Partial monosomy for chromosome 13 caused by either deletion of part of the long arm or ring 13 results in similar phenotypes including microcephaly and trigonocephaly; facial features include a prominent bridge of the nose; small chin; large, low-set, malformed ears; and facial asymmetry (see Fig. 40-4). Other features include absent or hypoplastic thumbs, cardiac and renal malformations, anal atresia, and features of Noonan syndrome; males may have hypospadias and undescended testes. Ophthalmic findings of interstitial deletions of 13q include hyper-telorism, up- or downward slanting and/or narrow palpebral fissures, epicanthus, strabismus, blepharoptosis, cataract, retinoblastoma, Rieger syndrome, and colobomatous micro-
The retinoblastoma gene is located in band 14 of the long arm of chromosome 13 and has become a prototype for the study of the hereditary predisposition to cancer. The initial evidence for the location of the gene came from children who had mental retardation, dysmorphic features, and retinoblasoma associated with a deletion in the long arm of chromosome 13332,412; approximately 6% of children with a retinoblastoma have been shown to have a detectable deletion in the same regions.36 The autosomal dominant form of the disease was later localized to the same region by the identification of linkage with esterase D,333 a red cell enzyme that had been previously mapped to the same region on the long arm of chromosome 13.334 Assay of esterase D may be a method for identification of small deletions.63 Thereafter, Benedict and colleagues23 demonstrated homozygosity for the locus (inactivation or both alleles) in tumor cell lines supporting a recessive pattern on a cellular level. The gene has been cloned,103,126,193 and risk assessment is greatly improved.48,142,381 Prenatal diagnosis may be possible for families with a germinal mutation or a balanced translocation.
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