A five-year experience with fragile X screening of high-risk gravid women

Who should be tested for fragile X carriership? A review of 1 center's pedigrees

OBJECTIVE

To determine whether following American College of Obstetricians and Gynecologists and American College of Medical Genetics recommendations would have detected carriers in pedigrees of patients diagnosed with fragile X.

STUDY DESIGN

Using a database of patients referred to the UT genetics clinic for evaluation of fragile X, pedigrees of cases of fragile X syndrome were analyzed.

RESULTS

Eight of 17 cases identified had a family history of unexplained mental retardation (MR) or fragile X MR and would have been diagnosed using current guidelines. Other findings noted in the pedigrees included autism, speech or hearing problems, attention deficit hyperactivity syndrome and behavioural disorders. No risk factors were found in 4 cases.

CONCLUSION

Using current guidelines, less than one half of fragile X carriers would have been identified during a prenatal assessment. Using other risk factors in screening would likely increase carrier detection rate.

Pre-conception and antenatal screening for the fragile site on the X-chromosome

BACKGROUND

Fragile X is the most common cause of mental retardation after Down syndrome. It is the commonest inherited cause of mental retardation, and results from a dynamic mutation in a gene on the long arm of the X chromosome. Various strategies are used for prenatal screening.

OBJECTIVES

To determine whether pre-conceptual or antenatal screening for Fragile X carrier status in apparently low risk women confers any additional benefit over the existing practice of offering testing to women thought to be at increased risk.

SEARCH STRATEGY

We searched the Cochrane Pregnancy and Childbirth Group trials register (November 2001), the Cochrane Controlled Trials Register (The Cochrane Library Issue 3, 2001), MEDLINE (1980 to 2001), and reference lists of articles.

SELECTION CRITERIA

Randomised clinical trials comparing women being tested regardless of family history (intervention group) with women tested only when there is a family history of either fragile X and/or other undiagnosed mental illness/impairment (control group).

DATA COLLECTION AND ANALYSIS

Three reviewers independently assessed trial quality and extracted data.

MAIN RESULTS

No trials were included.

REVIEWER'S CONCLUSIONS

No information is available from randomised trials to indicate whether routine pre-conceptual or antenatal screening for fragile X carrier status confers any benefit over testing women thought to be at increased risk.

FMR1 and the fragile X syndrome: human genome epidemiology review

The fragile X syndrome, an X-linked dominant disorder with reduced penetrance, is one of the most common forms of inherited mental retardation. The cognitive, behavioral, and physical phenotype varies by sex, with males being more severely affected because of the X-linked inheritance of the mutation. The disorder-causing mutation is the amplification of a CGG repeat in the 5' untranslated region of FMR1 located at Xq27.3. The fragile X CGG repeat has four forms: common (6-40 repeats), intermediate (41-60 repeats), premutation (61-200 repeats), and full mutation (>200-230 repeats). Population-based studies suggest that the prevalence of the full mutation, the disorder-causing form of the repeat, ranges from 1/3,717 to 1/8,918 Caucasian males in the general population. The full mutation is also found in other racial/ethnic populations; however, few population-based studies exist for these populations. No population-based studies exist for the full mutation in a general female population. In contrast, several large, population-based studies exist for the premutation or carrier form of the disorder, with prevalence estimates ranging from 1/246 to 1/468 Caucasian females in the general population. For Caucasian males, the prevalence of the premutation is approximately 1/1,000. Like the full mutation, little information exists for the premutation in other populations. Although no effective cure or treatment exists for the fragile X syndrome, all persons affected with the syndrome are eligible for early intervention services. The relatively high prevalence of the premutation and full mutation genotypes coupled with technological advances in genetic testing make the fragile X syndrome amenable to screening. The timing as well as benefits and harms associated with the different screening strategies are the subject of current research and discussion.