Small Supernumerary Marker Chromosomes (sSMC) A Guide for Human Gene
Human beings normally have a total of 46 chromosomes, with each chromosome present twice, apart from the X and Y chromosomes in males. Some three million people worldwide, however, have 47 chromosomes: they have a small supernumerary marker chromosome (sS
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Small Supernumerary Marker Chromosomes Known To Be Correlated with Specific Syndromes
Classically, four clinical syndromes are included in the cytogenetically defined group of patients with an sSMC: ES (see Sect. 5.1), CES (see Sect. 5.2), PKS (see Sect. 5.3), and i18pS (see Sect. 5.4). Besides, there is the special group of patients with mosaic karyotypes 45,X/46,X,+mar who can, but need not have, TS phenotype (see Sect. 5.5). Isochromosome 8p syndrome (see Sect. 6.8), isochromosome 9p syndrome (see Sect. 6.9), isochromosome 15 syndrome (see Sect. 6.15), and isochromosome 20p syndrome (see Sect. 6.20) are sometimes mentioned as well. However, these extra chromosomes are larger than chromosome 20 of the same metaphase spread and are thus, by definition, SMC because they can normally be identified by banding cytogenetics alone. Finally, UPD-related syndromes that might coincide with sSMC presence are also discussed in the corresponding subsections in Chap. 6.
5.1
Emanuel Syndrome
ES (OMIM #609029), also known as “derivative chromosome 22 syndrome”, is most often caused by a balanced translocation of chromosomes 11 and 22 in one of the parents. This translocation, cytogenetically described as t(11;22)(q23;q11.2), is the most frequently occurring recurrent non-Robertsonian constitutional translocation in humans (Zackai and Emanuel 1980). It was demonstrated that palindromemediated double-strand breaks in meiosis cause illegitimate recombination between subbands 11q23 and 22q11, resulting in this recurrent translocation (Kurahashi and Emanuel 2001). Also, one low copy repeat (LCR) on chromosome 22 has been shown to be involved (McDermid and Morrow 2002). There are hints that the ESspecific translocation of chromosomes 11 and 22 is predominantly formed de novo during male spermatogenesis (Kurahashi and Emanuel 2001; Kato et al. 2006). Carriers of the balanced constitutional t(11;22) translocation are phenotypically normal. However, there is a 2–6% risk of their having live-born progeny with ES (Medne et al. 2010). The latter is a result of malsegregation of the derivative T. Liehr, Small Supernumerary Marker Chromosomes (sSMC), DOI 10.1007/978-3-642-20766-2_5, # Springer-Verlag Berlin Heidelberg 2012
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5 Small Supernumerary Marker Chromosomes Known To Be Correlated
chromosome 22 during meiosis (Shaikh et al. 1999). In the literature more than 300 ES cases have been reported (Liehr 2011a). The ES-causing der(22)t(11;22)(q23; q11.2) is detected in cytogenetic analysis as a centric minute-shaped sSMC (see Sect. 3.2).
5.1.1
Clinical Characteristics
Clinical manifestations are/can be microcephaly (100% of ES patients), cardiac defects (60% of ES patients), cleft palate (50% of ES patients), renal malformations (30% of ES patients), anal atresia or stenosis (20% of ES patients), hip dysplasia, other skeletal complications, craniofacial abnormalities, gastroesophageal reflux, hearing loss, refractive errors, strabismus or other ophthalmologic issues, inguinal hernia, seizures, and other manifestations. Life-thr
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