What is Genomic Imprinting?
People normally inherit two copies of most genes, one from each parent. Usually, both of these genes are "turned on" (active). When a gene is active and producing a protein, we say the gene is being expressed. In certain genes, however, both copies of the gene are not always active. Which gene is "turned on" and which gene is "turned off" depends on which parent, mom or dad, the gene was inherited from. Some genes are only active when inherited from mom, while other genes are only active when inherited from dad. This parent-specific gene activation is called parental genomic imprinting.
Genomic imprinting is a dynamic, complicated process that we don't fully understand at this time. Researchers believe that one of the major mechanisms involved in imprinting is a chemical process called methylation. In methylation, small molecules called methyl groups are attached to certain segments of DNA, called CpG islands, but do not modify the genetic code (DNA sequence) itself. In genes that undergo genomic imprinting, methylation is one manner in which a gene's parental origin is "marked" during the formation of egg and sperm cells. Also, the addition and removal of these methyl groups may help control the activity of genes by "turning on" or "turning off" genetic expression.
Genomic Imprinting Facts
Only a small percentage of all human genes undergo genomic imprinting. Out of our 25,000 genes, about 100 genes have been identified as imprinted genes.
Researchers are not yet certain why some genes are imprinted and others are not. They do know that imprinted genes tend to cluster together in the similar chromosomal regions, and as such are probably subject to the same regulatory mechanisms.
Researchers also believe that imprinted genes likely play a role in multifactorial conditions, such as diabetes, bipolar disease, obesity and schizophrenia. Imprinting errors also play a role in the development of cancer.
Parental genomic imprinting was first linked to two genetic conditions: Prader-Willi syndrome and Angelman syndrome.
It has been suggested that for couples undergoing infertility treatment, using In Vitro Fertilization (IVF) with Intracytoplasmic Sperm Injection (ISCI) increases the risk to have a child with an imprinting error. There is much debate over this issue, and studies have been conflicting.
The phenomenon of genomic imprinting is not limited to humans. Other animal species and plant species have shown evidence of imprinting.
What is Uniparental Disomy (UPD)?
Uniparental disomy (UPD) occurs when an individual receives two copies of a chromosome, or part of a chromosome, from one parent and no copies from the other parent. UPD can occur as a random event during the formation of egg or sperm cells or may occur in early embryonic development.
- Maternal UPD is when both copies are inherited from mom, and there is no paternal copy.
- Paternal UPD is when both copies are inherited from dad, and there is no maternal copy.
As the vast majority of genes are not imprinted, it typically does not matter if both chromosomes (and subsequently both gene copies) are inherited from the same parent. As such, UPD typically has no effect on a person's health or development. However, in some cases, a person with UPD may lack any active copies of genes that are imprinted. This loss of gene function can lead to delayed development, mental retardation, certain genetic conditions, and/or other medical problems.
Genetic Conditions and Imprinting Errors/UPD
Several genetic disorders can result from UPD and/or an imprinting abnormality (error). Some of the most common of these conditions include:
- Angelman syndrome is associated with abnormalities of imprinted genes on chromosome 15.