Fragile X full mutation expansions are inhibited by one or more AGG interruptions in premutation carriers

Fragile X CGG repeat alleles often contain one or more AGG interruptions that influence allele stability and risk of a full mutation transmission from parent to child. We have examined transmissions of maternal and paternal alleles with 45-90 repeats to quantify the effect of AGG interruptions on fragile X repeat instability.

A novel FMR1 polymerase chain reaction assay was used to determine CGG repeat length and AGG interruptions for 1,040 alleles from 705 families.

We grouped transmissions into nine categories of five repeats by parental size and found that in every size category, alleles with no AGGs had the greatest risk for instability. For maternal alleles <75 repeats, 89% (24/27) that expanded to a full mutation had no 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).

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.