Non allelic homologous recombination
Encyclopedia
Non allelic homologous recombination (NAHR) is a form of homologous recombination
that occurs between two lengths of DNA
that have high sequence homology
, but are not allele
s.
It usually occurs between sequences of DNA that have been previously duplicated
through evolution, and therefore have low copy repeats (LCRs). When non allelic homologous recombination occurs between different LCRs, deletions or further duplications
of the DNA can occur. This can give rise to rare genetic disorder
s, caused by the loss or increased copy number of genes within the deleted or duplicated region. It can also contribute to the copy number variation seen in some gene clusters.
As LCRs are often found in "hotspots" in the human genome, some chromosomal regions are particularly prone to NAHR. Therefore multiple patients may manifest with similar deletions or duplications, resulting in the description of genetic syndrome
s. Examples of these include NF1 microdeletion syndrome
, 17q21.3 recurrent microdeletion syndrome
or 3q29 microdeletion syndrome
.
Homologous recombination
Homologous recombination is a type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical molecules of DNA. It is most widely used by cells to accurately repair harmful breaks that occur on both strands of DNA, known as double-strand breaks...
that occurs between two lengths of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
that have high sequence homology
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
, but are not allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s.
It usually occurs between sequences of DNA that have been previously duplicated
Segmental duplication
Segmental duplications are segments of DNA with near-identical sequence.Segmental duplications give rise to low copy repeats and are believed to have played a role in creating new primate genes as reflected in human genetic variation...
through evolution, and therefore have low copy repeats (LCRs). When non allelic homologous recombination occurs between different LCRs, deletions or further duplications
Gene duplication
Gene duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.The second copy of the gene is often free from selective pressure — that is, mutations of it have no...
of the DNA can occur. This can give rise to rare genetic disorder
Genetic disorder
A genetic disorder is an illness caused by abnormalities in genes or chromosomes, especially a condition that is present from before birth. Most genetic disorders are quite rare and affect one person in every several thousands or millions....
s, caused by the loss or increased copy number of genes within the deleted or duplicated region. It can also contribute to the copy number variation seen in some gene clusters.
As LCRs are often found in "hotspots" in the human genome, some chromosomal regions are particularly prone to NAHR. Therefore multiple patients may manifest with similar deletions or duplications, resulting in the description of genetic syndrome
Syndrome
In medicine and psychology, a syndrome is the association of several clinically recognizable features, signs , symptoms , phenomena or characteristics that often occur together, so that the presence of one or more features alerts the physician to the possible presence of the others...
s. Examples of these include NF1 microdeletion syndrome
Neurofibromatosis type I
Neurofibromatosis type I , formerly known as von Recklinghausen disease after the researcher who first documented the disorder, is a human genetic disorder. It is possibly the most common inherited disorder caused by a single gene...
, 17q21.3 recurrent microdeletion syndrome
17q21.3 recurrent microdeletion syndrome
17q21.31 microdeletion syndrome is a rare genetic disorder caused by a deletion of a segment of chromosome 17 which contains six genes. This deletion syndrome was discovered independently in 2006 by three different research groups.-Clinical description:...
or 3q29 microdeletion syndrome
3q29 microdeletion syndrome
3q29 microdeletion syndrome is a rare genetic disorder resulting from the deletion of a segment of chromosome 3. This syndrome was first described in 2005.- Clinical description :The clinical phenotype of 3q29 microdeletion syndrome is variable...
.