We investigated the effect of AGG interruptions on fragile X repeat instability upon transmission of fragile X intermediate and small premutation alleles with 45–69 CGG repeats. The FMR1 repeat structure was determined for 375 mothers, 48 fathers, and 538 offspring (457 maternal and 81 paternal transmissions) using a novel PCR assay to determine repeat length and AGG interruptions. The number of AGG interruptions and the length of uninterrupted CGG repeats at the 3′ end were correlated with repeat instability on transmission. Maternal alleles with no AGGs conferred the greatest risk for unstable transmissions. All nine full mutation expansions were inherited from maternal alleles with no AGGs. Furthermore, the magnitude of repeat expansion was larger for alleles lacking AGG interruptions. Transmissions from paternal alleles with no AGGs also exhibited greater instability than those with one or more AGGs. Our results demonstrate that characterization of the AGG structure within the FMR1 repeat allows more accurate risk estimates of repeat instability and expansion to full mutations for intermediate and small premutation alleles.

AGGs. Two contractions in maternal transmission were accompanied by loss of AGGs, suggesting a mechanism for generating alleles that lack AGG interruptions. Maternal age was examined as a factor in full mutation expansions using prenatal samples to minimize ascertainment bias, and a possible effect was observed though it was not statistically significant (P = 0.06). Conclusion:These results strengthen the association of AGG repeats with CGG repeat stability and provide more accurate risk estimates of full mutation expansions for women with 45-90 repeat alleles.

Objective To determine risks of expansion for normal, intermediate, and premutation FMR1 CGG repeats.Methods PCR was used to compare the FMR1 alleles in prenatal (chorionic villi and amniocytes) and parental samples collected from 1991 to 2010. Prenatal diagnoses were confirmed by Southern analysis.Results Fragile X analysis of 1112 pregnancies identified 558 normal, 106 intermediate, 216 premutation, and 232 full mutation fetuses. Of 509 maternal, intermediate, and premutation alleles, 350 (68.7%) were unstable on transmission with expansions ranging from one repeat to the full mutation. The smallest premutation alleles expanding to the full mutation were in mothers with 65 and 66 repeats. Transmissions from women with or without a family history of fragile X suggested greater instability in women from families that included full mutation expansions. ConclusionsThe maternal transmissions of alleles with 55 to 59 CGG repeats summarized here indicate that the risk for expansion to full mutation is substantially less than previous estimates for this size category. Most premutation alleles with no family history of fragile X exhibited less instability than those with a history of fragile X. Thus, lower risk estimates for full mutation expansion may be appropriate for women newly identified as premutation carriers through routine screening.

An SNP upstream of the CGG repeats located at a replication initiation site may contribute to origin inactivation, to altered replication fork progression through the CGG repeats, and repeat expansion to fragile X full mutation.