Maternal FMR1 premutation allele expansion and contraction in fraternal twins

Reversion to Normal of FMR1 Expanded Alleles: A Rare Event in Two Independent Fragile X Syndrome Families

Fragile X syndrome (FXS) is mostly due to the expansion and subsequent methylation of a polymorphic CGG repeat in the 5’ UTR of the FMR1 gene. Full mutation alleles (FM) have more than 200 repeats and result in FMR1 gene silencing and FXS. FMs arise from maternal premutations (PM) that have 56–200 CGGs; contractions of a maternal PM or FM are rare. Here, we describe two unaffected boys in two independent FXS families who inherited a non-mosaic allele in the normal and intermediate range, respectively, from their mothers who are carriers of an expanded CGG allele. The first boy inherited a 51 CGG allele (without AGG interruptions) from his mother, who carries a PM allele with 72 CGGs. The other boy inherited from his FM mother an unusual allele with 19 CGGs resulting from a deletion, removing 85 bp upstream of the CGG repeat. Given that transcription of the deleted allele was found to be preserved, we assume that the binding sites for FMR1 transcription factors are excluded from the deletion. Such unusual cases resulting in non-mosaic reduction of maternal CGG expansions may help to clarify the molecular mechanisms underlying the instability of the FMR1 gene.

Clinical and Molecular Differences between 4-Year-Old Monozygous Male Twins Mosaic for Normal, Premutation and Fragile X Full Mutation Alleles

This study describes monozygotic (MZ) male twins with fragile X syndrome (FXS), mosaic for normal size (NS: <44 CGGs), premutation (PM: 55–199 CGG) and full mutation (FM alleles ≥ 200) alleles, with autism. At 4 years of age chromosomal microarray confirmed monozygosity with both twins showing an XY sex complement. Normal size (30 CGG), PM (99 CGG) and FM (388–1632 CGGs) alleles were detected in Twin 1 (T1) by standard polymerase chain reaction (PCR) and Southern blot testing, while only PM (99 CGG) and FM (672–1025) alleles were identified in Twin 2 (T2). At ~5 years, T2 had greater intellectual impairments with a full scale IQ (FSIQ) of 55 and verbal IQ (VIQ) of 59, compared to FSIQ of 62 and VIQ of 78 for T1. This was consistent with the quantitative FMR1 methylation testing, revealing 10% higher methylation at 80% for T2; suggesting that less active unmethylated alleles were present in T2 as compared to T1. AmplideX methylation PCR also identified partial methylation, including an unmethylated NS allele in T2, undetected by standard testing. In conclusion, this report demonstrates significant differences in intellectual functioning between the MZ twins mosaic for NS, PM and FM alleles with partial FMR1 promoter methylation.

Prenatal Diagnosis of Fragile X Syndrome in a Twin Pregnancy Complicated by a Complete Retraction

Fragile X syndrome (FXS) is usually associated with a CGG repeat expansion >200 repeats within the FMR1 gene, known as a full mutation (FM). FM alleles produce abnormal methylation of the FMR1 promoter with reduction or silencing of FMR1 gene expression. Furthermore, premutation (PM: 55–199 CGGs) and full mutation alleles usually expand in size when maternally transmitted to progeny. This study describes a PM allele carried by the mother decreasing to a normal sized allele in a male from a dichorionic diamniotic (DCDA) twin pregnancy, with the female twin inheriting FM (200–790 CGGs), PM (130 CGGs) and normal-sized (39 CGGs) alleles. Further evidence of instability of the maternal PM allele was shown by a male proband (older brother) mosaic for PM (CGG 78 and 150 CGGs) and FM (200–813 CGGs), and a high level of FMR1 promoter methylation, between 50 and 70%, in multiple tissues. The fully-retracted, normal-sized allele was identified by PCR CGG sizing in the male twin, with no evidence of a FM allele identified using Southern blot analysis in multiple tissues collected postnatally and prenatally. Consistent with this, prenatal PCR sizing (35 CGGs) showed inconsistent inheritance of the maternal normal allele (30 CGGs), with single-nucleotide polymorphism (SNP) linkage analysis confirming that the abnormal FMR1 chromosome had been inherited from the mother’s PM chromosome. Importantly, the male twin showed no significant hypermethylation of the FMR1 promoter in all pre and postnatal tissues tested, as well as normal levels of FMR1 mRNA in blood. In summary, this report demonstrates the first postnatal follow up of a prenatal case in which FMR1 mRNA levels were approaching normal, with normal levels of FMR1 promoter methylation and normal CGG size in multiple pre and postnatally collected tissues.