Identification of small FRAXA premutations.
BACKGROUND
Diagnosis of fragile X syndrome in mentally retarded individuals is satisfactorily achieved using a Southern blot test that detects the typical triplet repeat expansion (>200 repeats) within the FMR1 gene. All such individuals inherit the mutation from a carrier, who usually shows a lower triplet repeat number and may be asymptomatic. Having identified a fragile X proband, it is necessary to identify related carriers of this familial X-linked dominant mutation to provide family counseling and testing.


RESULTS
For one family in which a fragile XA repeat expansion occurs, Southern blot hybridization did not give accurate sizing data because of the very small premutation associated with the unstable allele. PCR sizing methods and linkage analysis were adapted to identify family members with the premutation.


CONCLUSIONS
Although most carriers can be detected using Southern blot and/or direct PCR sizing tests, very small expansions (55-70 repeats) are difficult to distinguish from larger, normal alleles. We have used linkage analysis in combination with direct allele analysis to identify carriers of very small expansions of a fragile X chromosome in a four-generation family.
Francis
D
D
Cytogenetics Laboratory, Victorian Clinical Genetics Services, Royal Children's Hospital, Parkville, Victoria, Australia. francisd@cryptic.rch.unimelb.edu.au
Burgess
T
T
Mitchell
J
J
Slater
H
H
D015139:Blotting, Southern; D002675:Child, Preschool; D004252:DNA Mutational Analysis; D051860:Fragile X Messenger Ribonucleoprotein 1; D005600:Fragile X Syndrome / Q000175:diagnosis* / Q000235:genetics; D008040:Genetic Linkage; D006579:Heterozygote; D006801:Humans; D008297:Male; D009419:Nerve Tissue Proteins / Q000235:genetics*; D010375:Pedigree; D016133:Polymerase Chain Reaction; D016601:RNA-Binding Proteins; D019680:Trinucleotide Repeat Expansion
D051860:Fragile X Messenger Ribonucleoprotein 1; D009419:Nerve Tissue Proteins; D016601:RNA-Binding Proteins; C497683:FMR1 protein, human