome's 3D structure, are a bigger player in imprinting than we knew. Most of the new imprinted genes identified in this research were linked to modifications in the histone H3 (the specific epigenetic modification is called H3K27me3) and only a few were associated with methylation.
We still have a lot to learn about imprinting, including how exactly one copy of a specific gene is deactivated or turned down as the other copy's activity moves in the opposite direction. This work has suggested that imprinted genes may not be tagged as such in gametes, but later on in development or possibly in adulthood.
While imprinting may only involve a handful of genes, it's physiologically importnt, especially if one copy of one of the imprinted genes is defective or picks up a mutation. Defects in imprinted genes have been linked to a variety of disorders including cancer and metabolic disease.
"We may underestimate how important the relationship between imprinting and disease is, as well as the relationship of imprinting to the inheritance of parentally-acquired disease, such as obesity," said Professor Perry. "Hopefully, this improved picture of imprinting will increase our understanding of disease."