Expanding the Traditional Definition of a Gene Through Epigenetics

Expanding Traditional Definition of a Gene

Expands Traditional Definition A Gene

Expanding the traditional definition of a gene through epigenetics:

Changes to the human gene through exposure to external forces in the environment

Increasingly, the field of epigenetics is challenging traditional conceptions of what constitutes the human gene. While it has long been acknowledged that parents can pass their DNA onto their offspring, epigenetics acknowledges the potential for changes in human DNA which subsequently causes a change in the genetic composition of the child. Epigenetics acknowledges "the external environment's effects upon genes can influence disease, and some of these effects can be inherited in humans" (Simmons 2008). While it is difficult to design a study to test environmental factors, historical and experimental research does support this hypothesis. "For example, Swedish scientists recently conducted investigations examining whether nutrition affected the death rate associated with cardiovascular disease and diabetes and whether these effects were passed from parents to their children and grandchildren" (Simmons 2008). Such insightful research holds the potential to explain why, for example, there may be escalating rates of diabetes and heart disease in this generation, versus the last generations. "Three systems, including DNA methylation, RNA-associated silencing and histone modification, are used to initiate and sustain epigenetic silencing" in human DNA (Egger 2004: 457). This paper will specifically discuss the phenomenon of epigenetics and its impact upon the MC4R gene, the FMR1 gene, and molecular RNA.

Epigenetics, weight, and disease

An unintentional study of the potential for epigenetics to influence disease and body weight was found during the infamous Dutch famine in which large numbers of the Dutch population were starved during World War II. The babies born to pregnant mothers who survived the Dutch famine showed a notably more marked tendency towards weight gain than those that did not, regardless of the genetic history of their parents. "Women exposed to famine in late pregnancy did not gain any weight in the third trimester, whereas women exposed in mid and early pregnancy gained more weight than non-exposed mothers did due to the immediate provision of food after the war. Consequently, women exposed in late pregnancy weighed less at their last prenatal visit" (Roseboom et al. 2001: 95). Late or mid-gestation-exposed famine babies had lower glucose tolerance and a lipid profile more associated with obesity, regardless of the weight of the mother (Roseboom et al. 2001: 95). Women were also exposed during the famine to extreme cold and stress which likewise had an impact upon the development of their offspring, suggesting that "chronic diseases originate through adaptations made by the fetus in response to undernutrition" (Roseboom et al. 2001: 99). The deprivation of nutrition thus creates a 'thriftier' gene profile in terms of caloric expenditure than would otherwise occur.

While over 40 genetic variants are associated with obesity and obesity-related diseases, such studies indicate that pure genetic transmission is only part of the story of how obesity can be provoked and why it is more common today than in previous generations (Herrera et al. 2011). Some specific diseases associated with obesity, such as the uncontrolled appetite of Prader -- Willi are known to be associated with a failure of imprinting of normal DNA but the exact role is still being determined of epigenetics (Herrera et al. 2011).

Epigenetics and cancer

One of the first examples of the effects of epigenetics upon the development of disease was found in regards to cancer. The phenomenon of DNA methylation refers to a "chemical process that adds a methyl group to DNA. It is highly specific and always happens in a region in which a cytosine nucleotide is located next to a guanine nucleotide that is linked by a phosphate; this is called a CpG site" (Simmons 2008). This insertion fundamentally changes the DNA through the insertion of new methyl groups which alters patterns of transcription and "DNA methylation is used in some genes to differentiate which gene copy is inherited from the father and which gene copy is inherited from the mother, a phenomenon known as imprinting" (Simmons 2008). Patients with colorectal cancer have diseased tissues with less DNA methylation than non-sufferers. "Because methylated genes are typically turned off, loss of DNA methylation can cause abnormally high gene activation by altering the arrangement of chromatin. On the other hand, too much methylation can undo the work of protective tumor suppressor genes" (Simmons 2008). Thus, the effects of the external environment can cause a real, demonstrable modification in the DNA of the gene from the original parent copy that markedly increases the offspring's risk of developing a specific type of cancer.

Other forms of methylation disturbance are associated with an increased risk of obesity (a condition itself associated with specific cancers) and "may arise during fetal development due to lack of availability of dietary methyl donors…interactions between the environment and epigenetic mechanisms mediating the expression of genes associated with increased BMI and adiposity, may also be possible as suggested for: for example, the MC4R gene which has reduced methylation following long-term exposure to a high fat diet" (Herrera et al. 2011). Obese mothers are also more likely to give birth to obese children, suggesting the mechanisms which trigger obesity may also vary depending on the specific conditions of the child in utero and the mother's diet and body composition.

Epigenetics and Fragile X syndrome

One of the most-studied phenomena of epigenetics is Fragile X syndrome. "Fragile X syndrome gets its name from the way the part of the X chromosome that contains the gene abnormality looks under a microscope; it usually appears as if it is hanging by a thread and easily breakable…an abnormality in the FMR1 (fragile X mental retardation 1) gene" (Simmons 2008). The disease is more commonly found in males and is more severe in males, causing developmental delays and autistic-like symptoms. An overabundance of repeats of the trinucleotide CGG will "cause the CpG islands at the promoter region of the FMR1 gene to become methylated; normally, they are not. This methylation turns the gene off, stopping the FMR1 gene from producing an important protein called fragile X mental retardation protein. Loss of this specific protein causes fragile X syndrome… FMR1 methylation is the real syndrome culprit" (Simmons 2008). The reasons for methylation, however, remain poorly understood.