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© 1994 Oxford University Press

RESEARCH-ARTICLE

The cloning of FRAXF: trinucleotide repeat expansion and methylation at a third fragile site in distal Xqter

Rachael J. Ritchie, Samantha J.L. Knight, Mark C. Hirst, Prabhjit K. Grewal, Martin Bobrow1, Gareth S. Cross2 and Kay E. Davies*

Molecular Genetics Group, Institute of Molecular Medicine, John Radciffe Hospital Headington, Oxford OX3 9DU 1Division of Medical and Molecular Genetics, United Medical and Dental School, Guy's Tower, London Bridge London SE1 9RT 2Department of Molecular Genetics, Centre for Medical Genetics, City Hospital Hucknall Road, Nottingham NG5 1PB, UK

*To whom correspondence should be addressed

Received October 10, 1994; Accepted October 13, 1994

Three fragile sites, FRAXA, FRAXE and FRAXF lie in the Xq27–28 region of the human X chromosome. The expression of FRAXA is associated with the fragile X syndrome, the most prevalent form of Inherited mental retardation whilst the expression of FRAXE Is associated with a rarer and comparatively milder form of mental handicap. Both the FRAXA and FRAXE sites have been cloned and the fragile site expression found to be due to the expansion of analogous CGG/GCC trinucleotide repeat arrays. We describe here the cloning of the third fragile site, FRAXF, and demonstrate that it Involves the expansion of a (GCCGTC)n(GCC)n compound array. PCR analyses across the repeat of normal individuals show that the number of triplets in the array ranges from 12–26 and the most common allele consists of 14 triplet units. Sequencing analyses show that 95% of normal individuals have three copies of the GCCGTC motif and In these individuals, the size variation observed by PCR is due to copy number alterations in the GCC array. In a cytogenetically positive male with developmental delay, the array is expanded by >900 triplets and the adjacent CpG-rich region is methylated. The array is also expanded In cytogenetically positive carrier females from the family originally used to define the FRAXF site. We conclude that the expanded array corresponds to the FRAXF fragile site.


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