Autor(es):
Ali, Javed 
; Ali, G. ; Caicedo, Lina ; Marques, Isabel ; Santos, Rosário ; Jorge, Paula
Data: 2012
Identificador Persistente: http://hdl.handle.net/10400.18/1241
Origem: Repositório Científico do Instituto Nacional de Saúde
Assunto(s): Doenças Genéticas; FRAXE
Descrição
FRAXE mental retardation is a form of mild to moderate intellectual disability generally associated with learning difficulties, communication deficits, attention problems, hyperactivity and autistic behavior. FRAXE (AFF2/ FMR2 gene) a folate-sensitive fragile site in Xq28 ~600 kb distal to the FRAXA (FMR1 gene) site is the most common form of inherited mental retardation. Molecular characterization revealed that individuals expressing FRAXE had amplifications of a CCG repeat adjacent to a CpG island. Normal individuals showed 4–39 copies of the polymorphic FRAXE CCG repeat, while individuals expressing the fragile site had >200 copies and their CpG island was fully methylated. These findings are similar to those found for folate-sensitive fragile X site FRAXA. Reports of FRAXE full expansions and pre-mutations are rarely documented. In this respect, it has been very difficult to determine to what extent the alleles, with CCG repeats in the range of 36 and 199, have a pathogenic effect. Intellectually disabled individuals are primarily referred for FRAXA screening and individuals who are negative for FRAXA are possible candidates for FRAXE screening. Traditionally in some laboratories AFF2 molecular analysis is performed by PCR, it is known that CCG repeats in the range of ~80 and above are not reliably amplified. We embarked on an effort to supplement our PCR analysis by Southern blot and cloned a segment of the AFF2 gene that can be used by appropriate labeling as a probe to determine expansion of the CCG repeats in the AFF2 gene. We have developed a probe to be used for Southern blot analysis that reliably detects the AFF2 CCG triple repeat amplification. We present data of AFF2 molecular analysis in a subpopulation of 5,000 individuals referred for FRAXA screening. The presence of pre-mutated and fully expanded alleles in either gender, were confirmed by Southern blot analysis, which also enabled exclusion of methylation or repeat number mosaics as well as PCR failure. We recommend the use of this probe as suitable for genotyping of pre-mutations, full mutations, and mosaics specifically for individuals presented for FRAXA screening with negative results to determine FRAXE status.
