The Principles of Clinical Cytogenetics - Extra Materials - Springer

Autosomal Aneuploidy 155
This marker contains two copies of a small segment of proximal long-arm euchromatin (22q11.2),
thus resulting in tetrasomy for 22q11.2. Clinically, these patients usually present with cat-eye syndrome
(260–262). Characteristic features include craniofacial anomalies (vertical coloboma of the
iris, which gives the syndrome its name, coloboma of the choroid or optic nerve, preauricular skin
tags/pits, down-slanting palpebral fissures), and anal atresia with rectovestibular fistula. Cardiac
defects are present in more than one-third of cases. Renal malformations include unilateral agenesis,
unilateral or bilateral hypoplasia, or dysplasia. Other less frequent findings include microphthalmia,
microtia, atresia of the external auditory canal, biliary atresia, and malrotation of the gut. Intelligence
is usually low normal to mildly deficient.
Other types of supernumerary marker, such as ring chromosomes derived from chromosome 22
resulting in either trisomy or tetrasomy for 22q11.2, can also cause various features of the cat-eye
syndrome. The critical region of this syndrome has been shown to lie within a 2.1-Mb DNA segment
defined distally by locus D22S57 and containing the ATP6E (the E subunit of vacuolar H-ATPase)
gene (263).
Clinically definable entities have not been observed for other markers, as each is typically unique.
However, this could change as data concerning the composition of marker chromosomes accumulates
through the use of FISH and other molecular technologies.
ACKNOWLEDGMENTS
The author is grateful to Martha Keagle, Leven Chen, and Craig McKee for preparation of the
diagrams and to Dr. Linda Randolph for providing patient photographs. Thanks are also given to Jo
Ann Rieger for her aid in preparation of the manuscript.
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