Molecular characterization of X chromosome fragility in idiopathic mental retardation

Abstract

Background: Fragile X syndrome (FXS) is the most common form of inherited mental retardation. Frequency of fragile X syndrome among male siblings and relatives of mentally retarded patients is relatively high. Cytogenetic diagnosis of FXS is unreliable since it is ineffective for the diagnosis of premutated males or females. Proper molecular diagnosis is a pre-requisite for providing proper counseling advice.

Subjects and methods: Sixty-four males with idiopathic mental retardation, ranging in age from 4.2 to 19 years (10.92± 4.00) were clinically pre-selected, based on scoring protocol comprising eight features of the syndrome, before molecular testing. A rapid polymerase chain reactionbased screening was applied for detection of expanded FMR1 alleles. Samples that did not yield the normal band lengths were subjected to a second PCR screen. The secondary screen utilizes a chimeric primer demonstrating the presence or absence of an expanded allele.

Results: Amplification of FMRI gene by PCR of tested patients revealed that 8 cases (12.5%) have full mutation and 6 cases (9.4%) have premutation. A wide range of Fra X-scoring ranging from 1 to 7 features was detected in examined cases. Significant clinical features included large prominent ears, hyperextensibility of joints and macroorchidism in post pubertal males.

Conclusions: A simplified checklist of fragile X should be used for patients with idiopathic MR and those patients above score 3 should be tested for FXS. The diagnostic assay may be used as a screening method for fragile X syndrome being rapid and cost effective compared to other techniques. In addition, screening of all relatives of proven patients should be performed to detect clinically unidentified cases for provision of proper counseling and optimal management of detected cases.

Keywords: X chromosome fragility; CGG repeats; FMRI gene methylation; Pre-mutation phenotype; Full mutation phenotype