Chromosomal translocation

Chromosomal translocation of the 4th and 20th chromosome.

In genetics, a chromosome translocation is a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes. A gene fusion may be created when the translocation joins two otherwise separated genes, the occurrence of which is common in cancer. It is detected on cytogenetics or a karyotype of affected cells. There are two main types, reciprocal (also known as non-Robertsonian) and Robertsonian. Also, translocations can be balanced (in an even exchange of material with no genetic information extra or missing, and ideally full functionality) or unbalanced (where the exchange of chromosome material is unequal resulting in extra or missing genes).

Reciprocal (non-Robertsonian) translocations

Reciprocal translocations are usually an exchange of material between nonhomologous chromosomes. Estimates of incidence range from about 1 in 500 ^[1] to 1 in 625 human newborns.^[2] Such translocations are usually harmless and may be found through prenatal diagnosis. However, carriers of balanced reciprocal translocations have increased risks of creating gametes with unbalanced chromosome translocations leading to miscarriages or children with abnormalities. Genetic counseling and genetic testing are often offered to families that may carry a translocation. Most balanced translocation carriers are healthy and do not have any symptoms. But about 6% of them have a range of symptoms which may include autism, intellectual disability, or congenital anomalies. A gene disrupted or disregulated at the breakpoint of the translocation carrier is likely the cause of these symptoms.

Robertsonian translocations

Main article: Robertsonian translocation

This type of rearrangement involves two acrocentric chromosomes that fuse near the centromere region with loss of the short arms. The resulting karyotype in humans leaves only 45 chromosomes since two chromosomes have fused together. This has no direct effect on the phenotype since the only genes on the short arms of acrocentrics are common to all of them and are present in variable copy number (nucleolar organiser genes). Robertsonian translocations have been seen involving all combinations of acrocentric chromosomes. The most common translocation in humans involves chromosomes 13 and 14 and is seen in about 0.97 / 1000 newborns.^[3] Carriers of Robertsonian translocations are not associated with any phenotypic abnormalities, but there is a risk of unbalanced gametes which lead to miscarriages or abnormal offspring. For example, carriers of Robertsonian translocations involving chromosome 21 have a higher chance of having a child with Down syndrome.This is known as a 'translocation Downs'. This is due to a mis-segregation (Nondisjunction) during gametogenesis. The mother has a higher (10%) risk of transmission than the father (1%). Robertsonian translocations involving chromosome 14 also carry a slight risk of uniparental disomy 14 due to trisomy rescue.

Some human diseases caused by translocations are:

• Cancer: several forms of cancer are caused by acquired translocations (as opposed to those present from conception); this has been described mainly in leukemia (acute myelogenous leukemia and chronic myelogenous leukemia). Translocations have also been described in solid malignancies such as Ewing's sarcoma.
• Infertility: one of the would-be parents carries a balanced translocation, where the parent is asymptomatic but conceived fetuses are not viable.
• Down syndrome is caused in a minority (5% or less) of cases by a Robertsonian translocation of the chromosome 21 long arm onto the long arm of chromosome 14.

By chromosome

Overview of some chromosomal translocations involved in different cancers, as well as implicated in some other conditions, e.g. schizophrenia,^[4] with chromosomes arranged in standard karyogram order. Abbreviations:
ALL - Acute lymphoblastic leukemia
AML - Acute myeloid leukemia
CML - Chronic myelogenous leukemia
DFSP - Dermatofibrosarcoma protuberans

Denotation

The International System for Human Cytogenetic Nomenclature (ISCN) is used to denote a translocation between chromosomes.^[5] The designation t(A;B)(p1;q2) is used to denote a translocation between chromosome A and chromosome B. The information in the second set of parentheses, when given, gives the precise location within the chromosome for chromosomes A and B respectively—with p indicating the short arm of the chromosome, q indicating the long arm, and the numbers after p or q refers to regions, bands and subbands seen when staining the chromosome with a staining dye. See also the definition of a genetic locus.

Examples

For an explanation of the symbols and abbreviations used in these examples, see Cytogenetic notation.

Translocation	Associated diseases	Fused genes/proteins
		First	Second
t(8;14)(q24;q32)	Burkitt's lymphoma	c-myc on chromosome 8, gives the fusion protein lymphocyte-proliferative ability	IGH@ (immunoglobulin heavy locus) on chromosome 14, induces massive transcription of fusion protein
t(11;14)(q13;q32)	Mantle cell lymphoma[6]	cyclin D1[6] on chromosome 11, gives fusion protein cell-proliferative ability	IGH@[6] (immunoglobulin heavy locus) on chromosome 14, induces massive transcription of fusion protein
t(14;18)(q32;q21)	Follicular lymphoma	IGH@[6] (immunoglobulin heavy locus) on chromosome 14, induces massive transcription of fusion protein	Bcl-2 on chromosome 18, gives fusion protein anti-apoptotic abilities
t(10;(various))(q11;(various))	Papillary thyroid cancer [7]	RET proto-oncogene[7] on chromosome 10	PTC (Papillary Thyroid Cancer) - Placeholder for any of several other genes/proteins [7]
t(2;3)(q13;p25)	Follicular thyroid cancer[7]	PAX8 - paired box gene 8[7] on chromosome 2	PPARγ1[7] (peroxisome proliferator-activated receptor γ 1) on chromosome 3
t(8;21)(q22;q22)	Acute myeloblastic leukemia with maturation	ETO on chromosome 8	AML1 on chromosome 21
t(9;22)(q34;q11) Philadelphia chromosome	Chronic myelogenous leukemia (CML), acute lymphoblastic leukemia (ALL)	Abl1 gene on chromosome 9[8]	BCR ("breakpoint cluster region" on chromosome 22 [8]
t(15;17)	Acute promyelocytic leukemia	PML protein on chromosome 15	RAR-α on chromosome 17
t(12;15)(p13;q25)	Acute myeloid leukemia, congenital fibrosarcoma, secretory breast carcinoma	TEL on chromosome 12	TrkC receptor on chromosome 15
t(9;12)(p24;p13)	CML, ALL	JAK on chromosome 9	TEL on chromosome 12
t(12;21)(p12;q22)	ALL	TEL on chromosome 12	AML1 on chromosome 21
t(11;18)(q21;q21)	MALT lymphoma[9]	BCL-2[9]	MLT[9]
t(1;11)(q42.1;q14.3)	Schizophrenia [4]
t(2;5)(p23;q35)	Anaplastic large cell lymphoma
t(11;22)(q24;q11.2-12)	Ewing's sarcoma
t(17;22)	DFSP	Collagen I on chromosome 17	Platelet derived growth factor B on chromosome 22
t(1;12)(q21;p13)	Acute myelogenous leukemia
t(X;18)(p11.2;q11.2)	Synovial sarcoma
t(1;19)(q10;p10)	Oligodendroglioma and oligoastrocytoma
t(17;19)(q22;p13)	ALL

History

In 1938 Karl Sax, at the Harvard University Biological Laboratories, published a paper entitled "Chromosome Aberrations Induced by X-rays," which demonstrated that radiation could induce major genetic changes by affecting chromosomal translocations. The paper is thought to mark the beginning of the field of radiation cytology, and led him to be called "the father of radiation cytology".

See also

• Accipitridae
• Aneuploidy
• Chromosome abnormalities
• DbCRID
• Fusion gene
• Takifugu rubripes

References

1. ^ Caroline Mackie Ogilvie and Paul N Scriven (December 2002). "Meiotic outcomes in reciprocal translocation carriers ascertained in 3-day human embryos". European Journal of Human Genetics (European Society of Human Genetics) 10 (12): 801–806. doi:10.1038/sj.ejhg.5200895. PMID 12461686. http://www.nature.com/ejhg/journal/v10/n12/full/5200895a.html. Retrieved 2008-12-26. 
2. ^ M. Oliver-Bonet; J. Navarro1, M. Carrera, J. Egozcue, J. Benet (October 2002). "Aneuploid and unbalanced sperm in two translocation carriers: evaluation of the genetic risk". Molecular Human Reproduction (Oxford University Press for the European Society for Human Reproduction and Embryology) 8 (10): 958–963. doi:10.1093/molehr/8.10.958. ISSN 1460-2407. PMID 12356948. http://molehr.oxfordjournals.org/cgi/content/full/8/10/958?ijkey=e61cf0fc0c0b4228aa0188dc434b99ffa9625359. Retrieved 2008-12-26. 
3. ^ E. Anton; J. Blanco, J. Egozcue, F. Vidal (April 29, 2004). "Sperm FISH studies in seven male carriers of Robertsonian translocation t(13;14)(q10;q10)". Human Reproduction (Oxford University Press) 19 (6): 1345–1351. doi:10.1093/humrep/deh232. ISSN 1460-2350. PMID 15117905. http://humrep.oxfordjournals.org/cgi/content/full/19/6/1345. Retrieved 2008-12-25. 
4. ^ ^{a b} Semple CA, Devon RS, Le Hellard S, Porteous DJ (April 2001). "Identification of genes from a schizophrenia-linked translocation breakpoint region". Genomics 73 (1): 123–6. doi:10.1006/geno.2001.6516. PMID 11352574. 
5. ^ Schaffer, Lisa. (2005) International System for Human Cytogenetic Nomenclature S. Karger AG ISBN 978-3805580199
6. ^ ^{a b c d} Li JY, Gaillard F, Moreau A, et al. (May 1999). "Detection of translocation t(11;14)(q13;q32) in mantle cell lymphoma by fluorescence in situ hybridization". Am. J. Pathol. 154 (5): 1449–52. doi:10.1016/S0002-9440(10)65399-0. PMC 1866594. PMID 10329598. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1866594. 
7. ^ ^{a b c d e f} Chapter 20 in: Mitchell, Richard Sheppard; Kumar, Vinay; Abbas, Abul K.; Fausto, Nelson. Robbins Basic Pathology. Philadelphia: Saunders. ISBN 1-4160-2973-7.  8th edition.
8. ^ ^{a b} Kurzrock R, Kantarjian HM, Druker BJ, Talpaz M (May 2003). "Philadelphia chromosome-positive leukemias: from basic mechanisms to molecular therapeutics". Ann. Intern. Med. 138 (10): 819–30. PMID 12755554. http://www.annals.org/cgi/pmidlookup?view=long&pmid=12755554. 
9. ^ ^{a b c} Page 626 in: Mitchell, Richard Sheppard; Kumar, Vinay; Abbas, Abul K.; Fausto, Nelson. Robbins Basic Pathology. Philadelphia: Saunders. ISBN 1-4160-2973-7.  8th edition.