Identification and characterisation of genetic variation that modifies age at onset in Huntington’s disease

Huntington’s disease (HD) is a progressive and ultimately fatal neurodegeneration caused by a CAG repeat expansion in the huntingtin gene (HTT). The length of the CAG repeat is strongly inversely correlated with disease onset, but there remains considerable onset variation that is unexplained, even between individuals with the same repeat length. This thesis details the investigation of genetic modifiers of HD onset using next-generation sequencing (NGS), with an emphasis on rare coding variant identification. Chapter 1 gives a general introduction. Chapter 2 details the experimental procedures used in this study. Chapter 3 explores several HD onset phenotypes in the Registry-HD study, derived using the clinician’s estimate of onset and the HD clinical characteristics questionnaire. An extreme HD onset cohort (N=500) is then selected using residual age at motor onset. Chapter 4 uses whole-exome sequencing to sequence the 500 HD patients selected in the previous chapter. I identified rare damaging variation in several DNA repair genes associated with altered disease onset, including FAN1, EXO1, MSH3, LIG1 and PMS1. Unbiased whole-exome burden and SKAT(-O) analyses identified NOP14 as an exome-wide significant gene. Investigation revealed NOP14 strongly tagged HTT allele structure, identifying it as a major modifier of disease onset. Chapter 5 confirms the HTT allele structures using an independent NGS method. HTT alleles possessing additional interruptions were associated with late disease onset; whereas alleles lacking interruptions were uniformly found in early onset individuals. I also detail three novel HTT alleles associated with extremely delayed onset and explore MiSeq-based instability measurements. In Chapter 6, I discuss the study generally. I give an overall model describing genetic factors that modify HD onset, and present my two-fated pathway model for somatic instability. I then highlight future studies and approaches in HD given what we have learned from this work. The results presented here have important implications for understanding the mechanisms underlying HD onset, underscoring DNA and DNA repair as critical components in HD

Mutant huntingtin confers cell-autonomous phenotypes on Huntington’s disease iPSC-derived microglia

Huntington’s disease (HD) is a neurodegenerative disorder caused by a dominantly inherited CAG repeat expansion in the huntingtin gene (HTT). Neuroinflammation and microglia have been implicated in HD pathology, however it has been unclear if mutant HTT (mHTT) expression has an adverse cell-autonomous effect on microglial function, or if they are only activated in response to the neurodegenerative brain environment in HD. To establish a human cell model of HD microglia function, we generated isogenic controls for HD patient-derived induced pluripotent stem cells (iPSC) with 109 CAG repeats (Q109). Q109 and isogenic Q22 iPSC, as well as non-isogenic Q60 and Q33 iPSC lines, were differentiated to iPSC-microglia. Our study supports a model of basal microglia dysfunction in HD leading to elevated pro-inflammatory cytokine production together with impaired phagocytosis and endocytosis capacity, in the absence of immune stimulation. These findings are consistent with early microglia activation observed in pre-manifest patients and indicate that mHTT gene expression affects microglia function in a cell-autonomous way

Huntington’s disease age at motor onset is modified by the tandem hexamer repeat in TCERG1

Huntington’s disease is caused by an expanded CAG tract in HTT. The length of the CAG tract accounts for over half the variance in age at onset of disease, and is influenced by other genetic factors, mostly implicating the DNA maintenance machinery. We examined a single nucleotide variant, rs79727797, on chromosome 5 in the TCERG1 gene, previously reported to be associated with Huntington’s disease and a quasi-tandem repeat (QTR) hexamer in exon 4 of TCERG1 with a central pure repeat. We developed a method for calling perfect and imperfect repeats from exome-sequencing data, and tested association between the QTR in TCERG1 and residual age at motor onset (after correcting for the effects of CAG length in the HTT gene) in 610 individuals with Huntington’s disease via regression analysis. We found a significant association between age at onset and the sum of the repeat lengths from both alleles of the QTR (p = 2.1 × 10−9), with each added repeat hexamer reducing age at onset by one year (95% confidence interval [0.7, 1.4]). This association explained that previously observed with rs79727797. The association with age at onset in the genome-wide association study is due to a QTR hexamer in TCERG1, translated to a glutamine/alanine tract in the protein. We could not distinguish whether this was due to cis-effects of the hexamer repeat on gene expression or of the encoded glutamine/alanine tract in the protein. These results motivate further study of the mechanisms by which TCERG1 modifies onset of HD

The onset and prevalence of motor and psychiatric symptoms in Huntington’s disease

Background Huntington’s disease is characterised by a range of motor, psychiatric and cognitive symptoms. These present in different combinations through the disease course and impact on daily life and functioning. Huntington’s disease is caused by a dominant CAG repeat expansion in the HTT gene, and longer repeats are associated with earlier onset of motor symptoms. Objectives To investigate the onset, prevalence and functional impact of motor and psychiatric symptoms of Huntington’s disease. Methods We analysed clinical phenotype data from the European REGISTRY study for 6316 individuals with manifest Huntington’s disease. Onset and prevalence data for motor and psychiatric symptoms were extracted from the clinical history part of REGISTRY and the detailed Clinical Characteristics Questionnaire. Generalised linear models were constructed to assess relationships between symptoms and functional outcomes. Results As age at first presentation of Huntington’s disease increases, the likelihood that the initial presenting symptom is motor also increases. This is not associated with pathogenic CAG repeat length. At a population level there were conserved relationships between symptoms across different repeat lengths, with depression often occurring early followed by motor and then cognitive symptoms. There were significant relationships between all individual psychiatric and cognitive symptoms and reduced functional capacity. Conclusions There are conserved patterns of symptoms in HD that can be quantified. Psychiatric and behavioural symptoms significantly impair daily functioning and should be considered part of the disease trajectory at any age

Timing and impact of psychiatric, cognitive, and motor abnormalities in Huntington disease

Objective To assess the prevalence, timing and functional impact of psychiatric, cognitive and motor abnormalities in Huntington’s disease (HD) gene carriers, we analysed retrospective clinical data from individuals with manifest HD. Methods Clinical features of HD patients were analysed for 6316 individuals in the European REGISTRY study from 161 sites across 17 countries. Data came from clinical history and the patient-completed Clinical Characteristics Questionnaire that assessed eight symptoms: motor, cognitive, apathy, depression, perseverative/obsessive behavior, irritability, violent/aggressive behavior, and psychosis. Multiple logistic regression was used to analyse relationships between symptoms and functional outcomes. Results The initial manifestation of HD is increasingly likely to be motor, and less likely to be psychiatric, as age at presentation increases, and is independent of pathogenic CAG repeat length. The Clinical Characteristics Questionnaire captures data on non-motor symptom prevalence that correlate specifically with validated clinical measures. Psychiatric and cognitive symptoms are common in HD gene carriers, with earlier onsets associated with longer CAG repeats. 42.4% of HD patients reported at least one psychiatric or cognitive symptom before motor symptoms, with depression most common. Each non-motor symptom was associated with significantly reduced total functional capacity scores. Conclusions Psychiatric and cognitive symptoms are common and functionally debilitating in HD gene carriers. They require recognition and targeting with clinical outcome measures and treatments. However, as it is impossible to distinguish confidently between non-motor symptoms arising from HD and primary psychiatric disorders, particularly in younger pre-manifest patients, non-motor symptoms should not be used to make a clinical diagnosis of HD

Modification of Huntington's disease by short tandem repeats

Expansions of glutamine-coding CAG trinucleotide repeats cause a number of neurodegenerative diseases, including Huntington's disease (HD) and several of the spinocerebellar ataxias (SCAs). In general, age-at-onset of the polyglutamine diseases is inversely correlated with the size of the respective inherited expanded CAG repeat. Expanded CAG repeats are also somatically unstable in certain tissues, and age-at-onset of HD corrected for individual HTT CAG repeat length (i.e., residual age-at-onset), is modified by repeat instability-related DNA maintenance/repair genes as demonstrated by recent genome-wide association studies (GWAS). Modification of one polyglutamine disease (e.g., HD) by the repeat length of another (e.g., ATXN3, CAG expansions in which cause SCA3) has also been hypothesized. Consequently, we determined whether age-at-onset in HD is modified by the CAG repeats of other polyglutamine disease genes. We found that the CAG measured repeat sizes of other polyglutamine disease genes were polymorphic in HD participants but did not influence HD age-at-onset. Additional analysis focusing specifically on ATXN3 in a larger sample set (n = 1,388) confirmed the lack of association between HD residual age-at-onset and ATXN3 CAG repeat length. Additionally, neither our HD onset modifier GWAS single nucleotide polymorphism (SNP) data nor imputed short tandem repeat (STR) data supported involvement of other polyglutamine disease genes in modifying HD. By contrast, our GWAS based on imputed STRs revealed significant modification signals for other genomic regions. Together, our STR GWAS show that modification of HD is associated with STRs that do not involve other polyglutamine disease-causing genes, refining the landscape of HD modification and highlighting the importance of rigorous data analysis, especially in genetic studies testing candidate modifiers

CAG repeat not polyglutamine length determines timing of Huntington’s disease onset

Variable, glutamine-encoding, CAA interruptions indicate that a property of the uninterrupted HTT CAG repeat sequence, distinct from the length of huntingtin’s polyglutamine segment, dictates the rate at which Huntington’s disease (HD) develops. The timing of onset shows no significant association with HTT cis-eQTLs but is influenced, sometimes in a sex-specific manner, by polymorphic variation at multiple DNA maintenance genes, suggesting that the special onset-determining property of the uninterrupted CAG repeat is a propensity for length instability that leads to its somatic expansion. Additional naturally occurring genetic modifier loci, defined by GWAS, may influence HD pathogenesis through other mechanisms. These findings have profound implications for the pathogenesis of HD and other repeat diseases and question the fundamental premise that polyglutamine length determines the rate of pathogenesis in the “polyglutamine disorders.

Repeat Detector: versatile sizing of expanded tandem repeats and identification of interrupted alleles from targeted DNA sequencing

Targeted DNA sequencing approaches will improve how the size of short tandem repeats is measured for diagnostic tests and preclinical studies. The expansion of these sequences causes dozens of disorders, with longer tracts generally leading to a more severe disease. Interrupted alleles are sometimes present within repeats and can alter disease manifestation. Determining repeat size mosaicism and identifying interruptions in targeted sequencing datasets remains a major challenge. This is in part because standard alignment tools are ill-suited for repetitive and unstable sequences. To address this, we have developed Repeat Detector (RD), a deterministic profile weighting algorithm for counting repeats in targeted sequencing data. We tested RD using blood-derived DNA samples from Huntington’s disease and Fuchs endothelial corneal dystrophy patients sequenced using either Illumina MiSeq or Pacific Biosciences single-molecule, real-time sequencing platforms. RD was highly accurate in determining repeat sizes of 609 blood-derived samples from Huntington’s disease individuals and did not require prior knowledge of the flanking sequences. Furthermore, RD can be used to identify alleles with interruptions and provide a measure of repeat instability within an individual. RD is therefore highly versatile and may find applications in the diagnosis of expanded repeat disorders and in the development of novel therapies

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