Screening for Fragile X Syndrome

Fragile X Syndrome is an inherited condition which presents with mental retardation, delayed milestones, high forehead, large testicles, facial asymmetry, large jaw, long ears and short temper. The diagnosis is usually made before the child is one year old.^1,2

Fragile X syndrome is what is known as a repeat expansion disorder. In DNA coding it is common to see repeated sequences of the nucleotides that make up the genetic strand. In Fragile X Syndrome there is an expansion of the number of repeat sequences in the Fragile X mental retardation (FMR1) gene. The nucleotides involved are cytosine (C) and guanine (G) and the repeated sequence is CGG. In the commonest form of the condition, the CGG sequence is repeated more than 200 times. The metabolic result of this is to block production of a substance called fragile X mental retardation protein (FMRP).³

Family studies have identified both a full mutation and a "premutation" (less than 50 CGG repeats per segment) which is unstable and can become a full mutation on female transmission (but not on male transmission of the X chromosome to their daughters). This happens more frequently the larger the mutation and the mutation may become larger in subsequent generations until the gene expression is blocked. About a third of female carriers of the full mutation have mild mental retardation.

MicroRNAs (miRNAs), a newly discovered class of small noncoding RNAs, are thought to be involved in the aetiology of this condition.⁴

For further details concerning epidemiology, presentation, diagnosis and management see Fragile X Syndrome.

The UK National Screening Program Committee decided not to institute a national newborn screening programme in 2003 and did not change their position when the subject was revisited in 2006. The next review is due in 2009/10.⁵

Currently, the policy is to restrict screening to carrier identification within affected families. It may also be worthwhile screening mentally retarded children aiming to identify more families and enable carriers to have prenatal counselling. Screening thus has the potential for reducing the numbers born with the condition (primary prevention). Parents who are carriers may choose not to have children, use donor eggs, or use prenatal diagnosis to terminate affected pregnancies.

Methods available

• A blood sample (or chorionic villus biopsy) can be sent for DNA analysis. Most laboratories currently use a combination of Southern blotting (detects full mutations) and polymer chain reaction (PCR) tests (identifies pre-mutations and smaller CGG repeats). Southern blotting (named after Edward Southern, who developed the technique at Edinburgh University in the 1970s) involves transferring DNA material from an agar gel onto a membrane. Electrophoresis applied to this membrane can then be used to identify a particular DNA sequence.⁶ PCR used as polymerase enzyme to amplify a particular DNA region, making identification easier.⁷
• A refinement of this technique, using capillary action to separate DNA fragments of differing size, enables the rapid testing of large numbers of samples, making the method suitable as a newborn screening test.^8,9,10
• FMRP antibody testing is available on a blood smear (this can detect affected males)¹¹ and has also been developed on a pulled hair sample (this can detect males and females with a full mutation).¹²
• A method recently developed, called methylation-specific melting curve analysis (MS-MCA), relies on the fact that the transcription errors seen on the FR1 gene results in hypermethylation of the genetic material. It can be used it males only, but allows rapid and reliable identification of patients.¹³