The Principles of Clinical Cytogenetics - Extra Materials - Springer

208 Cynthia Powell
Fig. 1. Ideograms of the X and Y chromosomes showing the pseudoautosomal regions (PAR1 and PAR2),
the locations of the X-inactive-specific transcript (XIST) gene, the critical region on Xq, the sex-determining
region Y (SRY), the azoospermia factor region (AZF), the heterochromatic region of the Y, and the malespecific
region located between PAR1 and PAR2.
gene that seems to control X inactivation is XIST (X-inactive-specific transcript) and is located at the
X-inactivation center (XIC) at band Xq13 (see Fig. 1). Only the inactive X expresses this gene. XIST
codes for Xist RNA, which appears to coat or paint the inactive X chromosome and is responsible for
inactivation (4). About 15% of genes on the X chromosome escape inactivation and remain active on
both X chromosomes in females (5). Many more genes on Xp escape inactivation as compared to Xq
(3). In individuals with extra or missing X chromosomes, a single X remains active. However, in
triploids, there are usually two active X chromosomes, suggesting a counting mechanism protecting
a single X-chromosome inactivation for every two autosome sets (3).
Early evidence for the existence of the inactive X was the observation of the Barr body, named for
the Canadian cytologist Murray Barr (6). This is a dark-staining chromatin body, present in one copy
in normal females, which is the condensed, inactive X chromosome. Normal males have no Barr
body. Initially, a buccal smear was obtained from patients to look for Barr bodies. Because of
improved methods for looking at sex chromosomes and the inaccuracy of the buccal smear technique,
it is now considered an obsolete test. The sex chromatin body in polymorphonuclear leukocytes
takes the form of the “drumstick,” seen attached to the nucleus in approximately 2% of these
cells in women, but not in men (7).
Techniques for detecting the inactive X have been based on the fact that it is late replicating. The
most commonly used cytogenetic method involves the use of bromodeoxyuridine (BrdU) (8). Newer
methods for detecting the inactive X involve molecular techniques often using differential methylation
analysis (9–13).