Epigenetics

When one considers inheritance, typically it is at the level of the DNA, gene, or chromosome: typical Mendelian genetics. However, not all hereditary [Page 336]information is passed on in our genes. The field that studies these heritable differences in gene expression that do not involve a change in the DNA sequence is called epigenetics. This field can include studies of how gene expression is modified as cells divide and differentiate into specific types; how environmental factors (such as chemicals or radiation) change the way genes are expressed; and how patterns of gene expression are passed on from parent to daughter cells.

All multicellular organisms start off from a single cell, which contains all the genetic information present in an organism. However, from this single cell, vastly different daughter cells arise, eventually going on to form all the different tissues in our bodies. One way these differences are possible is via epigenetic regulation of individual cells. The first phenomenon of this type to be described was genetic imprinting, whereby one set of genes in a diploid individual is silenced. This silencing is based on its parental origin: either maternal or paternal. Epigenetic modifications can also include methylation (the addition of methyl groups) to the DNA backbone. The addition of these methyl groups changes the structure and appearance of the DNA itself, therefore, altering the interaction of methylated genes with messengers (such as regulatory proteins) that typically control gene expression.

It has also been suggested that epigenetic regulation plays a role in the development of some cancers and other diseases. Disorders caused by epigenetic mechanisms include Prader-Willi syndrome, which results in short stature and a complete lack of sexual development in adulthood, and Angelman syndrome, symptoms of which include developmental delays, seizures, and frequent laughter and smiling. Both diseases are caused by the loss of expression of a part of chromosome 15, which can be due to loss of a portion of the chromosome or epigenetic imprinting abnormalities eliminating expression of these genes. Beckwith-Wiedemann syndrome, which causes large size, an enlarged tongue, abdominal wall defects, and hypoglycemia, has also has been associated with an epigenetic imprinting abnormality. Epigenetic processes have also been suggested to be involved in various psychiatric disorders, including schizophrenia, bipolar illness, and depression, though these have not been fully characterized.

Research also suggests that certain cancers, including Wilm's tumor associated with Beckwith-Weidemann syndrome, may have an epigenetic origin. By examining DNA samples collected from tumors and normal tissues, researchers have found that the development and proliferation of cancerous tumors may be associated with a loss of imprinting at a number of different loci. As epigenetics is a quickly growing field of research, it is likely that additional epigeneticassociated diseases will be identified in the future.