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    An intronic GAA repeat expansion in FGF14 causes the autosomal-dominant adult-onset ataxia SCA50/ATX-FGF14
    Rafehi, H ; Read, J ; Szmulewicz, DJ ; Davies, KC ; Snell, P ; Fearnley, LG ; Scott, L ; Thomsen, M ; Gillies, G ; Pope, K ; Bennett, MF ; Munro, JE ; Ngo, KJ ; Chen, L ; Wallis, MJ ; Butler, EG ; Kumar, KR ; Wu, KHC ; Tomlinson, SE ; Tisch, S ; Malhotra, A ; Lee-Archer, M ; Dolzhenko, E ; Eberle, MA ; Roberts, LJ ; Fogel, BL ; Bruggemann, N ; Lohmann, K ; Delatycki, MB ; Bahlo, M ; Lockhart, PJ (CELL PRESS, 2023-01-05)
    Adult-onset cerebellar ataxias are a group of neurodegenerative conditions that challenge both genetic discovery and molecular diagnosis. In this study, we identified an intronic (GAA) repeat expansion in fibroblast growth factor 14 (FGF14). Genetic analysis of 95 Australian individuals with adult-onset ataxia identified four (4.2%) with (GAA)>300 and a further nine individuals with (GAA)>250. PCR and long-read sequence analysis revealed these were pure (GAA) repeats. In comparison, no control subjects had (GAA)>300 and only 2/311 control individuals (0.6%) had a pure (GAA)>250. In a German validation cohort, 9/104 (8.7%) of affected individuals had (GAA)>335 and a further six had (GAA)>250, whereas 10/190 (5.3%) control subjects had (GAA)>250 but none were (GAA)>335. The combined data suggest (GAA)>335 are disease causing and fully penetrant (p = 6.0 × 10-8, OR = 72 [95% CI = 4.3-1,227]), while (GAA)>250 is likely pathogenic with reduced penetrance. Affected individuals had an adult-onset, slowly progressive cerebellar ataxia with variable features including vestibular impairment, hyper-reflexia, and autonomic dysfunction. A negative correlation between age at onset and repeat length was observed (R2 = 0.44, p = 0.00045, slope = -0.12) and identification of a shared haplotype in a minority of individuals suggests that the expansion can be inherited or generated de novo during meiotic division. This study demonstrates the power of genome sequencing and advanced bioinformatic tools to identify novel repeat expansions via model-free, genome-wide analysis and identifies SCA50/ATX-FGF14 as a frequent cause of adult-onset ataxia.
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    Unexpected diagnosis of myotonic dystrophy type 2 repeat expansion by genome sequencing
    Rafehi, H ; Green, C ; Bozaoglu, K ; Gillies, G ; Delatycki, MB ; Lockhart, PJ ; Scheffer, IE ; Bahlo, M (SPRINGERNATURE, 2023-01)
    Several neurological disorders, such as myotonic dystrophy are caused by expansions of short tandem repeats (STRs) which can be difficult to detect by molecular tools. Methodological advances have made repeat expansion (RE) detection with whole genome sequencing (WGS) feasible. We recruited a multi-generational family (family A) ascertained for genetic studies of autism spectrum disorder. WGS was performed on seven children from four nuclear families from family A and analyzed for REs of STRs known to cause neurological disorders. We detected an expansion of a heterozygous intronic CCTG STR in CNBP in two siblings. This STR causes myotonic dystrophy type 2 (DM2). The expansion did not segregate with the ASD phenotype. Repeat-primed PCR showed that the DM2 CCTG motif was expanded above the pathogenic threshold in both children and their mother. On subsequent examination, the mother had mild features of DM2. We show that screening of STRs in WGS datasets has diagnostic utility, both in the clinical and research domain, with potential management and genetic counseling implications.
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    Sporadic hypothalamic hamartoma is a ciliopathy with somatic and bi-allelic contributions
    Green, TE ; Motelow, JE ; Bennett, MF ; Ye, Z ; Bennett, CA ; Griffin, NG ; Damiano, JA ; Leventer, RJ ; Freeman, JL ; Harvey, AS ; Lockhart, PJ ; Sadleir, LG ; Boys, A ; Scheffer, IE ; Major, H ; Darbro, BW ; Bahlo, M ; Goldstein, DB ; Kerrigan, JF ; Heinzen, EL ; Berkovic, SF ; Hildebrand, MS (OXFORD UNIV PRESS, 2022-07-21)
    Hypothalamic hamartoma with gelastic seizures is a well-established cause of drug-resistant epilepsy in early life. The development of novel surgical techniques has permitted the genomic interrogation of hypothalamic hamartoma tissue. This has revealed causative mosaic variants within GLI3, OFD1 and other key regulators of the sonic-hedgehog pathway in a minority of cases. Sonic-hedgehog signalling proteins localize to the cellular organelle primary cilia. We therefore explored the hypothesis that cilia gene variants may underlie hitherto unsolved cases of sporadic hypothalamic hamartoma. We performed high-depth exome sequencing and chromosomal microarray on surgically resected hypothalamic hamartoma tissue and paired leukocyte-derived DNA from 27 patients. We searched for both germline and somatic variants under both dominant and bi-allelic genetic models. In hamartoma-derived DNA of seven patients we identified bi-allelic (one germline, one somatic) variants within one of four cilia genes-DYNC2I1, DYNC2H1, IFT140 or SMO. In eight patients, we identified single somatic variants in the previously established hypothalamic hamartoma disease genes GLI3 or OFD1. Overall, we established a plausible molecular cause for 15/27 (56%) patients. Here, we expand the genetic architecture beyond single variants within dominant disease genes that cause sporadic hypothalamic hamartoma to bi-allelic (one germline/one somatic) variants, implicate three novel cilia genes and reconceptualize the disorder as a ciliopathy.
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    Germline variants in tumor suppressor FBXW7 lead to impaired ubiquitination and a neurodevelopmental syndrome
    Stephenson, SEM ; Costain, G ; Blok, LER ; Silk, MA ; Nguyen, TB ; Dong, X ; Alhuzaimi, DE ; Dowling, JJ ; Walker, S ; Amburgey, K ; Hayeems, RZ ; Rodan, LH ; Schwartz, MA ; Picker, J ; Lynch, SA ; Gupta, A ; Rasmussen, KJ ; Schimmenti, LA ; Klee, EW ; Niu, Z ; Agre, KE ; Chilton, I ; Chung, WK ; Revah-Politi, A ; Au, PYB ; Griffith, C ; Racobaldo, M ; Raas-Rothschild, A ; Ben Zeev, B ; Barel, O ; Moutton, S ; Morice-Picard, F ; Carmignac, V ; Cornaton, J ; Marle, N ; Devinsky, O ; Stimach, C ; Wechsler, SB ; Hainline, BE ; Sapp, K ; Willems, M ; Bruel, A ; Dias, K-R ; Evans, C-A ; Roscioli, T ; Sachdev, R ; Temple, SEL ; Zhu, Y ; Baker, JJ ; Scheffer, IE ; Gardiner, FJ ; Schneider, AL ; Muir, AM ; Mefford, HC ; Crunk, A ; Heise, EM ; Millan, F ; Monaghan, KG ; Person, R ; Rhodes, L ; Richards, S ; Wentzensen, IM ; Cogne, B ; Isidor, B ; Nizon, M ; Vincent, M ; Besnard, T ; Piton, A ; Marcelis, C ; Kato, K ; Koyama, N ; Ogi, T ; Goh, ES-Y ; Richmond, C ; Amor, DJ ; Boyce, JO ; Morgan, AT ; Hildebrand, MS ; Kaspi, A ; Bahlo, M ; Fridriksdottir, R ; Katrinardottir, H ; Sulem, P ; Stefansson, K ; Bjornsson, HT ; Mandelstam, S ; Morleo, M ; Mariani, M ; Scala, M ; Accogli, A ; Torella, A ; Capra, V ; Wallis, M ; Jansen, S ; Waisfisz, Q ; de Haan, H ; Sadedin, S ; Lim, SC ; White, SM ; Ascher, DB ; Schenck, A ; Lockhart, PJ ; Christodoulou, J ; Tan, TY (CELL PRESS, 2022-04-07)
    Neurodevelopmental disorders are highly heterogenous conditions resulting from abnormalities of brain architecture and/or function. FBXW7 (F-box and WD-repeat-domain-containing 7), a recognized developmental regulator and tumor suppressor, has been shown to regulate cell-cycle progression and cell growth and survival by targeting substrates including CYCLIN E1/2 and NOTCH for degradation via the ubiquitin proteasome system. We used a genotype-first approach and global data-sharing platforms to identify 35 individuals harboring de novo and inherited FBXW7 germline monoallelic chromosomal deletions and nonsense, frameshift, splice-site, and missense variants associated with a neurodevelopmental syndrome. The FBXW7 neurodevelopmental syndrome is distinguished by global developmental delay, borderline to severe intellectual disability, hypotonia, and gastrointestinal issues. Brain imaging detailed variable underlying structural abnormalities affecting the cerebellum, corpus collosum, and white matter. A crystal-structure model of FBXW7 predicted that missense variants were clustered at the substrate-binding surface of the WD40 domain and that these might reduce FBXW7 substrate binding affinity. Expression of recombinant FBXW7 missense variants in cultured cells demonstrated impaired CYCLIN E1 and CYCLIN E2 turnover. Pan-neuronal knockdown of the Drosophila ortholog, archipelago, impaired learning and neuronal function. Collectively, the data presented herein provide compelling evidence of an F-Box protein-related, phenotypically variable neurodevelopmental disorder associated with monoallelic variants in FBXW7.
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    Pathogenic Variants in GPC4 Cause Keipert Syndrome
    Amor, DJ ; Stephenson, SEM ; Mustapha, M ; Mensah, MA ; Ockeloen, CW ; Lee, WS ; Tankard, RM ; Phelan, DG ; Shinawi, M ; de Brouwer, APM ; Pfundt, R ; Dowling, C ; Toler, TL ; Sutton, VR ; Agolini, E ; Rinelli, M ; Capolino, R ; Martinelli, D ; Zampino, G ; Dumic, M ; Reardon, W ; Shaw-Smith, C ; Leventer, RJ ; Delatycki, MB ; Kleefstra, T ; Mundlos, S ; Mortier, G ; Bahlo, M ; Allen, NJ ; Lockhart, PJ (CELL PRESS, 2019-05-02)
    Glypicans are a family of cell-surface heparan sulfate proteoglycans that regulate growth-factor signaling during development and are thought to play a role in the regulation of morphogenesis. Whole-exome sequencing of the Australian family that defined Keipert syndrome (nasodigitoacoustic syndrome) identified a hemizygous truncating variant in the gene encoding glypican 4 (GPC4). This variant, located in the final exon of GPC4, results in premature termination of the protein 51 amino acid residues prior to the stop codon, and in concomitant loss of functionally important N-linked glycosylation (Asn514) and glycosylphosphatidylinositol (GPI) anchor (Ser529) sites. We subsequently identified seven affected males from five additional kindreds with novel and predicted pathogenic variants in GPC4. Segregation analysis and X-inactivation studies in carrier females provided supportive evidence that the GPC4 variants caused the condition. Furthermore, functional studies of recombinant protein suggested that the truncated proteins p.Gln506∗ and p.Glu496∗ were less stable than the wild type. Clinical features of Keipert syndrome included a prominent forehead, a flat midface, hypertelorism, a broad nose, downturned corners of mouth, and digital abnormalities, whereas cognitive impairment and deafness were variable features. Studies of Gpc4 knockout mice showed evidence of the two primary features of Keipert syndrome: craniofacial abnormalities and digital abnormalities. Phylogenetic analysis demonstrated that GPC4 is most closely related to GPC6, which is associated with a bone dysplasia that has a phenotypic overlap with Keipert syndrome. Overall, we have shown that pathogenic variants in GPC4 cause a loss of function that results in Keipert syndrome, making GPC4 the third human glypican to be linked to a genetic syndrome.
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    Rapid Diagnosis of Spinocerebellar Ataxia 36 in a three-Generation Family Using Short-Read Whole-Genome Sequencing Data
    Rafehi, H ; Szmulewicz, DJ ; Pope, K ; Wallis, M ; Christodoulou, J ; White, SM ; Delatycki, MB ; Lockhart, PJ ; Bahlo, M (WILEY, 2020-09)
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    Cerebrospinal fluid liquid biopsy for detecting somatic mosaicism in brain
    Ye, Z ; Chatterton, Z ; Pflueger, J ; Damiano, JA ; McQuillan, L ; Harvey, AS ; Malone, S ; Do, H ; Maixner, W ; Schneider, A ; Nolan, B ; Wood, M ; Lee, WS ; Gillies, G ; Pope, K ; Wilson, M ; Lockhart, PJ ; Dobrovic, A ; Scheffer, IE ; Bahlo, M ; Leventer, RJ ; Lister, R ; Berkovic, SF ; Hildebrand, MS (OXFORD UNIV PRESS, 2021)
    Brain somatic mutations are an increasingly recognized cause of epilepsy, brain malformations and autism spectrum disorders and may be a hidden cause of other neurodevelopmental and neurodegenerative disorders. At present, brain mosaicism can be detected only in the rare situations of autopsy or brain biopsy. Liquid biopsy using cell-free DNA derived from cerebrospinal fluid has detected somatic mutations in malignant brain tumours. Here, we asked if cerebrospinal fluid liquid biopsy can be used to detect somatic mosaicism in non-malignant brain diseases. First, we reliably quantified cerebrospinal fluid cell-free DNA in 28 patients with focal epilepsy and 28 controls using droplet digital PCR. Then, in three patients we identified somatic mutations in cerebrospinal fluid: in one patient with subcortical band heterotopia the LIS1 p. Lys64* variant at 9.4% frequency; in a second patient with focal cortical dysplasia the TSC1 p. Phe581His*6 variant at 7.8% frequency; and in a third patient with ganglioglioma the BRAF p. Val600Glu variant at 3.2% frequency. To determine if cerebrospinal fluid cell-free DNA was brain-derived, whole-genome bisulphite sequencing was performed and brain-specific DNA methylation patterns were found to be significantly enriched (P = 0.03). Our proof of principle study shows that cerebrospinal fluid liquid biopsy is valuable in investigating mosaic neurological disorders where brain tissue is unavailable.
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    Reducing the exome search space for Mendelian diseases using genetic linkage analysis of exome genotypes
    Smith, KR ; Bromhead, CJ ; Hildebrand, MS ; Shearer, AE ; Lockhart, PJ ; Najmabadi, H ; Leventer, RJ ; McGillivray, G ; Amor, DJ ; Smith, RJ ; Bahlo, M (BIOMED CENTRAL LTD, 2011)
    Many exome sequencing studies of Mendelian disorders fail to optimally exploit family information. Classical genetic linkage analysis is an effective method for eliminating a large fraction of the candidate causal variants discovered, even in small families that lack a unique linkage peak. We demonstrate that accurate genetic linkage mapping can be performed using SNP genotypes extracted from exome data, removing the need for separate array-based genotyping. We provide software to facilitate such analyses.
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    Neuropathology of childhood-onset basal ganglia degeneration caused by mutation of VAC14
    Stutterd, C ; Diakumis, P ; Bahlo, M ; Fernandez, MF ; Leventer, RJ ; Delatycki, M ; Amor, D ; Chow, CW ; Stephenson, S ; Meisler, MH ; Mclean, C ; Lockhart, PJ (WILEY, 2017-12)
    OBJECTIVE: To characterize the clinical features and neuropathology associated with recessive VAC14 mutations. METHODS: Whole-exome sequencing was used to identify the genetic etiology of a rapidly progressive neurological disease presenting in early childhood in two deceased siblings with distinct neuropathological features on post mortem examination. RESULTS: We identified compound heterozygous variants in VAC14 in two deceased siblings with early childhood onset of severe, progressive dystonia, and neurodegeneration. Their clinical phenotype is consistent with the VAC14-related childhood-onset, striatonigral degeneration recently described in two unrelated children. Post mortem examination demonstrated prominent vacuolation associated with degenerating neurons in the caudate nucleus, putamen, and globus pallidus, similar to previously reported ex vivo vacuoles seen in the late-endosome/lysosome of VAC14-deficient neurons. We identified upregulation of ubiquitinated granules within the cell cytoplasm and lysosomal-associated membrane protein (LAMP2) around the vacuole edge to suggest a process of vacuolation of lysosomal structures associated with active autophagocytic-associated neuronal degeneration. INTERPRETATION: Our findings reveal a distinct clinicopathological phenotype associated with recessive VAC14 mutations.
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    Diagnostic and cost utility of whole exome sequencing in peripheral neuropathy
    Walsh, M ; Bell, KM ; Chong, B ; Creed, E ; Brett, GR ; Pope, K ; Thorne, NP ; Sadedin, S ; Georgeson, P ; Phelan, DG ; Day, T ; Taylor, JA ; Sexton, A ; Lockhart, PJ ; Kiers, L ; Fahey, M ; Macciocca, I ; Gaff, CL ; Oshlack, A ; Yiu, EM ; James, PA ; Stark, Z ; Ryan, MM (WILEY, 2017-05)
    OBJECTIVE: To explore the diagnostic utility and cost effectiveness of whole exome sequencing (WES) in a cohort of individuals with peripheral neuropathy. METHODS: Singleton WES was performed in individuals recruited though one pediatric and one adult tertiary center between February 2014 and December 2015. Initial analysis was restricted to a virtual panel of 55 genes associated with peripheral neuropathies. Patients with uninformative results underwent expanded analysis of the WES data. Data on the cost of prior investigations and assessments performed for diagnostic purposes in each patient was collected. RESULTS: Fifty patients with a peripheral neuropathy were recruited (median age 18 years; range 2-68 years). The median time from initial presentation to study enrollment was 6 years 9 months (range 2 months-62 years), and the average cost of prior investigations and assessments for diagnostic purposes AU$4013 per patient. Eleven individuals received a diagnosis from the virtual panel. Eight individuals received a diagnosis following expanded analysis of the WES data, increasing the overall diagnostic yield to 38%. Two additional individuals were diagnosed with pathogenic copy number variants through SNP microarray. CONCLUSIONS: This study provides evidence that WES has a high diagnostic utility and is cost effective in patients with a peripheral neuropathy. Expanded analysis of WES data significantly improves the diagnostic yield in patients in whom a diagnosis is not found on the initial targeted analysis. This is primarily due to diagnosis of conditions caused by newly discovered genes and the resolution of complex and atypical phenotypes.