Biology of Disease

structure of each chromosome. Karyotyping can be performed on white cells

from whole blood as described above or from amniotic fluid, which contains

cells from the developing fetus. When a patient’s chromosomes are examined

using a microscope, it is possible to identify aberrations in chromosome

number and structure.

The phenotype of a patient with a chromosomal disorder depends on the

nature of their chromosomal defect. The first human chromosomal disorder

was discovered in 1959 when three copies of chromosome 21 were found

to be associated with Down syndrome (Section 15.9). The development of

chromosomal banding in 1970 has markedly increased the ability to resolve

small chromosomal aberrations.

Fragile Sites

When human cells are grown in culture, some of the chromosomes in cells

derived from certain individuals fail to stain in particular regions, giving the

appearance of a gap. These sites are known as fragile sites, since they are

susceptible to breakage when the cells are cultured in the absence of certain

chemicals such as folic acid, which is normally present in the culture medium.

More than 80 such sites have been identified since they were first discovered

in 1965. The cause of the fragility at these sites is not known with certainty

but they may represent regions where the DNA has been incompletely

replicated.

Almost all studies on fragile sites have been carried out in vitro on cells halted

in mitosis. Initially they were not considered to be clinically relevant and,

indeed, most fragile sites do not appear to be associated with any clinical

syndrome. However, a strong association has been shown to exist between

a form of mental retardation called fragile X or Martin-Bell syndrome and a

fragile site on the X chromosome at position Xq27.3 (Figure 15.21) associated

with the FMR1 gene. It is a dominant trait but fortunately fails to be fully

expressed (incomplete penetrance) in many individuals. However, it is the

commonest cause of mental retardation and has been estimated to affect one

in about 4100 males in the UK. Most suffer mental retardation to the point

that they are unable to live an independent life, and have a distinct physical

appearance, including long, narrow faces with protruding chins, enlarged

ears, and increased testicular size particularly after puberty. The syndrome

also affects about one in 8000 females, who tend to suffer milder forms of

retardation. Most humans carry a stable version of FMR1 which has about 30

CGG repeats (Box 15.2). Individuals who have genes with about 45 to about

55 CGG repeats are in a gray zone; they do not have fragile X syndrome and,

while they are likely to pass on a stable gene to their children, they have an

increased chance of having children with a larger number of CGG repeats.

People with about 55 to about 200 are said to have a premutation since

although they generally have few or lack symptoms of fragile X syndrome, they

can have children with more than 200 CGG repeats. This is the full mutation

that initiates inappropriate methylation of cytosine bases and unfortunately

can lead to the full blown syndrome.

The FMR1 gene is carried on the X chromosome and shows Mendelian

patterns of inheritance for X-linked disorders (Figure 15.12), although if it

reaches the premutation stage it has a high probability of mutating (by repeat

amplification) from one generation to the next. However, amplification of

the CGG repeats can only occur in females, not males. Thus although there

may be no family history of fragile X syndrome, it can suddenly appear in a

number of offspring. The patterns of inheritance of fragile X syndrome are

also somewhat complicated compared with a disorder like phenylketonuria,

described in Section 15.6. Given that FMR1 is on the X chromosome, a father

cannot pass on any form of it to male offspring. However, daughters generally

X]VeiZg&*/ GENETIC DISEASES

)(% W^dad\nd[Y^hZVhZ

Figure 15.21 Schematic of a human X

chromosome with a fragile site (arrowed).