FAN1-MLH1 interaction affects repair of DNA interstrand cross-links and slipped-CAG/CTG repeats

A FAN1 point mutation associated with accelerated Huntington's disease progression alters its PCNA-mediated assembly on DNA

FAN1 is an endo- and exo-nuclease involved in DNA and interstrand crosslink repair. Genome-wide association studies of people with Huntington's disease revealed a strong association between the FAN1 R507H mutation and early disease onset, however the underlying mechanism(s) remains unclear. FAN1 has previously been implicated in modulating triplet repeat expansion in a PCNA dependent manner. To examine the role of PCNA on FAN1 activation, we solved the cryo-EM structures of a PCNA-FAN1-DNA complex. Our findings reveal that the FAN1 R507 residue directly interacts with PCNA D232. Biophysical interaction studies demonstrated that FAN1 enhances the binding affinity of PCNA for DNA, a synergistic effect disrupted in mutants carrying the R507H mutation. In contrast, PCNA does not affect the affinity of FAN1 for DNA but does modulate FAN1 activity upon ternary complex formation. The weakened and functionally altered FAN1 R507H-PCNA-DNA complex may partly impair the FAN1-mediated repair of CAG extrahelical extrusions, providing a potential explanation for the mutation's role in accelerating disease progression.

Comprehensive genetic and epigenetic characterization of Lynch-like syndrome patients

Lynch-like syndrome (LLS) presents very similar clinicopathological characteristics to Lynch syndrome (LS) but the mechanism for cancer predisposition remains unknown. The present study aims to investigate the causal mechanism of LLS by a comprehensive genetic and epigenetic approach. Thirty-two LLS and 34 LS patients with colorectal cancer (CRC) fitting the Amsterdam and Bethesda criteria were included, along with 29 CRC sporadic patients, and analyzed for the presence of pathogenic variants in 94 genes associated with hereditary tumors. The cohorts were also characterized for the methylation profile and examined through a sample group analysis and a Stochastic Epigenetic Mutations (SEMs) analysis in comparison with 29 age-matched healthy controls. The multigene panel analysis revealed the presence of pathogenic variants in non-mismatch repair (MMR) genes and three variants classified as pathogenic/likely pathogenic possibly predisposing to LLS. The epigenetic analysis showed epivariations targeting genes associated with LS or DNA repair, most of them associated with the Fanconi Anemia pathway, which could explain the susceptibility to cancer. Our results highlight the need for using extended genetic and epigenetic analyses to understand the causal mechanism of LLS.

Therapeutic targeting of mismatch repair proteins in triplet repeat expansion diseases

Triplet repeat expansion diseases are a class of ∼20 inherited neurological disorders. Many of these diseases are debilitating, sometimes fatally so, and they have unfortunately proved difficult to treat. New compelling evidence shows that somatic repeat expansions in some diseases are essential to the pathogenic process, accelerating the age of onset and the rate of disease progression. Inhibiting somatic repeat expansions, therefore, provides a therapeutic opportunity to delay or block disease onset and/or slow progression. Several key aspects enhance the appeal of this therapeutic approach. First, the proteins responsible for promoting expansions are known from human genetics and model systems, obviating the need for lengthy target searches. They include the mismatch repair proteins MSH3, PMS1 and MLH3. Second, inhibiting or downregulating any of these three proteins is attractive due to their good safety profiles. Third, having three potential targets helps mitigate risk. Fourth, another protein, the nuclease FAN1, protects against expansions; in principle, increasing FAN1 activity could be therapeutic. Fifth, therapies aimed at inhibiting somatic repeat expansions could be used against several diseases that display this shared mechanistic feature, offering the opportunity for one treatment against multiple diseases. This review will address the underlying findings and the recent therapeutic advances in targeting MSH3, PMS1, MLH3 and FAN1 in triplet repeat expansion diseases.

CellFIE: Integrating Pathway Discovery With Pooled Profiling of Perturbations Uncovers Pathways of Huntington's Disease, Including Genetic Modifiers of Neuronal Development and Morphology

Genomic screens and GWAS are powerful tools for identifying disease-modifying genes, but it is often challenging to understand the pathways by which these genes function. Here, we take an integrated approach that combines network analysis and an imaging-based pooled genetic perturbation study to examine modifiers of Huntington's disease (HD). The computational analysis highlighted several genes in a subnetwork enriched for modifiers of neuronal development and morphology. To test the functional roles of these genes, we developed an experimental pipeline that allows pooled CRISPRi KD of 21 genes in human iPSC-derived neurons followed by optical analysis of genotypes, neuronal arborization, multiplexed pathway activity and morphological fingerprint readout. This approach recovered known genes involved in morphology and confirmed unexpected links from the network between several genetic modifiers of HD and morphology. Our approach overcomes challenges in pooled measurement of neuronal function and health and could be adapted for other phenotypes in HD and other neurological diseases.

Genomic and Transcriptomic Approaches Advance the Diagnosis and Prognosis of Neurodegenerative Diseases

Neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS), represent a growing societal challenge due to their irreversible progression and significant impact on patients, caregivers, and healthcare systems. Despite advances in clinical and imaging-based diagnostics, these diseases are often detected at advanced stages, limiting the effectiveness of therapeutic interventions. Recent breakthroughs in genomic and transcriptomic technologies, including whole-genome sequencing, single-cell RNA sequencing (scRNA-seq), and CRISPR-based screens, have revolutionized the field, offering new avenues for early diagnosis and personalized prognosis. Genomic approaches have elucidated disease-specific genetic risk factors and molecular pathways, while transcriptomic studies have identified stage-specific biomarkers that correlate with disease progression and severity. Furthermore, genome-wide association studies (GWAS), polygenic risk scores (PRS), and spatial transcriptomics are enabling the stratification of patients based on their risk profiles and prognostic trajectories. Advances in functional genomics have uncovered actionable targets, such as ATXN2 in ALS and TREM2 in AD, paving the way for tailored therapeutic strategies. Despite these achievements, challenges remain in translating genomic discoveries into clinical practice due to disease heterogeneity and the complexity of neurodegenerative pathophysiology. Future integration of genetic technologies holds promise for transforming diagnostic and prognostic paradigms, offering hope for improved patient outcomes and precision medicine approaches.

Advances on the Mechanisms and Therapeutic Strategies in Non-coding CGG Repeat Expansion Diseases

Non-coding CGG repeat expansions within the 5' untranslated region are implicated in a range of neurological disorders, including fragile X-associated tremor/ataxia syndrome, oculopharyngeal myopathy with leukodystrophy, and oculopharyngodistal myopathy. This review outlined the general characteristics of diseases associated with non-coding CGG repeat expansions, detailing their clinical manifestations and neuroimaging patterns, which often overlap and indicate shared pathophysiological traits. We summarized the underlying molecular mechanisms of these disorders, providing new insights into the roles that DNA, RNA, and toxic proteins play. Understanding these mechanisms is crucial for the development of targeted therapeutic strategies. These strategies include a range of approaches, such as antisense oligonucleotides, RNA interference, genomic DNA editing, small molecule interventions, and other treatments aimed at correcting the dysregulated processes inherent in these disorders. A deeper understanding of the shared mechanisms among non-coding CGG repeat expansion disorders may hold the potential to catalyze the development of innovative therapies, ultimately offering relief to individuals grappling with these debilitating neurological conditions.

Repair of genomic interstrand crosslinks

Genomic interstrand crosslinks (ICLs) are formed by reactive species generated during normal cellular metabolism, produced by the microbiome, and employed in cancer chemotherapy. While there are multiple options for replication dependent and independent ICL repair, the crucial step for each is unhooking one DNA strand from the other. Much of our insight into mechanisms of unhooking comes from powerful model systems based on plasmids with defined ICLs introduced into cells or cell free extracts. Here we describe the properties of exogenous and endogenous ICL forming compounds and provide an historical perspective on early work on ICL repair. We discuss the modes of unhooking elucidated in the model systems, the concordance or lack thereof in drug resistant tumors, and the evolving view of DNA adducts, including ICLs, formed by metabolic aldehydes.

Beyond CAG Repeats: The Multifaceted Role of Genetics in Huntington Disease

Huntington disease (HD) is a dominantly inherited neurodegenerative disorder caused by a CAG expansion on the huntingtin (HTT) gene and is characterized by progressive motor, cognitive, and neuropsychiatric decline. Recently, new genetic factors besides CAG repeats have been implicated in the disease pathogenesis. Most genetic modifiers are involved in DNA repair pathways and, as the cause of the loss of CAA interruption in the HTT gene, they exert their main influence through somatic expansion. However, this mechanism might not be the only driver of HD pathogenesis, and future studies are warranted in this field. The aim of the present review is to dissect the many faces of genetics in HD pathogenesis, from cis- and trans-acting genetic modifiers to RNA toxicity, mitochondrial DNA mutations, and epigenetics factors. Exploring genetic modifiers of HD onset and progression appears crucial to elucidate not only disease pathogenesis, but also to improve disease prediction and prevention, develop biomarkers of disease progression and response to therapies, and recognize new therapeutic opportunities. Since the same genetic mechanisms are also described in other repeat expansion diseases, their implications might encompass the whole spectrum of these disorders.

Posttranscriptional regulation of FAN1 by miR-124-3p at rs3512 underlies onset-delaying genetic modification in Huntington's disease

Many Mendelian disorders, such as Huntington's disease (HD) and spinocerebellar ataxias, arise from expansions of CAG trinucleotide repeats. Despite the clear genetic causes, additional genetic factors may influence the rate of those monogenic disorders. Notably, genome-wide association studies discovered somewhat expected modifiers, particularly mismatch repair genes involved in the CAG repeat instability, impacting age at onset of HD. Strikingly, FAN1, previously unrelated to repeat instability, produced the strongest HD modification signals. Diverse FAN1 haplotypes independently modify HD, with rare genetic variants diminishing DNA binding or nuclease activity of the FAN1 protein, hastening HD onset. However, the mechanism behind the frequent and the most significant onset-delaying FAN1 haplotype lacking missense variations has remained elusive. Here, we illustrated that a microRNA acting on 3'-UTR (untranslated region) SNP rs3512, rather than transcriptional regulation, is responsible for the significant FAN1 expression quantitative trait loci signal and allelic imbalance in FAN1 messenger ribonucleic acid (mRNA), accounting for the most significant and frequent onset-delaying modifier haplotype in HD. Specifically, miR-124-3p selectively targets the reference allele at rs3512, diminishing the stability of FAN1 mRNA harboring that allele and consequently reducing its levels. Subsequent validation analyses, including the use of antagomir and 3'-UTR reporter vectors with swapped alleles, confirmed the specificity of miR-124-3p at rs3512. Together, these findings indicate that the alternative allele at rs3512 renders the FAN1 mRNA less susceptible to miR-124-3p-mediated posttranscriptional regulation, resulting in increased FAN1 levels and a subsequent delay in HD onset by mitigating CAG repeat instability.

Attenuated huntingtin gene CAG nucleotide repeat size in individuals with Lynch syndrome

DNA mismatch repair (MMR) is thought to contribute to the onset and progression of Huntington disease (HD) by promoting somatic expansion of the pathogenic CAG nucleotide repeat in the huntingtin gene (HTT). Here we have studied constitutional HTT CAG repeat size in two cohorts of individuals with Lynch syndrome (LS) carrying heterozygous loss-of-function variants in the MMR genes MLH1 (n = 12/60; Lund cohort/Bochum cohort, respectively), MSH2 (n = 15/88), MSH6 (n = 21/23), and controls (n = 19/559). The sum of CAG repeats for both HTT alleles in each individual was calculated due to unknown segregation with the LS allele. In the larger Bochum cohort, the sum of CAG repeats was lower in the MLH1 subgroup compared to controls (MLH1 35.40 CAG repeats ± 3.6 vs. controls 36.89 CAG repeats ± 4.5; p = 0.014). All LS genetic subgroups in the Bochum cohort displayed lower frequencies of unstable HTT intermediate alleles and lower HTT somatic CAG repeat expansion index values compared to controls. Collectively, our results indicate that MMR gene haploinsufficiency could have a restraining impact on constitutional HTT CAG repeat size and support the notion that the MMR pathway is a driver of nucleotide repeat expansion diseases.

Genetic modifiers of repeat expansion disorders

Repeat expansion disorders (REDs) are monogenic diseases caused by a sequence of repetitive DNA expanding above a pathogenic threshold. A common feature of the REDs is a strong genotype-phenotype correlation in which a major determinant of age at onset (AAO) and disease progression is the length of the inherited repeat tract. Over a disease-gene carrier's life, the length of the repeat can expand in somatic cells, through the process of somatic expansion which is hypothesised to drive disease progression. Despite being monogenic, individual REDs are phenotypically variable, and exploring what genetic modifying factors drive this phenotypic variability has illuminated key pathogenic mechanisms that are common to this group of diseases. Disease phenotypes are affected by the cognate gene in which the expansion is found, the location of the repeat sequence in coding or non-coding regions and by the presence of repeat sequence interruptions. Human genetic data, mouse models and in vitro models have implicated the disease-modifying effect of DNA repair pathways via the mechanisms of somatic mutation of the repeat tract. As such, developing an understanding of these pathways in the context of expanded repeats could lead to future disease-modifying therapies for REDs.

The role of tandem repeat expansions in brain disorders

The human genome contains numerous genetic polymorphisms contributing to different health and disease outcomes. Tandem repeat (TR) loci are highly polymorphic yet under-investigated in large genomic studies, which has prompted research efforts to identify novel variations and gain a deeper understanding of their role in human biology and disease outcomes. We summarize the current understanding of TRs and their implications for human health and disease, including an overview of the challenges encountered when conducting TR analyses and potential solutions to overcome these challenges. By shedding light on these issues, this article aims to contribute to a better understanding of the impact of TRs on the development of new disease treatments.

Small Molecule Pytren-4QMn Metal Complex Slows down Huntington's Disease Progression in Male zQ175 Transgenic Mice

Huntington's disease (HD) is an inherited neurodegenerative disorder considered a rare disease with a prevalence of 5.7 per 100,000 people. It is caused by an autosomal dominant mutation consisting of expansions of trinucleotide repeats that translate into poly-glutamine enlarged mutant huntingtin proteins (mHTT), which are particularly deleterious in brain tissues. Since there is no cure for this progressive fatal disease, searches for new therapeutic approaches are much needed. The small molecule pytren-4QMn (4QMn), a highly water-soluble mimic of the enzyme superoxide dismutase, has shown in vivo beneficial anti-inflammatory activity in mice and was able to remove mHTT deposits in a C. elegans model of HD. In this study, we assessed 4QMn therapeutic potential in zQ175 neo-deleted knock-in mice, a model of HD that closely mimics the heterozygosity, genetic injury, and progressive nature of the human disease. We provide evidence that 4QMn has good acute and chronic tolerability, and can cross the blood-brain barrier, and in male, but not female, zQ175 mice moderately ameliorate HD-altered gene expression, mHtt aggregation, and HD disease phenotype. Our data highlight the importance of considering sex-specific differences when testing new therapies using animal models and postulate 4QMn as a potential novel type of small water-soluble metal complex that could be worth further investigating for its therapeutic potential in HD, as well as in other polyglutamine diseases.

FAN1 removes triplet repeat extrusions via a PCNA- and RFC-dependent mechanism

Human genome-wide association studies have identified FAN1 and several DNA mismatch repair (MMR) genes as modifiers of Huntington's disease age of onset. In animal models, FAN1 prevents somatic expansion of CAG triplet repeats, whereas MMR proteins promote this process. To understand the molecular basis of these opposing effects, we evaluated FAN1 nuclease function on DNA extrahelical extrusions that represent key intermediates in triplet repeat expansion. Here, we describe a strand-directed, extrusion-provoked nuclease function of FAN1 that is activated by RFC, PCNA, and ATP at physiological ionic strength. Activation of FAN1 in this manner results in DNA cleavage in the vicinity of triplet repeat extrahelical extrusions thereby leading to their removal in human cell extracts. The role of PCNA and RFC is to confer strand directionality to the FAN1 nuclease, and this reaction requires a physical interaction between PCNA and FAN1. Using cell extracts, we show that FAN1-dependent CAG extrusion removal relies on a very short patch excision-repair mechanism that competes with MutSβ-dependent MMR which is characterized by longer excision tracts. These results provide a mechanistic basis for the role of FAN1 in preventing repeat expansion and could explain the antagonistic effects of MMR and FAN1 in disease onset/progression.

Loss of TDP-43 promotes somatic CAG repeat expansion in Huntington's disease knock-in mice

TAR binding protein 43 (TDP-43) is normally present in the nucleus but mislocalized in the cytoplasm in a number of neurodegenerative diseases including Huntington's disease (HD). The nuclear loss of TDP-43 impairs gene transcription and regulation. However, it remains to be investigated whether loss of TDP-43 influences trinucleotide CAG repeat expansion in the HD gene, a genetic cause for HD. Here we report that CRISPR/Cas9 mediated-knock down of endogenous TDP-43 in the striatum of HD knock-in mice promoted CAG repeat expansion, accompanied by the increased expression of the DNA mismatch repair genes, Msh3 and Mlh1, which have been reported to increase trinucleotide repeat instability. Furthermore, suppressing Msh3 and Mlh1 by CRISPR/Cas9 targeting diminished the CAG repeat expansion. These findings suggest that nuclear TDP-43 deficiency may dysregulate the expression of DNA mismatch repair genes, leading to CAG repeat expansion and contributing to the pathogenesis of CAG repeat diseases. DATA AVAILABILITY: The key data supporting the findings of this study are presented within the article and the Supplemental Information. The RNA sequencing reported in this paper can be found at https://doi.org/10.6084/m9.figshare.22639429.

Replication dependent and independent mechanisms of GAA repeat instability

Trinucleotide repeat instability is a driver of human disease. Large expansions of (GAA)n repeats in the first intron of the FXN gene are the cause Friedreich's ataxia (FRDA), a progressive degenerative disorder which cannot yet be prevented or treated. (GAA)n repeat instability arises during both replication-dependent processes, such as cell division and intergenerational transmission, as well as in terminally differentiated somatic tissues. Here, we provide a brief historical overview on the discovery of (GAA)n repeat expansions and their association to FRDA, followed by recent advances in the identification of triplex H-DNA formation and replication fork stalling. The main body of this review focuses on the last decade of progress in understanding the mechanism of (GAA)n repeat instability during DNA replication and/or DNA repair. We propose that the discovery of additional mechanisms of (GAA)n repeat instability can be achieved via both comparative approaches to other repeat expansion diseases and genome-wide association studies. Finally, we discuss the advances towards FRDA prevention or amelioration that specifically target (GAA)n repeat expansions.

De novo mutations, genetic mosaicism and human disease

Mosaicism—the existence of genetically distinct populations of cells in a particular organism—is an important cause of genetic disease. Mosaicism can appear as de novo DNA mutations, epigenetic alterations of DNA, and chromosomal abnormalities. Neurodevelopmental or neuropsychiatric diseases, including autism—often arise by de novo mutations that usually not present in either of the parents. De novo mutations might occur as early as in the parental germline, during embryonic, fetal development, and/or post-natally, through ageing and life. Mutation timing could lead to mutation burden of less than heterozygosity to approaching homozygosity. Developmental timing of somatic mutation attainment will affect the mutation load and distribution throughout the body. In this review, we discuss the timing of de novo mutations, spanning from mutations in the germ lineage (all ages), to post-zygotic, embryonic, fetal, and post-natal events, through aging to death. These factors can determine the tissue specific distribution and load of de novo mutations, which can affect disease. The disease threshold burden of somatic de novo mutations of a particular gene in any tissue will be important to define.

Potential disease-modifying therapies for Huntington's disease: lessons learned and future opportunities

Huntington's disease is the most frequent autosomal dominant neurodegenerative disorder; however, no disease-modifying interventions are available for patients with this disease. The molecular pathogenesis of Huntington's disease is complex, with toxicity that arises from full-length expanded huntingtin and N-terminal fragments of huntingtin, which are both prone to misfolding due to proteolysis; aberrant intron-1 splicing of the HTT gene; and somatic expansion of the CAG repeat in the HTT gene. Potential interventions for Huntington's disease include therapies targeting huntingtin DNA and RNA, clearance of huntingtin protein, DNA repair pathways, and other treatment strategies targeting inflammation and cell replacement. The early termination of trials of the antisense oligonucleotide tominersen suggest that it is time to reflect on lessons learned, where the field stands now, and the challenges and opportunities for the future.

Rad5 and Its Human Homologs, HLTF and SHPRH, Are Novel Interactors of Mismatch Repair

DNA mismatch repair (MMR) repairs replication errors, and MMR defects play a role in both inherited cancer predisposition syndromes and in sporadic cancers. MMR also recognizes mispairs caused by environmental and chemotherapeutic agents; however, in these cases mispair recognition leads to apoptosis and not repair. Although mutation avoidance by MMR is fairly well understood, MMR-associated proteins are still being identified. We performed a bioinformatic analysis that implicated Saccharomyces cerevisiae Rad5 as a candidate for interacting with the MMR proteins Msh2 and Mlh1. Rad5 is a DNA helicase and E3 ubiquitin ligase involved in post-replicative repair and damage tolerance. We confirmed both interactions and found that the Mlh1 interaction is mediated by a conserved Mlh1-interacting motif (MIP box). Despite this, we did not find a clear role for Rad5 in the canonical MMR mutation avoidance pathway. The interaction of Rad5 with Msh2 and Mlh1 is conserved in humans, although each of the Rad5 human homologs, HLTF and SHPRH, shared only one of the interactions: HLTF interacts with MSH2, and SHPRH interacts with MLH1. Moreover, depletion of SHPRH, but not HLTF, results in a mild increase in resistance to alkylating agents although not as strong as loss of MMR, suggesting gene duplication led to specialization of the MMR-protein associated roles of the human Rad5 homologs. These results provide insights into how MMR accessory factors involved in the MMR-dependent apoptotic response interact with the core MMR machinery and have important health implications into how human cells respond to environmental toxins, tumor development, and treatment choices of tumors with defects in Rad5 homologs.

Exome sequencing of individuals with Huntington's disease implicates FAN1 nuclease activity in slowing CAG expansion and disease onset

The age at onset of motor symptoms in Huntington's disease (HD) is driven by HTT CAG repeat length but modified by other genes. In this study, we used exome sequencing of 683 patients with HD with extremes of onset or phenotype relative to CAG length to identify rare variants associated with clinical effect. We discovered damaging coding variants in candidate modifier genes identified in previous genome-wide association studies associated with altered HD onset or severity. Variants in FAN1 clustered in its DNA-binding and nuclease domains and were associated predominantly with earlier-onset HD. Nuclease activities of purified variants in vitro correlated with residual age at motor onset of HD. Mutating endogenous FAN1 to a nuclease-inactive form in an induced pluripotent stem cell model of HD led to rates of CAG expansion similar to those observed with complete FAN1 knockout. Together, these data implicate FAN1 nuclease activity in slowing somatic repeat expansion and hence onset of HD.

FAN1 exo- not endo-nuclease pausing on disease-associated slipped-DNA repeats: A mechanism of repeat instability

Ongoing inchworm-like CAG and CGG repeat expansions in brains, arising by aberrant processing of slipped DNAs, may drive Huntington's disease, fragile X syndrome, and autism. FAN1 nuclease modifies hyper-expansion rates by unknown means. We show that FAN1, through iterative cycles, binds, dimerizes, and cleaves slipped DNAs, yielding striking exo-nuclease pauses along slip-outs: 5'-C↓A↓GC↓A↓G-3' and 5'-C↓T↓G↓C↓T↓G-3'. CAG excision is slower than CTG and requires intra-strand A·A and T·T mismatches. Fully paired hairpins arrested excision, whereas disease-delaying CAA interruptions further slowed excision. Endo-nucleolytic cleavage is insensitive to slip-outs. Rare FAN1 variants are found in individuals with autism with CGG/CCG expansions, and CGG/CCG slip-outs show exo-nuclease pauses. The slip-out-specific ligand, naphthyridine-azaquinolone, which induces contractions of expanded repeats in vivo, requires FAN1 for its effect, and protects slip-outs from FAN1 exo-, but not endo-, nucleolytic digestion. FAN1's inchworm pausing of slip-out excision rates is well suited to modify inchworm expansion rates, which modify disease onset and progression.

FAN1's protection against CGG repeat expansion requires its nuclease activity and is FANCD2-independent

The Repeat Expansion Diseases, a large group of human diseases that includes the fragile X-related disorders (FXDs) and Huntington's disease (HD), all result from expansion of a disease-specific microsatellite via a mechanism that is not fully understood. We have previously shown that mismatch repair (MMR) proteins are required for expansion in a mouse model of the FXDs, but that the FANCD2 and FANCI associated nuclease 1 (FAN1), a component of the Fanconi anemia (FA) DNA repair pathway, is protective. FAN1's nuclease activity has been reported to be dispensable for protection against expansion in an HD cell model. However, we show here that in a FXD mouse model a point mutation in the nuclease domain of FAN1 has the same effect on expansion as a null mutation. Furthermore, we show that FAN1 and another nuclease, EXO1, have an additive effect in protecting against MSH3-dependent expansions. Lastly, we show that the loss of FANCD2, a vital component of the Fanconi anemia DNA repair pathway, has no effect on expansions. Thus, FAN1 protects against MSH3-dependent expansions without diverting the expansion intermediates into the canonical FA pathway and this protection depends on FAN1 having an intact nuclease domain.

SPYing on triplet repeat expansions: Insights into FAN1-MLH1 interaction and regulation

The DNA repair proteins FAN1 and MLH1 have opposing effects on triplet repeat expansions. New studies by Goold et al. (2021) and Porro et al. (2021) pinpoint interactions between FAN1 and MLH1 that cross-regulate each other's activities.