Premutation and intermediate-size FMR1 alleles in 10 572 males from the general population: loss of an AGG interruption is a late event in the generation of fragile X syndrome alleles

Abstract

We previously reported a 1:259 prevalence of female carriers of FMR1 premutation-size alleles (greater than 54 triplet repeats) in the general population. We now have screened 10 572 independent males from the same population for similar alleles using high-throughput Southern blotting. We identified 13 male carriers of an allele with more than 54 repeats. This corresponds to a prevalence of 1:813 males (95% confidence interval 1:527 to 1:1781). Haplotype analysis of four markers flanking the triplet array revealed that the prevalence of the major fragile X mutation-associated haplotype was increased among FMR1 alleles of 40-54 repeats. Although sequencing of highly unstable premutation alleles from fragile X families revealed only pure CGG tracts, this was not the case for alleles of similar size that were identified in males from the general population. Forty-eight out of forty-nine alleles of 40 or more triplets had one or two AGG interruptions. This observation, combined with the observation of the enrichment of major fragile X syndrome haplotypes in all alleles of this size, is evidence that the loss of an AGG interruption in the triplet repeat array is not necessary for expansion of normal alleles of 29-30 triplets to intermediate size. The loss of AGG interruptions thus appears to be a late event that leads to greatly increased instability and may be related to the haplotype background of specific FMR1 alleles.

Screening for Expanded Alleles of the FMR1 Gene in Blood Spots from Newborn Males in a Spanish Population

“…Lanes 1 and 4, the two full-mutation alleles identified in this study; lanes 2 and 3, normal alleles identified in this study; lanes 5 and 6, normal and full mutation controls; lane 7, negative control. Canadian cohorts, 6,20 the value of 1/251 for premutation alleles is three times larger than the value of 1/813 previously reported in males by Dombrowski et al, 6 and is in close agreement with the expectation derived from the female Israeli cohorts. 8 The premutation alleles ranged in size from 55 to 77 CGG repeats; thus, all were toward the small end of the premutation range, again consistent with previous observations.…”

“…For example, in the screens of males and females in Eastern Canada, allele frequencies were estimated to be ϳ1/800 males and ϳ1/260 females. 4,6 However, in the Israeli studies, 3,7 the frequency of premutation alleles in women is closer to ϳ1/130. 7,9 By contrast, in an Asian (Taiwanese) population, the frequency of premutation alleles in males was reported to be much lower (ϳ1/1670).…”

“…1 However, there remains some uncertainty regarding the premutation allele frequencies 1,[3][4][5][6][7] due in part to lingering issues of ascertainment bias, 8 but also to real frequency differences across ethnic and regional populations. For example, in the screens of males and females in Eastern Canada, allele frequencies were estimated to be ϳ1/800 males and ϳ1/260 females.…”

Intermediate FMR1 alleles and cognitive and/or behavioural phenotypes

“…Data reported in the literature show that IA present similar proportions of unstable transmissions. 25,26 In our series, 23 sequenced alleles had two or more AGG interruptions and 8 had none or only one AGG interruption, 6 of which remain stable in more than two generations. Even though the two IA that expanded to full mutation had lost the AGG interruptions, all alleles lacking an AGG interruption do not generally expand in the next generation.…”

“…27 The lack of AGG interruptions, together with other unknown genetic factors, probably contributed to repeat instability in the two cases as has been studied by Dombrowski and Morel. 26 A frequent concern in FXS screening is the genetic counselling to IA carriers. Currently, the follow-up of individuals with no AGG interruptions is indicated, despite the low risk of expansion in the next generation.…”