New York State screening program for fragile X syndrome: a progress report

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.

A clinical study of mentally retarded children with fragile X syndrome in Saudi Arabia

BACKGROUND

Studies on fragile X syndrome are scarce in Saudi Arabia and other countries of the Arabian Peninsula. The few studies previously done have either been in the form of case reports or those performed on institutionalized mentally retarded patients. The aim of this study was to determine the prevalence of fragile X syndrome among cases with mental retardation who have been referred to the pediatric neurology clinics of King Khalid University Hospital (KKUH) in Riyadh.

PATIENTS AND METHODS

Cytogenetic studies were performed in 94 cases who were referred to the pediatric neurology clinics of KKUH because of mental retardation and/or delayed milestones of development, from July 1995 to December 1997.

RESULTS

Six male probands (6.4%) showed the classic fragile X chromosome and another six (including a four year-old girl) were detected, following extension of the cytogenetic studies to all 32 first-degree relatives. Affection of more than one sibling was ascertained in four families. One family had four brothers with fragile X syndrome, whereas another formed part of a large kindred with twelve males and five females who were mentally retarded. A clinical, physical and psychological screening checklist was applied to the eleven affected males. Large testicular size, long face and short attention span were the most frequent features, and each was detected in nine patients (82%). Pes planus and history of delayed speech were found in eight patients (73%).

CONCLUSION

The study showed that the fragile X syndrome clinical screening checklist that has been applied in other populations might equally be valuable and applicable among the population of Saudi Arabia. However, the presence of pale blue eyes can be excluded and more weight given to positive family history of mental retardation. The study also highlighted the need for further emphasis on the orientation of primary care physicians to the most common clinical diagnostic features of fragile X syndrome.

Frequency of the fragile X syndrome in Chinese mentally retarded populations is similar to that in Caucasians

Fragile X syndrome is recognized as the most common inherited cause of mental retardation in western countries. The prevalence of the fragile X syndrome in Asian populations is uncertain. We report a multi-institutional collaborative study of molecular screening for the fragile X syndrome from 1,127 Chinese mentally retarded (MR) individuals. We found that 2.8% of the Chinese MR population screened by DNA analysis had the fragile X full mutation. Our screening indicated that the fragile X syndrome prevalence was very close to that of Caucasian subjects. In addition, we found that 62.5% of fragile X chromosomes had a single haplotype for DXS548-FRAXAC1 (21-18 repeats) which was present in only 9.7% of controls. This unique distribution of microsatellite markers flanking the FMR1 CGG repeats suggests that the fragile X syndrome in Chinese populations, as in the Caucasian, may also be derived from founder chromosomes.

DNA testing for fragile X syndrome in schools for learning difficulties

Fragile X syndrome is the most common inherited cause of mental retardation. Early diagnosis is important not only for appropriate management of individuals but also to identify carriers who are unaware of their high risk of having an affected child. The disorder is associated with a cytogenetically visible fragile site (FRAXA) at Xq27.3, caused by amplification of a (CGG)n repeat sequence within the gene at this locus designated FMR1. Clinical and molecular studies have been undertaken to screen for fragile X syndrome in 154 children with moderate and severe learning difficulties of previously unknown origin. Southern blot analysis of peripheral blood showed the characteristic abnormally large (CGG)n repeat sequence associated with fragile X syndrome in four of the 154 children. The findings were confirmed by cytogenetic observation of the fragile site and by further molecular studies. The families of the affected children were offered genetic counselling and DNA tests to determine their carrier status. These findings show that there are still unrecognised cases of fragile X syndrome. Given the difficulty of making a clinical diagnosis and the implications for families when the diagnosis is missed, screening in high risk populations may be justified. The issues involved in screening all children in special schools for fragile X syndrome are discussed.

The fragile X syndrome: implications of molecular genetics for the clinical syndrome

The fragile X syndrome of mental retardation is one of the most common genetic diseases. Characterization of the mutations involved has greatly improved our knowledge of the transmission of fragile X syndrome and new DNA-based diagnostics tools significantly outperform cytogenetic testing both for establishing the diagnosis and for determining carrier status. Fragile X mutations consist of an expansion of a CGG trinucleotide repeat localized in a gene (FMR-1) that is abnormally methylated in all affected individuals. They are classified as premutations (asymptomatic) and full mutations (associated with the disease). Several different DNA analysis protocols are used for fragile X genotyping but only a few have been tested on large samples of individuals. There are several clinical indications for direct DNA genotyping for fragile X including mental retardation, learning disability or hyperactivity in children with or without a family history of mental retardation, the establishment of carrier diagnosis in fragile X families and prenatal screening of children from carrier women.