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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.
Normally, a healthy child inherits two sets of chromosomes, one from the mother and one from the father. 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.
A process known as reprogramming occurs in the parent female or male when the egg or sperm is maturing. In many instances this is achieved through methylation of the DNA of genes or regulatory sequences, which results in the gene not being expressed. In other instances, phosphorylation or other chemical modification of histone proteins appears to lead to silencing.
Problems With Imprinting
Imprinting is known to cause problems in cloning, with clones having DNA that is not methylated in the right places. Some scientists think this is due to there not being enough time for reprogramming to be properly achieved. When a nucleus is added to an egg during somatic cell nuclear transfer, the egg starts dividing in minutes, as compared to the days or months it takes for reprogramming during embryonic development. If time is the responsible factor, it may be possible to delay cell division in clones, giving time for proper reprogramming to occur.
An allele of the "callipyge" (from the Greek for "beautiful buttocks"), or CLPG, gene in sheep produces large buttocks consisting of muscle with very little fat. 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.
Prader Willi vs. Angelman Syndrome
Several genetic diseases that map to 15q13 (locus 3 of section 1 of the long arm of chromosome 15) in humans are due to abnormal imprinting. The Prader-Willi syndrome is due to 2 copies of the chromosome 15 being inherited from the mother, and the locus is imprinted; the Angelman syndrome gene is due to 2 copies of the chromosome 15 inherited from the father and the locus is similarly imprinted. Thus, someone who inherits both chromosomes 15 from one parent (called uniparental disomy) has Prader-Willi or Angelman syndrome, depending on which parent they come from. But more common is the deletion of the region, 15q13, and when this deletion is inherited depending on from whom the affected person gets this deletion from, the disease varies, if it's the mother who transfers the deletion then the child will have Angelman syndrome and if it's the father who gives the deletion then the child will get Prader-Willi syndrome.
Other Imprinted Genes
NOEY2 is located on chromosome 1 in humans. It is maternally imprinted. Researchers have found that its lack of expression relates to ovarian and breast cancers; in 41% of breast and ovarian cancers the protein transcribed by NOEY2 is not expressed. This leads scientists to believe that it is a tumor suppressor gene- a gene that helps to prevent cancer by stopping uncontrolled cell growth. 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.
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- includeonly>"The Legacy of Solid Gold", Genome News Network, May 7, 2001.
- includeonly>"NOEY2 (ARHI), an imprinted putative tumor suppressor gene in ovarian and breast carcinomas", The National Academy of Sciences, January 5, 1999.