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Mol. Hum. Reprod. Advance Access published online on August 20, 2004
Molecular Human Reproduction, doi:10.1093/molehr/gah102
© 2004 by Oxford University Press
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Received June 1, 2004
Accepted July 24, 2004

Article

Analysis of CGG variation through 642 meioses in Fragile X families

M. Rifé 1, C. Badenas 2, Ll. Quintó 3, E. Puigoriol 3, B. Tazón 4, L. Rodriguez-Revenga 1, L. Jiménez 4, A. Sánchez 1, M. Milà 1*

1 Servei de Genètica, Centre de Diagnòstic Biomèdic, Hospital Clínic, Barcelona, Spain; IDIBAPS (Institut d'Investigacions Biomèdiques August Pi i Sunyer), Barcelona, Spain
2 Servei de Genètica, Centre de Diagnòstic Biomèdic, Hospital Clínic, Barcelona, Spain; Fundació Clínic per a la Recerca Biomèdica, Barcelona, Spain
3 Servei d'Epidemiologia, Hospital Clínic, Barcelona, Spain
4 Servei de Genètica, Centre de Diagnòstic Biomèdic, Hospital Clínic, Barcelona, Spain

* To whom correspondence should be addressed. E-mail: mmila{at}clinic.ub.es.


  Abstract

Fragile X syndrome is the commonest familial form of inherited mental retardation. The molecular defect is an expansion of the CGG trinucleotide repeats in the 5' untranslated region of the FMR1 gene that is inherited in an unstable fashion in fragile X families. In an attempt to provide more information about the CGG tract intergenerational variation, we have evaluated 642 transmissions in 175 Fragile X families. PCR and Southern blot (StB12.3) was used to analyse the CGG number. Among premutated alleles, 90.2% showed expansion, two-thirds to a full mutation while the rest remained in the premutation range, 5.5% of alleles did not vary and finally 4.3% of them reduced in size. Premutated females showed an increased risk of expansion to the full mutation depending on the CGG tract. The estimated risk for 80 triplets is more than seven times that of a woman carrying 59 CGG, the risk being 100% for alleles of >100 repeats. Fifty-nine repeats was the smallest allele that expanded to full mutation. Contractions were detected more frequently in males than in females, being statistically significant. This study contributes to the literature by increasing the data available regarding transmissions in Fragile X families and it allows us to perform more precise genetic counselling for women with the CGG repeat in the premutation range.

Keywords: CGG transmission; contraction; expansion; FMR1; Fragile X syndrome.
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