Molecular Test Detection

Test : Fragile X DNA detection


Indications : The test is ordered when a person presents with the following clinical symptoms:

• Family history of fragile X syndrome, autism
• Clinical symptoms that suggest fragile X associated tremor/ataxia syndrome (FXTAS)
• Clinical symptoms that suggest FXPOL (Fragile X-associated primary ovarian insufficiency)
• Person with developmental impairment
• Female with elevated FSH levels, premature ovarian failure, irregular menses or infertility
• Adult above 50 yrs with cognitive irregularities, personality changes, memory loss

A random request for fragile X testing for fetus by a pregnant woman.

The samples used may be peripheral whole blood, amniotic fluid, or chorionic villi. DNA is isolated from the sample through standard methods and it is subjected to the southern blotting technique in the following manner:

• DNA is digested using a restriction enzyme and is separated according to its molecular weight on an agarose gel electrophoresis

• Incubation with NaOH helps the DNA to denature into single strands

• Separated DNA is transferred to a special blotting paper (membrane)

• This blot is incubated with many copies of single stranded DNA probes. The probes are easily detected as they are radioactive and can be exposed on an X-ray film directly, or has an attached enzyme which converts to a colored product upon incubation with a colorless substrate

The Southern blot technique is very useful in determining full gene mutation of the fragile X gene. For various technical reasons the PCR has not been the method of choice. Of late it is being reconsidered as the technique is faster, cheaper and has undergone several desirable improvements which has increased its accuracy.

Physiology : The fragile X syndrome is a genetic condition that causes the X chromosome to become susceptible when exposed to certain agents, giving rise to a range of developmental problems including cognitive impairments and learning inabilities.

In the seventies and eighties the only diagnostic tool for fragile X syndrome was the cytogenetic test. Now the DNA test has replaced the cytogenetic tests as a reliable diagnostic method as it is able to detect 99 % of those affected and also those who are fragile X carriers.

Interpretation : Mutations in the fragile X gene may be indication of a fragile X syndrome.

Sample : Laboratory generally provides an interpretive report.

Test Method : Southern blotting technique.

Related Tests : Whole blood, amniotic fluid, chorionic villus

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