Molecular Inconsistencies in a Fragile X Male with Early Onset Ataxia

Abstract

Mosaicism for FMR1 premutation (PM: 55–199 CGG)/full mutation (FM: >200 CGG) alleles or the presence of unmethylated FM (UFM) have been associated with a less severe fragile X syndrome (FXS) phenotype and fragile X associated tremor/ataxia syndrome (FXTAS)—a late onset neurodegenerative disorder. We describe a 38 year old male carrying a 100% methylated FM detected with Southern blot (SB), which is consistent with complete silencing of FMR1 and a diagnosis of fragile X syndrome. However, his formal cognitive scores were not at the most severe end of the FXS phenotype and he displayed tremor and ataxic gait. With the association of UFM with FXTAS, we speculated that his ataxia might be related to an undetected proportion of UFM alleles. Such UFM alleles were confirmed by more sensitive PCR based methylation testing showing FM methylation between 60% and 70% in blood, buccal, and saliva samples and real-time PCR analysis showing incomplete silencing of FMR1. While he did not meet diagnostic criteria for FXTAS based on MRI findings, the underlying cause of his ataxia may be related to UFM alleles not detected by SB, and follow-up clinical and molecular assessment are justified if his symptoms worsen.

The Contribution of Pluripotent Stem Cell (PSC)-Based Models to the Study of Fragile X Syndrome (FXS)

“…In addition, it may suggest that the expansion is not evenly methylated or is not long enough in all cells of the preimplantation embryo. This is consistent with the realization that FMRP loss-of-function may not be the sole mechanism contributing to the clinical phenotype in FXS patients, because mosaicism for allele size and methylation within affected tissues [63] may lead to typical features of FXTAS by toxic gain-of-function mechanisms [57]. An alternative explanation for the extent of variability in FMR1 methylation among the different FXS hESC lines has to do with the pluripotency state of hESCs, which are heterogeneous in that some cells may reflect a less primitive ground state of pluripotency (primed) as compared to the inner cell mass (ICM) cells in the embryo (naïve) [42,64].…”

“…One limitation of the iPSC system which is also relevant to PHCFs, is that they are derived by clonal selection. As a result, single clones do not accurately reflect the composition of all alleles within individuals, particularly if the primary cells were derived from subjects who are mosaic for methylation or repeat size [57,58].…”

Relationships between UBE3A and SNORD116 expression and features of autism in chromosome 15 imprinting disorders

“…The molecular classes for each syndrome are presented in the supplementary material (Table S1 ). Samples from 20 de-identified controls, aged 8–52 years, from our earlier studies 21 , 22 were included as reference data for gene expression comparisons.…”

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

“…Assessed using Matrix-Assisted Laser Desorption/Ionisation-Time-Of-Flight Mass Spectrometry (MALDI-TOF MS) and Methylation Sensitive-Quantitative Melt Analysis (MS-QMA) [13]. Note: the error bars for reference ranges represent 1 standard deviation from the mean FREE2 methylation in blood of male controls (CGG < 44), PM males (56–170 CGGs), FM males with typical FXS (213–2000 CGGs) and 4 atypical ‘high functioning’ FM males unmethylated by Southern blot (UFM) (CGG 200–637 CGG) assessed as part of previous studies [13,21,22]. The reference samples were co-run with T1 and T2 DNA extracted from retrospectively retrieved newborn blood spots.…”