The Principles of Clinical Cytogenetics - Extra Materials - Springer

Fragile X 495
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Fragile X
From Cytogenetics to Molecular Genetics
Dana C. Crawford, PhD and Patricia N. Howard-Peebles, PhD
INTRODUCTION TO HUMAN FRAGILE SITES
The first fragile site identified in humans was on chromosome 9q, as described by Dekaban in 1965
(1). Fragile sites were an active area of cytogenetic research during the late 1970s and most of the 1980s
stimulated by: a link between the fragile site at Xq27 and X-linked mental retardation, the discovery
that fragile site expression was directly related to the tissue culture conditions used for cell preparations
(2), and a possible relationship between fragile sites and cancer/cancer cytogenetics. The application of
molecular techniques to fragile sites began in the early 1990s with the discovery of a new mutation
mechanism for fragile Xq27, which resulted in the identification of a new type of human disease.
In 1979, Sutherland defined a fragile site as a specific point on a chromosome, which appears as a
nonstaining gap, usually on both arms or chromatids (3). In a family, this site is always in the same
location, is inherited as a Mendelian co-dominant, and results in chromosome fragility under appropriate
tissue culturing conditions. Over 100 human fragile sites have been described, and they can be
classified into two major categories: rare or common (see Chapter 14).
Clinical significance has been established for two fragile sites: FRAXA (Xq27.3) and FRAXE
(Xq28). Both are rare, folate-sensitive fragile sites. FRAXA is the fragile X (fraX), which is associated
with the fragile X syndrome, the most common form of familial mental retardation. FRAXE is
associated with a mild form of X-linked mental retardation.
Fragile sites require an induction system for consistent cytogenetic expression. The majority of
cytogenetic studies have been performed on phytohemagglutinin (PHA)-stimulated lymphocytes,
due to induction problems in other tissue types. Tissue culture conditions and modifications for induction
of all fragile sites are detailed by Sutherland and Hecht (4,5).
The folate-sensitive fragile sites including fraX can be induced by multiple methods, as summarized
in Table 1. Method 4 has an advantage of being somewhat less cytotoxic to the cells during
culturing (7). Numerous physical and chemical factors affect the ultimate level of expression of fragile
sites. Sutherland and Hecht provided extensive details and documentation of requirements for
fragile site expression (5). Because of the stringent requirements for folate-sensitive fragile site
expression, basic guidelines were developed to assure quality clinical testing for fraX (8,9).
GENETICS OF FRAGILE X SYNDROME,
PRIOR TO THE AVAILABILITY OF MOLECULAR ANALYSIS
X-Linked Mental Retardation
In 1938, Penrose noted a higher incidence of mental retardation (MR) in males and reports of families
with only affected males (10). These observations were compatible with X-linked inheritance, and
From: The Principles of Clinical Cytogenetics, Second Edition
Edited by: S. L. Gersen and M. B. Keagle © Humana Press Inc., Totowa, NJ
495