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Next-generation sequencing (NGS) is increasingly being applied to clinical testing. This practice is predicted to grow especially in neurology clinics because many of their patients have monogenetic causes for their “diagnostic odyssey.” The cost of sequencing has been steadily decreasing, but the cost of DNA sequencing is a minor part of the total cost. Downstream data analysis, storage, and interpretation account for most of the total expense. In patients with nonspecific neurologic disorders in which an extensive number of genetic differential diagnoses exist, whole-genome sequencing (WGS) or whole-exome sequencing (WES) has shown promise in the identification of genetic causes. However, both WGS and WES have incomplete coverage and produce a large number of rare variants of unknown importance. In addition, ethical dilemmas are often created by unexpected findings in genes unrelated to the initial sequencing indication. Targeted-panel NGS starts with the capture of a set of disease-focused genes, followed by massive parallel sequencing. For many genetically heterogeneous neurologic disorders, a genetic panel that is disease focused yet inclusive of a large genetic differential diagnosis can be defined to reduce cost, increase turnaround time, and optimize performance. Targeted-panel NGS is currently the preferred first-tier approach because it provides a reliable clinical application while eliminating unexpected ethical dilemmas. Targeted-panel NGS is leading to a paradigm shift in the diagnosis of many neurologic disorders, enabling individualized precision medicine. In this review, we provide an overview of WGS, WES, and targeted-panel NGS in consideration of their utility in clinical testing for neurologic diseases.

Pathogenic mutations in APP, PSEN1, PSEN2, MAPT and GRN have previously been linked to familial early onset forms of dementia. Mutation screening in these genes has been performed in either very small series or in single families with late onset AD (LOAD). Similarly, studies in single families have reported mutations in MAPT and GRN associated with clinical AD but no systematic screen of a large dataset has been performed to determine how frequently this occurs. We report sequence data for 439 probands from late-onset AD families with a history of four or more affected individuals. Sixty sequenced individuals (13.7%) carried a novel or pathogenic mutation. Eight pathogenic variants, (one each in APP and MAPT, two in PSEN1 and four in GRN) three of which are novel, were found in 14 samples. Thirteen additional variants, present in 23 families, did not segregate with disease, but the frequency of these variants is higher in AD cases than controls, indicating that these variants may also modify risk for disease. The frequency of rare variants in these genes in this series is significantly higher than in the 1,000 genome project (p = 5.09 × 10⁻⁵; OR = 2.21; 95%CI = 1.49-3.28) or an unselected population of 12,481 samples (p = 6.82 × 10⁻⁵; OR = 2.19; 95%CI = 1.347-3.26). Rare coding variants in APP, PSEN1 and PSEN2, increase risk for or cause late onset AD. The presence of variants in these genes in LOAD and early-onset AD demonstrates that factors other than the mutation can impact the age at onset and penetrance of at least some variants associated with AD. MAPT and GRN mutations can be found in clinical series of AD most likely due to misdiagnosis. This study clearly demonstrates that rare variants in these genes could explain an important proportion of genetic heritability of AD, which is not detected by GWAS.

Late-onset Alzheimer's disease (LOAD) is the most common form of dementia in the elderly. The National Institute of Aging-Late Onset Alzheimer's Disease Family Study and the National Cell Repository for Alzheimer's Disease conducted a joint genome-wide association study (GWAS) of multiplex LOAD families (3,839 affected and unaffected individuals from 992 families plus additional unrelated neurologically evaluated normal subjects) using the 610 IlluminaQuad panel. This cohort represents the largest family-based GWAS of LOAD to date, with analyses limited here to the European-American subjects. SNPs near APOE gave highly significant results (e.g., rs2075650, p = 3.2×10(-81)), but no other genome-wide significant evidence for association was obtained in the full sample. Analyses that stratified on APOE genotypes identified SNPs on chromosome 10p14 in CUGBP2 with genome-wide significant evidence for association within APOE ε4 homozygotes (e.g., rs201119, p = 1.5×10(-8)). Association in this gene was replicated in an independent sample consisting of three cohorts. There was evidence of association for recently-reported LOAD risk loci, including BIN1 (rs7561528, p = 0.009 with, and p = 0.03 without, APOE adjustment) and CLU (rs11136000, p = 0.023 with, and p = 0.008 without, APOE adjustment), with weaker support for CR1. However, our results provide strong evidence that association with PICALM (rs3851179, p = 0.69 with, and p = 0.039 without, APOE adjustment) and EXOC3L2 is affected by correlation with APOE, and thus may represent spurious association. Our results indicate that genetic structure coupled with ascertainment bias resulting from the strong APOE association affect genome-wide results and interpretation of some recently reported associations. We show that a locus such as APOE, with large effects and strong association with disease, can lead to samples that require appropriate adjustment for this locus to avoid both false positive and false negative evidence of association. We suggest that similar adjustments may also be needed for many other large multi-site studies.

Background Alzheimer’s disease (AD) is a chronic progressive neurodegenerative disease impacting an estimated 44 million adults worldwide. The causal pathology of AD (accumulation of amyloid-beta and tau), precedes hallmark symptoms of dementia by more than a decade, necessitating development of early diagnostic markers of disease onset, particularly for new drugs that aim to modify disease processes. To evaluate differentially methylated positions (DMPs) as novel blood-based biomarkers of AD, we used a subset of 653 individuals with peripheral blood (PB) samples in the Alzheimer’s disease Neuroimaging Initiative (ADNI) consortium. The selected cohort of AD, mild cognitive impairment (MCI), and age-matched healthy controls (CN) all had imaging, genetics, transcriptomics, cerebrospinal protein markers, and comprehensive clinical records, providing a rich resource of concurrent multi-omics and phenotypic information on a well-phenotyped subset of ADNI participants. Results In this manuscript, we report cross-diagnosis differential peripheral DNA methylation in a cohort of AD, MCI, and age-matched CN individuals with longitudinal DNA methylation measurements. Epigenome-wide association studies (EWAS) were performed using a mixed model with repeated measures over time with a P value cutoff of 1 × 10 −5 to test contrasts of pairwise differential peripheral methylation in AD vs CN, AD vs MCI, and MCI vs CN. The most highly significant differentially methylated loci also tracked with Mini Mental State Examination (MMSE) scores. Differentially methylated loci were enriched near brain and neurodegeneration-related genes (e.g., BDNF, BIN1, APOC1 ) validated using the genotype tissue expression project portal (GTex). Conclusions Our work shows that peripheral differential methylation between age-matched subjects with AD relative to healthy controls will provide opportunities to further investigate and validate differential methylation as a surrogate of disease. Given the inaccessibility of brain tissue, the PB-associated methylation marks may help identify the stage of disease and progression phenotype, information that would be central to bringing forward successful drugs for AD.

Fetal alcohol spectrum disorders (FASD) describe ethanol-induced developmental defects including craniofacial malformations. While ethanol-sensitive genetic mutations contribute to facial malformations, the impacted cellular mechanisms remain unknown. Signaling via bone morphogenetic protein (Bmp) is a key regulatory step of epithelial morphogenesis driving facial development, providing a possible ethanol-sensitive mechanism. We found that zebrafish carrying mutants for Bmp signaling components are ethanol-sensitive and affect anterior pharyngeal endoderm shape and gene expression, indicating that ethanol-induced malformations of the anterior pharyngeal endoderm cause facial malformations. By integrating FASD patient data, we provide the first evidence that variants of the human Bmp receptor gene BMPR1B associate with ethanol-related differences in jaw volume. Our results show that ethanol exposure disrupts proper morphogenesis of, and tissue interactions between, facial epithelia that mirror overall viscerocranial shape changes and are predictive for Bmp−ethanol associations in human jaw development. Our data provide a mechanistic paradigm linking ethanol to disrupted epithelial cell behaviors that underlie facial defects in FASD.

The Genetics Core of the Alzheimer’s Disease Neuroimaging Initiative (ADNI), formally established in 2009, aims to provide resources and facilitate research related to genetic predictors of multidimensional Alzheimer’s disease (AD)-related phenotypes. Here, we provide a systematic review of genetic studies published between 2009 and 2012 where either ADNI APOE genotype or genome-wide association study (GWAS) data were used. We review and synthesize ADNI genetic associations with disease status or quantitative disease endophenotypes including structural and functional neuroimaging, fluid biomarker assays, and cognitive performance. We also discuss the diverse analytical strategies used in these studies, including univariate and multivariate analysis, meta-analysis, pathway analysis, and interaction and network analysis. Finally, we perform pathway and network enrichment analyses of these ADNI genetic associations to highlight key mechanisms that may drive disease onset and trajectory. Major ADNI findings included all the top 10 AD genes and several of these (e.g., APOE , BIN1 , CLU , CR1 , and PICALM ) were corroborated by ADNI imaging, fluid and cognitive phenotypes. ADNI imaging genetics studies discovered novel findings (e.g., FRMD6 ) that were later replicated on different data sets. Several other genes (e.g., APOC1, FTO, GRIN2B, MAGI2, and TOMM40 ) were associated with multiple ADNI phenotypes, warranting further investigation on other data sets. The broad availability and wide scope of ADNI genetic and phenotypic data has advanced our understanding of the genetic basis of AD and has nominated novel targets for future studies employing next-generation sequencing and convergent multi-omics approaches, and for clinical drug and biomarker development.

Collagen XXII (COL22A1) is a quantitatively minor collagen, which belongs to the family of fibril-associated collagens with interrupted triple helices. Its biological function has been poorly understood. Here, we used a genome-editing approach to generate a loss-of-function mutant in zebrafish col22a1. Homozygous mutant adults exhibit increased incidence of intracranial hemorrhages, which become more prominent with age and after cardiovascular stress. Homozygous col22a1 mutant embryos show higher sensitivity to cardiovascular stress and increased vascular permeability, resulting in a greater percentage of embryos with intracranial hemorrhages. Mutant embryos also exhibit dilations and irregular structure of cranial vessels. To test whether COL22A1 is associated with vascular disease in humans, we analyzed data from a previous study that performed whole-exome sequencing of 45 individuals from seven families with intracranial aneurysms. The rs142175725 single-nucleotide polymorphism was identified, which segregated with the phenotype in all four affected individuals in one of the families, and affects a highly conserved E736 residue in COL22A1 protein, resulting in E736D substitution. Overexpression of human wild-type COL22A1, but not the E736D variant, partially rescued the col22a1 loss-of-function mutant phenotype in zebrafish embryos. Our data further suggest that the E736D mutation interferes with COL22A1 protein secretion, potentially leading to endoplasmic reticulum stress. Altogether, these results argue that COL22A1 is required to maintain vascular integrity. These data further suggest that mutations in COL22A1 could be one of the risk factors for intracranial aneurysms in humans.

APOE e4 has been used to evaluate the risk for Alzheimer's diseases (AD) but there exist other AD risk genes, and their effects can be collectively measured by polygenic risk scores (PRS). In this study, we sought to use both PRS (APOE excluded) and APOE e4 to evaluate the AD risk. The discovery dataset was meta-analysis of three large-scale European ancestry AD GWAS (Kunkle et al, 2019, the UK Biobank, and the FinnGen consortium). SNPs within 500Kb from transcript starting and ending sites of APOE were excluded. PRS-CS was used to calculate PRS. Target datasets were European ancestry samples from NIA Alzheimer's disease centers (ADC, 2,413 cases and 3,423 controls) and All of Us research program (AOU, 1,177 cases and 60,607 controls). Participants having age at onset (cases) or age at the last interview (controls) <60 were excluded. The prevalence of AD were higher in ADC (41.35%) and lower in AOU (1.91%) than those in general populations; therefore, we combined ADC and AOU samples to approximate the PRS distribution in general populations. Then we dichotomized PRS as high (highest 10%) and other (the remaining 90%) based on the PRS distribution of combined sample. Cox proportional hazard model was used to test the effects of dichotomized PRS and e4 genotypes (0, 1, and 2 copies of e4 alleles) by adjusting for sex. Additionally, we performed sex stratified analyses in ADC only as numbers of high PRS and e4/e4 carriers in AOU in either sex were <5. Results are summarized in Table 1. We used those having other PRS and no e4 as the reference group. In both ADC and AOU, high PRS or e4 were significantly associated with the AD risk (PRS hazard ratios (HRs): 1.31-1.74, P-values≤0.01; e4 HRs: 1.39-8.68, P-values≤2.80E-06) but having both substantially increased the AD risk (HRs: 2.07-14.26, P-values≤6.80E-05). In ADC, both high PRS and e4 had larger effects in females than in males. The effects of PRS were modest and cannot be used alone to evaluate the AD risk; however, PRS can potentially be used with APOE e4 to evaluate the AD risk.

Despite substantial progress, causal variants are identified only for a minority of familial Parkinson’s disease (PD) cases, leaving high-risk pathogenic variants unidentified 1 , 2 . To identify such variants, we uniformly processed exome sequencing data of 2,184 index familial PD cases and 69,775 controls. Exome-wide analyses converged on RAB32 as a novel PD gene identifying c.213C > G/p.S71R as a high-risk variant presenting in ~0.7% of familial PD cases while observed in only 0.004% of controls (odds ratio of 65.5). This variant was confirmed in all cases via Sanger sequencing and segregated with PD in three families. RAB32 encodes a small GTPase known to interact with LRRK2 (refs. 3 , 4 ). Functional analyses showed that RAB32 S71R increases LRRK2 kinase activity, as indicated by increased autophosphorylation of LRRK2 S1292. Here our results implicate mutant RAB32 in a key pathological mechanism in PD—LRRK2 kinase activity 5 – 7 —and thus provide novel insights into the mechanistic connections between RAB family biology, LRRK2 and PD risk. Analysis of exome sequencing data identifies a missense variant in RAB32 associated with high risk of familial Parkinson’s disease. Functional studies show that this risk variant increases LRRK2 kinase activity.

As sequencing technologies allow for faster and easier access to genomic data, the incorporation of genetic information into research studies has become more routine. In conjunction, there is a push to return relevant genetic results to research participants. However, there are many considerations for genetic results return that may not apply to other medical information including CLIA-certification, genetic counseling, and familial implications. This project aims to create a scalable protocol for genetic results return that can be implemented into an existing project. The Alzheimer's disease Family-Based study (AD-FBS) started in 2003 and has recruited over 1,400 families. While research genetic sequencing was always a study component, participants did not historically receive these results. In the study's newest phase, six genes associated with autosomal dominant dementia (APP, PSEN1, PSEN2, MAPT, GRN, C9orf72) are returned to participants with early-onset dementia. The protocol consists of four components: (1) recruitment and consent (2) pre-test genetic counseling (3) genetic testing (4) result disclosure and post-test counseling. Eligibility criteria is a clinical diagnosis of AD or MCI and age of onset 65 or under. Since the FBS study recruits multiplex families, one individual (person with the earliest onset) per family is eligible for results disclosure. Informed consent is a key part of the protocol and the participant (or health care proxy) can decline the return of results at any point before results disclosure. Participants are provided with pre- and post-test genetic counseling to thoroughly understand the context of testing and implications of results for themselves and their family members. Genetic testing is performed at a CLIA-certified lab and positive, negative, and variants of uncertain significance are returned. Participants are given surveys before and after results disclosure to assess the psychosocial impact of the genetic testing process. Materials developed include a participant-facing handout on dementia genetic testing, genetic result return consent form, pre- and post- questionnaires, and genetic results letters. Preliminary responses indicate participants are generally satisfied with the genetic testing process, regardless of the result received. Our project demonstrates the feasibility of incorporating genetic results return in a pre-existing research study.