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The original test for the analysis of the CCG expansion at the FRAXE locus involves Southern blot analysis of HindIII digests. We show that, by using a different probe, the FRAXE mutation can be detected easily on the same EcoRI or EagI+EcoRI blots as are used for detection of FRAXA. Unexpectedly, we found that both the expansion and methylation status can be determined on a single EcoRI digest, because of the presence of a methylation-sensitive EcoRI site very close to the CCG repeat. We thus detected in a series of mentally retarded individuals previously tested for FRAXA expansion a FRAXE proband who led to the identification of a large sibship (7 of 10 children carrying a mutation). We also show that two fragile X families without FRAXA mutation that previously have been described by Oberlé et al. have the FRAXE expansion. In another family also ascertained initially by cytogenetic finding of a fragile X site, we performed the combined cytogenetic and molecular prenatal diagnosis of a mutated male fetus. All nine males (>3 years old) in whom we found a methylated mutation had mild mental retardation. Our results suggest that the threshold of repeat length for abnormal methylation and fragile-site expression may be smaller at FRAXE than at FRAXA.

Type

Journal article

Journal

Am J Hum Genet

Publication Date

10/1996

Volume

59

Pages

847 - 854

Keywords

Adolescent, Adult, Blotting, Southern, Child, Child, Preschool, Chromosome Fragile Sites, Chromosome Fragility, Deoxyribonuclease EcoRI, Deoxyribonucleases, Type II Site-Specific, Female, Fragile X Mental Retardation Protein, Fragile X Syndrome, Humans, Infant, Infant, Newborn, Intellectual Disability, Male, Nerve Tissue Proteins, Nuclear Proteins, Pedigree, Proteins, RNA-Binding Proteins, Trans-Activators