Frequency of fragile-x in x‑linked mental retardation
Introduction: Fragile X syndrome (FXS) is the most common form of inherited
mental retardation and accounts for about one third of all cases of X linked mental retardation (XLMR). It is inherited as an X‑linked dominant trait with a fragile site at Xq27.3 locus named fragile X mental retardation gene (FMR‑1). The FMR‑1 protein is widely expressed, with the highest expression in brain, testes, ovaries, esophagus, thymus, eye and spleen. Patient and Methods: This study was conducted on twenty mentally retarded
boys aged 8.5 ± 3.84 years, attending the genetic clinics at Menoufiya University
hospitals. They represented 11 families. All patients were subjected to detailed history, family pedigree, anthropometric measurements, thorough clinical examination with clinical scoring for the 13 items fragile X checklist, IQ assessment, routine investigations and cytogenetic studies which included conventional karyotyping using G banding and cytogenetic analysis for fragile X detection. Positive consanguineous marriage was found in 15% of our studied cases. Nine families out of total eleven families had positive family history most of them were second degree relative males through maternal cousins.
Results: Craniofacial abnormalities included high arched palate in 65% of patients,
large ears in 55%, prominent forehead in 45% and elongated face and abnormal teeth in 30% for each. Speech problems were present in 75% and hyperactivity in 55% of patients. Sixty five percent had mild mental retardation (IQ= 50‑70%). By applying the clinical scoring fragile X checklist, it was found that 3 patients (15%) had score more or equal to 19 and 3 (15%) had score from 16 to less than 19, while 14 (70%) had score less than 16.
As regards cytogenetic studies, 80% of our patients had normal karyotyping (46 XY) while four cases (20%) had positive fragile site on X‑chromosome of whom two cases from the same family had 46, Y, Frg (X) (q27.3), while the other two cases, also from a single family, had inversion of Y chromosome beside positive fragile X chromosome site 46, Fra(X) (q27.3), inv (Y). Conclusion: So, in a child with isolated mental retardation or autism of unknown
etiology with considerable fragile X dysmorphic features or established family history of fragile X syndrome, chromosomal study that identifies the fragile site at Xq27.3 in addition to other cytogenetic abnormalities could be useful or early diagnosis and intervention by a special services team. The present study revealed that the role of cytogenetic analysis in the diagnosis must be reevaluated since it can determine chromosomal abnormalities including the fragile X site with one single test, especially with unavailability of molecular techniques and their high costs. A national multicenter genetic study of fragile X syndrome among affected children and their families is recommended to define our indications and steps of early diagnosis, population screening strategy, genetic counseling guidelines for different phenotypes and early intervention policies.
Keywords: Fragile – X, cytogenetics, X linked mental retardation.
Egyptian Journal of Medical Human Genetics Vol. 9 (2) 2008: pp. 237-248