Chromosome 13

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Chromosome 13

Trisomy 13 or Patau syndrome is characterised by urogenital, cardiac, craniofacial, central nervous system and growth abnormalities. Defects include mental retardation, bilateral harelip and cleft palate, uni- or bilateral hexadactyly, growth retardation, polycystic kidney, ocular abnormalities, congenital heart disease and holoprosencephaly. Over 95% of human trisomy-13 conceptions spontaneously abort. Viable births rarely have prolonged survival. Approximately 80% of the cases involve free 13 trisomy. In 20% of the cases, either a mosaic or trisomy due to a translocation is involved. In cases of mosaicism, the severity of the clinical features can be diminished. A translocation, almost always t(13qDq) and more expressly t(13q14q), can occur de novo or can be transmitted by one of the parents. Rarely, more complex rearrangements are observed.

a number of observations of partial 13q trisomies are reported involving segments of variable length, with breakpoints occurring at different sites on 13q. One of the most frequent sites is the interface between q14 and q21. When the trisomy includes the q2 and q3 regions, it leads to a distinctive clinical syndrome. Facial dysmorphism resembles that of cornelia de lange syndrome. Respir 1000 atory distress and neonatal feeding difficulties are common. A small percentage of partial trisomies is due to de novo duplication.

The majority are the result of a malsegregation of a parental rearrangement (a reciprocal translocation or a pericentric inversion). Either total or partial monosomy 13q3 includes rings and terminal deletions and intercalary deletions which include band q14 and are accompanied by a retinoblastoma. The classical 13q- syndrome is associated with deletions in 13q32.

The most distinctive sign is the absence of a defined nasal bridge producing a greek profile. Microcephaly is often severe with brain malformations. Upper incisors set in a rabbitlike forward slant are highly characteristic. Hypoplasia or absence of the thumb, agenesis of the first metacarpal, fusion of the fourth and fifth metacarpals and syndactyly, eye malformations, bone and gi anomalies and considerable growth and mental retardation are frequently found. Deletions limited to bands more proximal to 13q32 are associated with growth retardation and moderate mental retardation, but not with major malformations. Deletions limited to bands distal to 13q32 have severe mental retardation without major malformations and usually without growth failure.

The characterization of 13q14 monosomy is justified by the existence of retinoblastoma. From the cytogenetic standpoint, this is a very heterogeneous group, with the deletion capable of extending on both sides of q14, from q11 to q22. The deletion usually occurs de novo 7c2

and can also result from a parental insertion. In ring 13, certain features of partial or complete 13 trisomy can be seen, due to partial duplication of the rings.

The study of patients afflicted with del(13)-retinoblastoma allowed the precise assignment of the gene for esterase D to 13q14.11. Other important genes on chromosome 13 include those for Wilson disease and propionyl CoA-carboxylase.