530
Thus, these traditional approaches had severe limitations for the identifi cation of
carriers and prenatal diagnosis. The tools of molecular genetics are beginning
to permit genotypic screening. The most practical outcome of the use of recombi-
nant DNA technology in medical genetics has been improved diagnosis and predic-
tion of inherited diseases, e.g. ALS, Huntington disease, cystic fi brosis, breast
cancer and hemophilia. Other applications are in disease management and
pharmacogenomics.
Molecular diagnostic methods are described in a special report on this topic (Jain
2015d ). A few that are used for diagnosis of genetic disorders will be mentioned in
this Chapter. Advantages of molecular diagnosis in genetic disorders are:
- The presence or absence of a mutation in an affected person or a carrier can be
determined without any ambiguity. - A distinction can be drawn between disorders with similar phenotypes.
- Diagnosis can be made in advance of clinical manifestations.
- The high specifi city of molecular diagnostics permits screening of large popula-
tions to identify carriers. - In this application, molecular genetic tests are not usually directed at a particular
anatomical lesion, as DNA obtained from anybody site is equally valid. - Tests can be used as the basis for gene therapy. If a normal gene has been cloned
for use as a probe for disease-causing mutations, the same normal sequence can
be used to replace the mutated sequence in the patient. This type of gene repair
has been tried experimentally in cystic fi brosis by using in situ PCR.
The main limitations of molecular diagnostics for genetic disorders are as
follows: - Because the genetic changes that underlie inherited disorders are so heteroge-
neous, the mutations can be so diverse that no two persons will demonstrate the
same change. This variability hinders the construction of a molecular diagnostic
test that is applicable to all patients with a certain disease. For example, two
distinct genes have been implicated in tuberous sclerosis in different families. - A normal counterpart may mask the gene deletion. Because non-sex-linked genes
come in pairs, the normal gene can potentially hide the loss of part or all of the other
copy. In conventional gene testing, PCR amplifi es the intact gene inherited from
one parent but the normal copy hides the mutated gene from the other parent.
Molecular Diagnostic Technologies
Cytogenetics
In the past decade, clinical cytogenetics has undergone remarkable advancement as
molecular biology techniques have been applied to conventional chromosome anal-
ysis. The limitations of conventional banding analysis in the accurate diagnosis and
16 Personalized Management of Genetic Disorders