Chapter

Insertions

R. J. McKinlay Gardner, Grant R. Sutherland and Lisa G. Shaffer

in Chromosome Abnormalities and Genetic Counseling

Fourth edition

Published on behalf of Oxford University Press

Published in print November 2011 | ISBN: 9780195375336
Published online October 2012 | e-ISBN: 9780199975174 | DOI: http://dx.doi.org/10.1093/med/9780195375336.003.0010

Series: Oxford Monographs on Medical Genetics

Insertions

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INSERTIONS ARE A TYPE OF TRANSLOCATION: sometimes the expression “insertional translocation” is used. In the common, simple insertion, three breaks are required. The first two breaks release an interstitial segment of chromosome, which is then inserted into the gap created by the third break. In the simple one-way interchromosomal insertion, a segment from one chromosome is intercalated into another chromosome. A more complicated four-break rearrangement is the reciprocal insertion, whereby two nonhomologous chromosomes exchange intercalary segments. In the intrachromosomal insertion, a segment is intercalated into another part of the same chromosome. The segment may be inserted “right way around”—that is, with the same orientation to the centromere as before; this is a direct insertion (dir ins). Or it may be reversed—an inverted insertion (inv ins). More complicated scenarios, which may involve both insertional and terminal translocated segments, are more appropriately dealt with in Chapter 12 (Complex Rearrangements).

Insertions are rare rearrangements, at the level of detection according to classical cytogenetics. With microarray technology, previously undetectable insertions of very small size are coming to light, de novo and familial, and it may prove that “uncommon” will be a more accurate adjective to describe frequency than is “rare.” Kang et al. (2010) found a 20-fold increased discovery of insertions, compared with earlier studies, in a large series of cases presenting with typical chromosomal clinical pictures. Many of these turned out to be (probably) harmless polymorphisms (CNVs), upon the recognition of a parent carrying the same insertion. But a fraction were, in all probability, truly pathogenic. It is notable that most of these would not have been detectable on classical cytogenetics.

In this chapter, we consider the case of the phenotypically normal heterozygote, in whom the rearrangement is assumed to be balanced.

Chapter.  6660 words.  Illustrated.

Subjects: Clinical Genetics

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