Biology of Disease

Detection of Hereditary Diseases

Tests based on recombinant DNA technology have increased in recent years

because of their high sensitivity and accuracy in detecting genetic disorders

at the prenatal stage. The detection of hereditary diseases in a given genome

was traditionally a long and laborious task. However, modern techniques of

chromosome and DNA analyses have resulted in a dramatic increase in the

number of tests for genetic diseases with a considerable improvement in

the time required for analysis. For example, using PCR, polymerase chain

reaction (Chapter 3), virtually any portion of a gene or even whole genes

can be amplified for analysis by electrophoretic techniques or sequenced to

detect mutations. This not only allows for diagnosing patients with inherited

disorders but also the detection of mutations in carriers, even though

they do not express any symptom of the disorder. It is also a direct way of

distinguishing different mutations within a single gene, each of which can

lead to disorders, for example muscular dystrophies which are described

inChapter 16. It is hoped that eventually predictive tests for disorders that

have only some genetic component, such as heart disease and cancers,

will eventually be developed. Indeed, PCR analysis of cells lost in feces has

demonstrated premalignant changes in the gastrointestinal tract and allowed

patients at risk of developing colon cancer to be identified. This is of clinical

importance since the earlier treatments of malignant conditions are started,

the more favorable the prognosis.

Cloned DNA sequences have expanded the range of prenatal testing because

they allow the fetal genotype to be examined directly, rather than relying

on secondary tests for the products of the normal or mutant genes. Thus,

mutations in DNA can be detected in those cases where an aberrant product

cannot be detected prior to birth, even though a test is available. A test for

sickle cell anemia using a restriction enzyme has been described in Chapter

13. However, RFLP analysis requires that the mutation in the gene alters the

restriction sites recognized by restriction endonucleases and this is not always

the case. In contrast, allele-specific oligonucleotides (ASOs) are synthetic

nucleotide probes, which bind only to their complementary DNA. They

will not hybridize to other sequences and, in appropriate highly stringent

conditions, differentially bind to and distinguish between alleles that differ

by as little as one nucleotide. They can thus distinguish between the native

and mutant forms of a gene with excellent resolution and potential versatility.

X]VeiZg&*/ GENETIC DISEASES

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Placenta

Uterine wall

Amniotic fluid

Villi of

chorion

frondosum

Stop

Vagina

Uterus

Catheter

A) B)

Figure 15.37 Outlines of (A) amniocentesis and (B) chorionic villus sampling to obtain specimens for genetic analysis of the fetus.