Fragile X syndrome FMR1 CGG retractions; Not so rare?

David I Francis1, Matthew Hunter2, Yael Prawer2, Zornitza Stark1, Robin Forbes1, David Amor 1,Mike Field3, Carolyn Rogers3, Ralph Oertel1, Sara Cronin1, Amber Boys1, Olivia Giouzeppos1, Essra Bartlett1, David Godler4

1 Victorian Clinical Genetics Service, Murdoch Childrens Research Institute, Flemington Road, Parkville, VIC, 3052.
2 Genetics Dept., Monash Health, Monash Medical Centre, 246 Clayton Rd, Clayton VIC 3168.
3 GOLD Service, PO Box 84, Waratah NSW 2298
4 Murdoch Childrens Research Institute, Flemington Road, Parkville, VIC, 3052.

FMR1 premutation (PM: 55-199 CCGs) alleles are highly unstable upon maternal transmission expanding at high frequency into the full mutation (FM: = or >200 CGGs) range. However there is increasing evidence that FM’s are highly unstable postzygotically, retracting into smaller size alleles, including the intermediate (45-54 CGGs) and normal (<45 CGGs) range. This has important implications for counselling, as mosaic individuals are less severely affected due to expression of FMR1; thought to be silent in non-mosaic fully methylated FM alleles. I present conclusive evidence in monozygous Fragile X twins of postzygotic retraction as well as three other FXS cases demonstrating complete retraction into the normal range. Further evidence of instability is confirmed by analysis of all VCGS male Southern Blot detected positive cases showing a high incidence of PM/FM mosaics of 25%, comparable to the literature which is variable (average of 26%).  This is likely to be an under representation of mosaicism as Southern blot cannot detect mosaic alleles if present in less than 20% of cells (Aliaga et al 2016 Clin Chem). Further studies of 100 males with typical FXS (manuscript in preparation) showed a significantly higher proportion of mosaic FMR1 alleles was detected in FM males using a more sensitive test, termed MS-QMA, than by Southern blot. Together, this suggests that mosaicism is currently under diagnosed by standard testing and retraction from FM to smaller size alleles is not as uncommon as previously thought.


Over 20 years of experience in the Cytogenetics/Molecular diagnostic field. I have published widely in the cyto/molecular area and currently involved in Fragile X syndrome and Prader Willi syndrome research projects. Recently appointed as the Head of the Postnatal Cytogenetics Laboratory at VCGS and was the past/last President of ASoC.

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