The premutation of the fragile X messenger ribonucleoprotein 1 (FMR1) gene is characterized by an expansion of the CGG trinucleotide repeats (55 to 200 CGGs) in the 5’ untranslated region and increased levels of FMR1 mRNA. Molecular mechanisms leading to fragile X-premutation-associated conditions (FXPAC) include cotranscriptional R-loop formations, FMR1 mRNA toxicity through both RNA gelation into nuclear foci and sequestration of various CGG-repeat-binding proteins, and the repeat-associated non-AUG (RAN)-initiated translation of potentially toxic proteins. Such molecular mechanisms contribute to subsequent consequences, including mitochondrial dysfunction and neuronal death. Clinically, premutation carriers may exhibit a wide range of symptoms and phenotypes. Any of the problems associated with the premutation can appropriately be called FXPAC. Fragile X-associated tremor/ataxia syndrome (FXTAS), fragile X-associated primary ovarian insufficiency (FXPOI), and fragile X-associated neuropsychiatric disorders (FXAND) can fall under FXPAC. Understanding the molecular and clinical aspects of the premutation of the FMR1 gene is crucial for the accurate diagnosis, genetic counseling, and appropriate management of affected individuals and families. This paper summarizes all the known problems associated with the premutation and documents the presentations and discussions that occurred at the International Premutation Conference, which took place in New Zealand in 2023.

Approximately 20-30% of women with an FMR1 premutation experience fragile X-associated primary ovarian insufficiency (FXPOI); however, current risk estimates based on repeat size only identify women with the midrange of repeats to be at the highest risk. To better understand the risk by repeat size, we collected self-reported reproductive histories on 1,668 women and divided them into high-resolution repeat size bins of ~5 CGG repeats to determine a more accurate risk for FXPOI in relation to CGG repeat length. As previously reported, women with 70-100 CGG repeats were at the highest risk for FXPOI using various statistical models to compare average age at menopause and risk of FXPOI, with women with 85-89 repeats being at the highest risk. Importantly, women with 120 repeats did not have a significantly increased risk for FXPOI compared to women with <45 repeats. Using a large cross-section study on 1,668 women, we have provided more personalized risk assessment for FXPOI using high-resolution repeat size bins. Understanding the variability in risk has important implications for family planning and overall health among women with a premutation.

GGC repeat expansions within NOTCH2NLC have been identified as the genetic cause of neuronal intranuclear inclusion disease (NIID). To understand the molecular pathogenesis of NIID, here, we established both a transgenic mouse model and a human neural progenitor cells (hNPCs) model. Expression of the NOTCH2NLC with expanded GGC repeats produced widespread intranuclear and perinuclear polyglycine (polyG), polyalanine (polyA), and polyarginine (polyR) inclusions, leading to behavioral deficits and severe neurodegeneration, which faithfully mimicked the clinical and pathological features associated with NIID. Furthermore, conserved alternative splicing events were identified between the NIID mouse and hNPC models, among which was the enrichment of the binding motifs of hnRNPM, an RNA binding protein known as alternative splicing regulator. Expanded NOTCH2NLC-polyG and NOTCH2NLC-polyA could interact with and sequester hnRNPM, while overexpression of hnRNPM could ameliorate the cellular toxicity. These results together suggested that dysfunction of hnRNPM could play an important role in the molecular pathogenesis of NIID.

Purpose Emerging evidence indicates that women who carry an FMR1 premutation can experience complex health profiles beyond the two well-established premutation-associated disorders: fragile X–associated primary ovarian insufficiency (FXPOI, affects ~20–30% carriers) and fragile X–associated tremor–ataxia syndrome (FXTAS, affects ~6–15% carriers). Methods To better understand premutation-associated health profiles, we collected self-reported medical histories on 355 carrier women. Results Twenty-two health conditions were reported by at least 10% of women. Anxiety, depression, and headaches were reported by more than 30%. The number of comorbid conditions was significantly associated with body mass index (BMI) and history of smoking, but not age. Survival analysis indicated that women with FXPOI had an earlier age at onset for anxiety and osteoporosis than women without FXPOI. Cluster analysis identified eight clusters of women who reported similar patterns of comorbid conditions. The majority of carriers (63%) fell into three categories primarily defined by the presence of only a few conditions. Interestingly, a single cluster defined women with symptoms of FXTAS, and none of these women had FXPOI. Conclusion Although some women with a premutation experience complex health outcomes, most carriers report only minimal comorbid conditions. Further, women with symptoms of FXTAS appear to be distinct from women with symptoms of FXPOI.

Major depression disorder is one of the most common psychiatric diseases. Recent evidence supports that environmental stress affects gene expression and promotes the pathological process of depression through epigenetic mechanisms. Three ten-eleven translocation (Tet) enzymes are epigenetic regulators of gene expression that promote 5-hydroxymethylcytosine (5hmC) modification of genes. Here, we show that the loss of Tet2 can induce depression-like phenotypes in mice. Paradoxically, using the paradigms of chronic stress, such as chronic mild stress and chronic social defeat stress, we found that depressive behaviors were associated with increased Tet2 expression but decreased global 5hmC level in hippocampus. We examined the genome-wide 5hmC profile in the hippocampus of Tet2 knockout mice and identified 651 dynamically hydroxymethylated regions, some of which overlapped with known depression-associated loci. We further showed that chronic stress could induce the abnormal nuclear translocation of Tet2 protein from cytosol. Through Tet2 immunoprecipitation and mass spectrum analyses, we identified a cellular trafficking protein, Abelson helper integration site-1 (Ahi1), which could interact with Tet2 protein. Ahi1 knockout or knockdown caused the accumulation of Tet2 in cytosol. The reduction of Ahi1 protein under chronic stress explained the abnormal Ahi1-dependent nuclear translocation of Tet2. These findings together provide the evidence for a critical role of modulating Tet2 nuclear translocation in regulating stress response.