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Opinion

Fragile X Syndrome (FXS) is the most common inherited form of intellectual disability, caused by a dynamic mutation in the FMR1 gene on chromosome Xq27.3. The disease is associated with the expansion of CGG trinucleotide repeats in the 5’ UTR: normal alleles have up to 54 repeats, premutation alleles range from 55 to 200, and full mutations exceed 200.

The expansion of premutation alleles during maternal transmission is well documented. Yrigollen et al. [1,2] showed that the risk of expansion increases steeply with the number of repeats, approaching 100% beyond 99. A striking case by Fernández-Carvajal et al. [3] reported a 56-repeat premutation in a mother expanding to 538 repeats in her son within a single generation.

By contrast, contraction events — in which CGG repeats are reduced from full or premutation to normal range — have received limited attention. In a prenatal diagnosis study, Manor et al. [4] identified a fetus who inherited a 19-repeat allele from a mother with a mosaic full mutation (>200 repeats), suggesting a reversion to normal range via contraction.

Repeat instability is modulated by AGG interruptions within the CGG tract. These interruptions enhance repeat stability and reduce expansion risk but their role in contraction remains unclear [5,6]. It is plausible that specific repeat-AGG configurations may predispose to contractions under certain conditions, yet this hypothesis remains largely unexplored.

The rarity of reported contractions may reflect not only true biological infrequency, but also a profound diagnostic bias. Current genetic testing protocols primarily target symptomatic individuals or those with a family history of FXS. Silent contractions that revert the allele to normal range are clinically invisible and unlikely to trigger testing. As a result, these events may go undetected, contributing to a skewed perception of repeat dynamics [7,8].

To address this knowledge gap, we propose systematic investigations of FMR1 transmission across extended pedigrees — including asymptomatic individuals — and the use of advanced technologies such as long-read sequencing, AGG profiling, and methylation-sensitive assays. These tools could identify rare contraction events and clarify their frequency and molecular drivers [9,10].

Furthermore, the epigenetic implications of such contractions deserve attention. Could contraction restore a normal methylation profile and prevent silencing of FMR1? If so, contraction might not only normalize repeat size, but also gene function, a possibility that opens new avenues in understanding FMR1 gene regulation.

Ultimately, a deeper exploration of contraction events may improve genetic counseling for families with FMR1 premutations or mosaicism and reshape our understanding of dynamic repeat disorders. What we perceive as rare might simply be unobserved.

References

1. Yrigollen CM, Durbin-Johnson B, Gane L, Nelson DL, Hagerman R, et al. (2012) AGG interruptions within the maternal FMR1 gene reduce the risk of offspring with fragile X syndrome. Genet Med 14(8): 729-736.
2. Yrigollen CM, Martorell L, Durbin-Johnson B, Naudo M, Genoves J, et al. (2014) AGG interruptions and maternal age affect FMR1 CGG repeat allele stability during transmission. J Neurodev Disord 6(1): 24.
3. Fernandez-Carvajal I, Lopez Posadas B, Pan R, Raske C, Hagerman PJ, et al. (2009) Expansion of an FMR1 grey-zone allele to a full mutation in two generations. Am J Med Genet A 149A(10): 2106-2110.
4. Manor E, Jabareen A, Magal N, Kofman A, Hagerman RJ, et al. (2017) Prenatal diagnosis of fragile X: Can a full mutation allele in the FMR1 gene contract to a normal size? Clin Genet 91(4): 578-581.
5. Eichler EE, Holden JJ, Popovich BW, Reiss AL, Snow K, et al. (1994) Length of uninterrupted CGG repeats determines instability in the FMR1 gene. Nat Genet 8(1): 88-94.
6. Kunst CB, Warren ST (1994) Cryptic and polar variation of the fragile X repeat could result in predisposing normal alleles. Cell 77(6): 853-861.
7. Oberlé I, Rousseau F, Heitz D, Kretz C, Devys D, et al. (1991) Instability of a 550-base pair DNA segment and abnormal methylation in fragile X syndrome. Science 252(5009): 1097-1102.
8. Pieretti M, Zhang FP, Fu YH, Warren ST, Oostra BA, et al. (1991) Absence of expression of the FMR-1 gene in fragile X syndrome. Cell 66(4): 817-822.
9. Verkerk AJ, Pieretti M, Sutcliffe JS, Fu YH, Kuhl DP, et al. (1991) Identification of a gene (FMR-1) containing a CGG repeat coincident with a breakpoint cluster region exhibiting length variation in fragile X syndrome. Cell 65(5): 905-914.
10. Zhong N, Yang W, Dobkin C, Brown WT (1995) Fragile X gene instability: anchoring AGGs and linked microsatellites. Am J Hum Genet 57(2): 351-361.