chromosomal translocations
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Definition: Translocations are chromosomal abnormalities which occur when chromosomes break and the fragments rejoin to other chromosomes. By this way, a segment of one chromosome is transferred to another. There are many structurally different types of translocations.
As with inversions, there is no loss of genetic material, although the breakpoint can cause disruption of a critical gene or juxtapose pieces of two genes to create a fusion gene that induces cancer.
Types
reciprocal translocation
centric fusions
constitutional translocation
tumoral translocation
Balanced reciprocal translocation
In balanced reciprocal translocation, there are single breaks in each of two chromosomes, with exchange of material. Such a translocation might not be disclosed without banding techniques. Because there has been no loss of genetic material, the individual is likely to be phenotypically normal.
A balanced translocation carrier is at increased risk for producing abnormal gametes. Such a gamete would be unbalanced because it would not contain the normal complement of genetic material. Subsequent fertilization by a normal gamete would lead to the formation of an abnormal (unbalanced) zygote, resulting in spontaneous abortion or birth of a malformed child.
Robertsonian translocation
The other important pattern of translocation is called a robertsonian translocation (or centric fusion), a translocation between two acrocentric chromosomes. Typically the breaks occur close to the centromeres of each chromosome. Transfer of the segments then leads to one very large chromosome and one extremely small one.
Usually the small product is lost; however, because it carries only highly redundant genes (e.g., ribosomal RNA genes), this loss is compatible with a normal phenotype. Robertsonian translocation between two chromosomes is encountered in 1 in 1000 apparently normal individuals.
See also
oncogenic chromosomal translocation
References
Baptista J, Mercer C, Prigmore E, Gribble SM, Carter NP, Maloney V, Thomas NS, Jacobs PA, Crolla JA. Breakpoint mapping and array CGH in translocations: comparison of a phenotypically normal and an abnormal cohort. Am J Hum Genet. 2008 Apr;82(4):927-36. PMID: 18371933
Reviews
Rabbitts TH, Stocks MR. Chromosomal translocation products engender new intracellular therapeutic technologies. Nat Med. 2003 Apr;9(4):383-6. PMID: 12669051
Aplan PD. Causes of oncogenic chromosomal translocation. Trends Genet. 2006 Jan;22(1):46-55. PMID: 16257470
Greaves MF, Wiemels J. Origins of chromosome translocations in childhood leukaemia. Nat Rev Cancer. 2003 Sep;3(9):639-49. PMID: 12951583
Rego EM, Pandolfi PP. Reciprocal products of chromosomal translocations in human cancer pathogenesis: key players or innocent bystanders? Trends Mol Med. 2002 Aug;8(8):396-405. PMID: 12127726
Tomescu O, Barr FG. Chromosomal translocations in sarcomas: prospects for therapy. Trends Mol Med. 2001 Dec;7(12):554-9. PMID: 11733218