Clinical Evidence for Variegated Silencing in Patients With Friedreich Ataxia

Abstract

Background and ObjectivesFriedreich ataxia (FRDA) is a neurodegenerative disease caused by a GAA triplet repeat (GAA-TR) expansion in intron 1 of the FXN gene. Patients have 100–1,300 GAA triplets compared with less than 30 in healthy controls. The GAA-TR expansion leads to FXN silencing, and consequent frataxin protein deficiency results in progressive ataxia, scoliosis, cardiomyopathy, and diabetes. The overt heterogeneity in age at onset and disease severity is explained partly by the length of the GAA-TR, in which shorter repeats correlate with milder disease. Evidence of variegated silencing in FRDA suggests that patients with shorter repeats retain a significant proportion of cells with FXN genes that have escaped GAA-TR expansion–induced silencing, explaining the less severe frataxin deficiency in this subpopulation. In ex vivo experiments, the proportion of spared cells negatively correlates with GAA-TR length until it plateaus at 500 triplets, an indication that the maximal number of silenced cells has been reached. In this study, we assessed whether an analogous ceiling effect occurs in severity of clinical features of FRDA by analyzing clinical outcome data.MethodsThe FRDA Clinical Outcome Measures Study database was used for a cross-sectional analysis of 1,000 patients with FRDA. Frataxin levels were determined by lateral flow immunoassays.ResultsThe length of the GAA-TR in our cohort predicted frataxin level (R2 = 0.38, p < 0.0001) and age at onset (R2 = 0.46, p < 0.0001) but only with GAA-TRs with ≤700 triplets. Age and disease duration predicted performance on clinical outcome measures, and such predictions in linear regression models statistically improved in the subcohort of patients with >700 GAA triplets. The prevalence of cardiomyopathy and scoliosis increased as GAA-TR length increased up to 700 GAA triplets where prevalence plateaued.DiscussionOur data suggest that there is a ceiling effect on the clinical consequences of GAA-TR length in FRDA, as would be predicted by variegated silencing. Patients with GAA-TRs of >700 triplets represent a subgroup in which the severity of clinical manifestations based on GAA-TR length have reached maximal levels and therefore display limited clinical variability in disease progression.

Liquid Chromatography–Mass Spectrometry Analysis of Frataxin Proteoforms in Whole Blood as Biomarkers of the Genetic Disease Friedreich’s Ataxia

“…FRDA disease severity is predicted by the number of GAA repeats in intron-1 of the FXN gene with more repeats leading to more severe disease and earlier onset. , There was a linear relationship between the two proteoforms and total frataxin with GAA repeats (Figure S5) and with the age of onset (Figure S4). Intriguingly, the correlations with the GAA repeat length and age of onset were higher for extra-mitochondrial frataxin-E than those for mitochondrial frataxin-M. Frataxin-E has thus far only been found in cells lacking mitochondria, whereas frataxin-M is only found in cells with mitochondria.…”

Direct utility of natural history data in analysis of clinical trials: Propensity match-based analysis of Omaveloxolone in Friedreich ataxia using the FA-COMS dataset

“…Covariates used for determining the propensity scores were well-balanced between the groups ( Table 3 ) as were other demographics and baseline characteristics ( Table 4 ). Slight differences observed in GAA1 and GAA2 (the longer of the two FXN GAA intron 1 repeats) repeat length were not clinically meaningful based on ceiling effects of the GAA1 length [26–28]. Demographics and baseline characteristics for the Primary Placebo-Omav and Primary Omav-Omav populations were generally well-balanced between the groups in both populations, although differences as in the Primary Pooled population in GAA1 and GAA2 repeat length were observed ( Supplementary Tables 1,2 respectively).…”

“…These were not included in the model due to lack of availability of GAA1 in all patients and differences in the method of evaluation of pes cavus between studies. However, baseline characteristics for mean GAA1 repeat length in both FA-COMS and MOXIe Extension patients were in a range in which the consequence of longer GAA1 repeat length on disease severity has reached a relative ceiling effect [25][26][27].…”

Retinal hypoplasia and degeneration result in vision loss in Friedreich ataxia

“…The modified Friedreich Ataxia Rating Scale (mFARS) and FA Functional Disease Stage score provide two validated metrics of neurological severity and can detect temporal progression of the disease. mFARS correlates with both GAA1 length and disease duration but is best predicted by the product of the two, called disease burden 16–22 . This measure predicts many features of FRDA better than its two components alone 22 .…”

“…mFARS correlates with both GAA1 length and disease duration but is best predicted by the product of the two, called disease burden. [16][17][18][19][20][21][22] This measure predicts many features of FRDA better than its two components alone. 22 Similarly, in linear regression models, outcome measures that capture progression are predicted by both disease duration and GAA repeat length, such that those with longer GAA1 lengths and longer disease duration have more severe disease including visual acuity deficits in some cases.…”