Screening of the Fragile X Syndromes with DNA and Cytogenetic Analysis in Populations with Learning Difficulties and Mental Retardation

Abstract

Fragile X syndrome is the most common inherited cause of mental retardation. 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 locus designated FMR1. This study was conducted on a total of 154 subjects, 82 of them are from fragile X suspected group, others from normal control group. The suspected group was selected from the special "subclasses" for students with learning difficulties, of primary schools under the auspice of the National Ministry of Education (of the Republic of Türkiye) in Samsun, and from the centers for special training, education and rehabilitation of mentally retarded individuals. In this study, DNA and cytogenetics analyses were used to screen individuals for CGG amplifications. For DNA analysis, DNAs were extracted from peripheral blood samples; normal and small premutation (carrier) individuals were identified by PCR amplification. The carriers (premutation) and affected (full mutation) were identified by Southern blot and non-radioactive hybridization. According to the PCR results, CGG repeat numbers of the fragile X suspected population ranged between 7 to 60; 94%alleles having fewer than 40 repeats. The most frequent allele (15%) had 31 repeats. Also, there was a male child with a premutation allele (60 repeats). In control group, the CGG alleles ranged from 9 to 55 repeats. The most frequent allele (15%) had 30 repeats. Also, in this group included was a male with a premutation allele (55 repeats). A male child with a full mutation was identified in the fragile X suspected group by Southern blot and non-radioactive hybridization. Both of the proband's two brothers and one of his two sisters had full mutations: his mother and his other sister had premutations. In cytogenetic analysis, the male child with full mutation was found fragile X positive (12%). In the fragile X suspected group, another male child and his mother were found to carry pericentric inversions on one of their chromosomes 9s. The incidence of the fragile X syndrome, which is an important public health concern, in our suspected study group was found to be approximately 1%, as determined by this study carried out in Samsun.