Polysomy

Polysomy is a condition found in many species, including fungi, plants, insects, and mammals, in which an organism has at least one more chromosome than normal, i.e., there may be three or more copies of the chromosome rather than the expected two copies. Most eukaryotic species are diploid, meaning they have two sets of chromosomes, whereas prokaryotes are haploid, containing a single chromosome in each cell. Aneuploids possess chromosome numbers that are not exact multiples of the haploid number and polysomy is a type of aneuploidy. A karyotype is the set of chromosomes in an organism and the suffix -somy is used to name aneuploid karyotypes. This is not to be confused with the suffix -ploidy, referring to the number of complete sets of chromosomes. Polysomy is a condition found in many species, including fungi, plants, insects, and mammals, in which an organism has at least one more chromosome than normal, i.e., there may be three or more copies of the chromosome rather than the expected two copies. Most eukaryotic species are diploid, meaning they have two sets of chromosomes, whereas prokaryotes are haploid, containing a single chromosome in each cell. Aneuploids possess chromosome numbers that are not exact multiples of the haploid number and polysomy is a type of aneuploidy. A karyotype is the set of chromosomes in an organism and the suffix -somy is used to name aneuploid karyotypes. This is not to be confused with the suffix -ploidy, referring to the number of complete sets of chromosomes. Polysomy is usually caused by non-disjunction (the failure of a pair of homologous chromosomes to separate) during meiosis, but may also be due to a translocation mutation (a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes). Polysomy is found in many diseases, including Down syndrome in humans where affected individuals possess three copies (trisomy) of chromosome 21. Polysomic inheritance occurs during meiosis when chiasmata form between more than two homologous partners, producing multivalent chromosomes. Autopolyploids may show polysomic inheritance of all the linkage groups, and their fertility may be reduced due to unbalanced chromosome numbers in the gametes. In tetrasomic inheritance, four copies of a linkage group rather than two (tetrasomy) assort two-by-two. Polysomy types are categorized based on the number of extra chromosomes in each set, noted as a diploid (2n) with an extra chromosome of various numbers. For example, a polysomy with three chromosomes is called a trisomy, a polysomy with four chromosomes is called tetrasomy, etc.: Polysomy plays a role in canine leukemia, hemangiopericytomas, and thyroid tumors. Abnormalities of chromosome 13 have been observed in canine osteoid chondrosarcoma and lymphosarcoma. Trisomy 13 in dogs with lymphosarcoma show a longer duration of first remission (medicine) and survival, responding well to treatments with chemotherapeutic agents. Polysomy of chromosome 13 (Polysomy 13) is significant in the development of prostate cancer and is often caused by centric fusions. Since canine chromosome 13 is similar to human chromosome 8q, research could provide insight to treatment for prostate cancer in humans.Polysomy of chromosomes 1, 2, 4, 5, and 25 are also frequently involved in canine tumors. Chromosome 1 may contain a gene responsible for tumor development and lead to changes in the karyotype, including fusion of the centromere, or centric fusions. Aneuploidy due to nondisjunction is a common feature in tumor cells. Some of the most frequent genetic disorders are abnormalities of sex chromosomes, but polysomies rarely occur. 49,XXXXY chromosome polysomy occurs every 1 in 85,000 newborn males. The incidence of other X polysomies (48,XXXX, 48,XXXY, 48,XXYY) is more rare than 49,XXXXY. Polysomy Y (47,XYY; 48,XYYY; 48,XXYY; 49,XXYYY) occurs in 1 out of 975 males and may cause psychiatric, social, and somatic abnormalities. Polysomy X may cause mental and developmental retardation and physical malformation. Klinefelter syndrome is an example of human polysomy X with the karyotype 47, XXY. X chromosome polysomies can be inherited from either a single maternal (49, X polysomies) or paternal (48, X polysomies) X chromosome. Polysomy of sex chromosomes is caused by successive nondisjunctions in meiosis I and II. In squamous cell carcinoma, a protein from the epidermal growth factor receptor (EGFR) gene is often overexpressed in conjunction with polysomy of chromosome 7, so chromosome 7 can be used to predict the presence of EGFR in squamous cell carcinoma. In colorectal cancer, EGFR expression is decreased with polysomy 7, which makes polysomy 7 easier to detect and could be used to prevent patients from having unnecessary cancer treatment. Tetrasomy and hexasomy 8 are rare compared to trisomy 8, which is the most common karyotypic finding in acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS). AML, MDS, or myeloproliferative disorder (MPD) with a high incidence of secondary diseases and a six-month survival rate are associated with a polysomy 8 syndrome. Overexpression of the HER2/neu gene on chromosome 17 and some type of polysomy has been reported in 8-68% of breast carcinomas. If theHER-2/neu gene does not amplify in the case of polysomy, proteins may be overexpressed and could lead to tumerogenesis. Polysomy 17 may complicate the interpretation of HER2 testing results in cancer patients. Chromosome 17 polysomy may not be present when the centromere is amplified, so it was later discovered that polysomy 17 is rare. This was discovered using array comparative genomic hybridization, a DNA-based alternative for clinical evaluation of HER2 gene copy number.