imprinting (genetics) - Other imprinted genes

The specific silencing of some DNA regions from a parent during meiosis and the formation of sperm or egg. In the resulting offspring the genes in these areas will not be able to be active. Disease can occur where a DNA deletion from the non-imprinted parent is matched by imprinting on the corresponding chromosome from the other parent. Examples of diseases caused by imprinting are Angelman and Prader-Willi syndromes. It is not fully understood why imprinting takes place.

Genomic imprinting is the phenomenon whereby a small subset of all the genes in the genome are expressed according to their parent of origin. Some imprinted genes are expressed from a maternally inherited chromosome and silenced on the paternal chromosome; while other imprinted genes show the opposite expression pattern and are only expressed from a paternally inherited chromosome. Contrary to expectation, 'imprints' can act as a silencer or an activator for imprinted genes. A living child (this applies to all mammals) cannot be produced when both sets of chromosomes come from the same parent because imprinted gene expression will be unbalanced. Because of the way imprints work, a fetus that has two maternal sets of chromosomes will have twice the normal level of some imprinted genes, and completely lack expression of other imprinted genes. No naturally occurring cases of parthenogenesis exist in mammals because of imprinted genes. Experimental manipulation of a paternal methylation imprint controlling the Igf2 gene has, however, recently allowed the creation of rare individual mice with two maternal sets of chromosomes - but this is not a true parthenogenote. Hybrid offspring of two species may exhibit unusual growth due to the novel combination of imprinted genes (summary article on this topic).

Problems with imprinting

Imprinting is known to cause problems in cloning, with clones having DNA that is not methylated in the right places. The large-buttocked phenotype only occurs when the allele is present on the copy of chromosome 18 inherited from a sheep's father and is not on the copy of chromosome 18 inherited from that sheep's mother. Angelman Syndrome

Several genetic diseases that map to 15q13 (band 13 of the long arm of chromosome 15) in humans are due to abnormal imprinting. This region is differently imprinted in maternal and paternal chromosomes, and both imprintings are needed for normal development. It is possible for an individual to fail to inherit a properly imprinted 15q13 from one parent, as a result either of deletion of the 15q13 region from that parent's chromosome 15 or, less frequently, of uniparental disomy (in which both copies have been taken from the other parent's genes).

If neither copy of 15q13 has paternal imprinting, the result is Prader-Willi syndrome (characterized by hypotonia, obesity, and hypogonadism).

Other imprinted genes

NOEY2

NOEY2 is located on chromosome 1 in humans. Therefore, if a person inherits both chromosomes from the mother, the gene will not be expressed and the individual is put at a greater risk for breast and ovarian cancer.