Faculty Bibliography

OBJECTIVE: Age at onset of diagnostic motor manifestations in Huntington disease (HD) is strongly correlated with an expanded CAG trinucleotide repeat. The length of the normal CAG repeat allele has been reported also to influence age at onset, in interaction with the expanded allele. Due to profound implications for disease mechanism and modification, we tested whether the normal allele, interaction between the expanded and normal alleles, or presence of a second expanded allele affects age at onset of HD motor signs. METHODS: We modeled natural log-transformed age at onset as a function of CAG repeat lengths of expanded and normal alleles and their interaction by linear regression. RESULTS: An apparently significant effect of interaction on age at motor onset among 4,068 subjects was dependent on a single outlier data point. A rigorous statistical analysis with a well-behaved dataset that conformed to the fundamental assumptions of linear regression (e.g., constant variance and normally distributed error) revealed significance only for the expanded CAG repeat, with no effect of the normal CAG repeat. Ten subjects with 2 expanded alleles showed an age at motor onset consistent with the length of the larger expanded allele. CONCLUSIONS: Normal allele CAG length, interaction between expanded and normal alleles, and presence of a second expanded allele do not influence age at onset of motor manifestations, indicating that the rate of HD pathogenesis leading to motor diagnosis is determined by a completely dominant action of the longest expanded allele and as yet unidentified genetic or environmental factors.

OBJECTIVE: Genome-wide association (GWAS) methods have identified genes contributing to Parkinson's disease (PD); we sought to identify additional genes associated with PD susceptibility. METHODS: A 2-stage design was used. First, individual level genotypic data from 5 recent PD GWAS (Discovery Sample: 4,238 PD cases and 4,239 controls) were combined. Following imputation, a logistic regression model was employed in each dataset to test for association with PD susceptibility and results from each dataset were meta-analyzed. Second, 768 single-nucleotide polymorphisms (SNPs) were genotyped in an independent Replication Sample (3,738 cases and 2,111 controls). RESULTS: Genome-wide significance was reached for SNPs in SNCA (rs356165; G: odds ratio [OR]=1.37; p=9.3x10(-21)), MAPT (rs242559; C: OR=0.78; p=1.5x10(-10)), GAK/DGKQ (rs11248051; T: OR=1.35; p=8.2x10(-9)/rs11248060; T: OR=1.35; p=2.0x10(-9)), and the human leukocyte antigen (HLA) region (rs3129882; A: OR=0.83; p=1.2x10(-8)), which were previously reported. The Replication Sample confirmed the associations with SNCA, MAPT, and the HLA region and also with GBA (E326K; OR=1.71; p=5x10(-8) Combined Sample) (N370; OR=3.08; p=7x10(-5) Replication sample). A novel PD susceptibility locus, RIT2, on chromosome 18 (rs12456492; p=5x10(-5) Discovery Sample; p=1.52x10(-7) Replication sample; p=2x10(-10) Combined Sample) was replicated. Conditional analyses within each of the replicated regions identified distinct SNP associations within GBA and SNCA, suggesting that there may be multiple risk alleles within these genes. INTERPRETATION: We identified a novel PD susceptibility locus, RIT2, replicated several previously identified loci, and identified more than 1 risk allele within SNCA and GBA.

BACKGROUND: Essential tremor (ET) is among the most misdiagnosed neurological diseases, and overdiagnosis is especially common. As many as 30-50% of supposed 'ET' cases have other diagnoses, with dystonia prominent among these. Therefore, the recognition of differences in tremor phenomenology has potential diagnostic value for practising clinicians. There is an anecdotal sense that head tremor in ET is positional rather than resting, a feature which could aid in the diagnosis, yet no published data exist. METHODS: In this cross-sectional study of ET and spasmodic torticollis (ST) patients (3:1 matching) who had head tremor while upright, the prevalence of supine (ie, resting) head tremor was compared. RESULTS: There were 60 ET cases and 19 ST cases with head tremor while seated. When supine, head tremor persisted in only 5/60 (8.3%) ET versus 13/19 (68.4%) ST cases (p<0.001), indicating that essential head tremor is more likely to resolve in the supine position than is the head tremor of ST. Supine head tremor, when present in ET, did not seem to preferentially occur in patients with more severe disease (p>0.05). CONCLUSIONS: These results indicate that there is some validity to the anecdotal sense that head tremor in ET is a postural tremor that dissipates when a patient lies down; by contrast, in ST, head tremor more often persists. In a clinical context, these results have potential implications in diagnostically ambiguous patients with head tremor. Physicians should consider asking their tremor patients to lie down to assess whether head tremor resolves.

Paroxysmal kinesigenic dyskinesia with infantile convulsions (PKD/IC) is an episodic movement disorder with autosomal-dominant inheritance and high penetrance, but the causative genetic mutation is unknown. We have now identified four truncating mutations involving the gene PRRT2 in the vast majority (24/25) of well-characterized families with PKD/IC. PRRT2 truncating mutations were also detected in 28 of 78 additional families. PRRT2 encodes a proline-rich transmembrane protein of unknown function that has been reported to interact with the t-SNARE, SNAP25. PRRT2 localizes to axons but not to dendritic processes in primary neuronal culture, and mutants associated with PKD/IC lead to dramatically reduced PRRT2 levels, leading ultimately to neuronal hyperexcitability that manifests in vivo as PKD/IC.

BACKGROUND: Careful characterization of the phenotype and genotype of Huntington disease (HD) can foster better understanding of the condition. METHODS: We conducted a cohort study in the United States, Canada, and Australia of members of families affected by HD. We collected demographic and clinical data, conducted the Unified Huntington's Disease Rating Scale and Mini-Mental State Examination, and determined Huntingtin trinucleotide CAG repeat length. We report primarily on cross-sectional baseline data from this recently completed prospective, longitudinal, observational study. RESULTS: As of December 31, 2009, 2,318 individuals enrolled; of these, 1,985 (85.6%) were classified into six analysis groups. Three groups had expanded CAG alleles (36 repeats or more): individuals with clinically diagnosed HD [n = 930], and clinically unaffected first-degree relatives who had previously pursued [n = 248] or not pursued [n = 112] predictive DNA testing. Three groups lacked expanded alleles: first-degree relatives who had previously pursued [n = 41] or not pursued [n = 224] genetic testing, and spouses and caregivers [n = 430]. Baseline mean performance differed across groups in all motor, behavioral, cognitive, and functional measures (p<0.001). Clinically unaffected individuals with expanded alleles weighed less (76.0 vs. 79.6 kg; p = 0.01) and had lower cognitive scores (28.5 vs. 29.1 on the Mini Mental State Examination; p = 0.008) than individuals without expanded alleles. The frequency of "high normal" repeat lengths (27 to 35) was 2.5% and repeat lengths associated with reduced penetrance (36 to 39) was 2.7%. CONCLUSION: Baseline analysis of COHORT study participants revealed differences that emerge prior to clinical diagnosis. Longitudinal investigation of this cohort will further characterize the natural history of HD and genetic and biological modifiers. TRIAL REGISTRATION: Clinicaltrials.gov NCT00313495.

BACKGROUND: Mutations in parkin are a known genetic risk factor for early onset Parkinson's disease (EOPD) but their role in non-motor manifestations is not well established. Genetic factors for depression are similarly not well characterized. We investigate the role of parkin mutations in depression among those with EOPD and their relatives. METHODS: We collected psychiatric information using the Patient Health Questionnaire and Beck Depression Inventory II on 328 genotyped individuals including 88 probands with early onset PD (41 with parkin mutations, 47 without) and 240 first and second-degree relatives without PD. RESULTS: Genotype was not associated with depression risk among probands. Among unaffected relatives of EOPD cases, only compound heterozygotes (n = 4), and not heterozygotes, had significantly increased risk of depressed mood (OR = 14.1; 95% CI 1.2-163.4), moderate to severe depression (OR = 17.8; 95% CI 1.0-332.0), depression (score >/= 15) on the Beck Depression Inventory II (BDI-II) (OR = 51.9; 95% CI 4.1-657.4), and BDI-II total depression score (beta = 8.4; 95% CI 2.4-11.3) compared to those without parkin mutations. CONCLUSIONS: Relatives of EOPD cases with compound heterozygous mutations and without diagnosed PD may have a higher risk of depression compared to relatives without parkin mutations. These findings support evidence of a genetic contribution to depression and may extend the phenotypic spectrum of parkin mutations to include non-motor manifestations that precede the development of PD.

OPINION STATEMENT: Sleep disturbance is an important feature of Parkinson's disease (PD) that deserves clinical attention. Various disorders need to be considered and treatment should be customized to the patient's specific symptoms and lifestyle. Evaluation of a PD patient complaining of difficulty sleeping begins with a detailed history from the patient and bed partner about the specific problem the patient is experiencing. It is important to inquire about difficulties with sleep onset, frequent awakenings, increased movements during sleep, acting out dreams, uncomfortable motor symptoms, disordered breathing, and nocturia. Current medications should be reviewed to determine whether the recent addition of a drug or a change in dose may be contributing to sleep difficulties, and one should ask about daytime sleepiness, frequent daytime naps, and sleep hygiene in the evening that can contribute to problems sleeping at night. The goal of therapy is to restore quality nighttime sleep without excessive daytime sedation and to improve the patient's daily mental and physical function and overall quality of life.

OBJECTIVE: The lack of generally accepted diagnostic criteria for neuroleptic malignant syndrome (NMS) impedes research and clinical management of patients receiving antipsychotic medications. The purpose of this study was to develop NMS diagnostic criteria reflecting a broad consensus among clinical knowledge experts, represented by an international multispecialty physician panel. PARTICIPANTS: Eleven psychiatrists, 2 neurologists, 2 anesthesiologists, and 2 emergency medicine specialists participated in a formal Delphi consensus procedure. EVIDENCE: A core bibliography consisting of 12 prominent, current reviews of the NMS literature was identified by an objective, comprehensive electronic search strategy. Each panel member was given a copy of these references and asked to examine them before commencing the survey process. CONSENSUS PROCESS: After reviewing the core bibliography, panel members were asked to list any clinical signs or symptoms or diagnostic studies that they believed, on the basis of their knowledge and clinical experience, were useful in making a diagnosis of NMS. In subsequent survey rounds, panel members assigned priority points to these items, and items that failed to receive a minimum priority score were eliminated from the next round. Information about individual panel member responses was fed back to the group anonymously in the form of the group median or mean and the number of members who had ranked or scored each survey item. The a priori consensus endpoint was defined operationally as a change of 10% or less in the mean priority score for any individual item, and an average absolute value change of 5% or less across all items, between consecutive rounds. The survey was conducted from January 2009 through September 2009. RESULTS: Consensus was reached on the fifth round regarding the following criteria: recent dopamine antagonist exposure, or dopamine agonist withdrawal; hyperthermia; rigidity; mental status alteration; creatine kinase elevation; sympathetic nervous system lability; tachycardia plus tachypnea; and a negative work-up for other causes. The panel also reached a consensus on the relative importance of these criteria and on the following critical values for quantitative criteria: hyperthermia, > 100.4 degrees F or > 38.0 degrees C on at least 2 occasions; creatine kinase elevation, at least 4 times the upper limit of normal; blood pressure elevation, >/= 25% above baseline; blood pressure fluctuation, >/= 20 mm Hg (diastolic) or >/= 25 mm Hg (systolic) change within 24 hours; tachycardia, >/= 25% above baseline; and tachypnea, >/= 50% above baseline. CONCLUSIONS: These diagnostic criteria significantly advance the field because they represent the consensus of an international multispecialty expert panel, include critical values, provide guidance regarding the relative importance of individual elements, and are less influenced by particular theoretical biases than most previously published criteria. They require validation before being applied in clinical settings.

BACKGROUND: To date, nine Parkinson disease (PD) genome-wide association studies in North American, European and Asian populations have been published. The majority of studies have confirmed the association of the previously identified genetic risk factors, SNCA and MAPT, and two studies have identified three new PD susceptibility loci/genes (PARK16, BST1 and HLA-DRB5). In a recent meta-analysis of datasets from five of the published PD GWAS an additional 6 novel candidate genes (SYT11, ACMSD, STK39, MCCC1/LAMP3, GAK and CCDC62/HIP1R) were identified. Collectively the associations identified in these GWAS account for only a small proportion of the estimated total heritability of PD suggesting that an 'unknown' component of the genetic architecture of PD remains to be identified. METHODS: We applied a GWAS approach to a relatively homogeneous Ashkenazi Jewish (AJ) population from New York to search for both 'rare' and 'common' genetic variants that confer risk of PD by examining any SNPs with allele frequencies exceeding 2%. We have focused on a genetic isolate, the AJ population, as a discovery dataset since this cohort has a higher sharing of genetic background and historically experienced a significant bottleneck. We also conducted a replication study using two publicly available datasets from dbGaP. The joint analysis dataset had a combined sample size of 2,050 cases and 1,836 controls. RESULTS: We identified the top 57 SNPs showing the strongest evidence of association in the AJ dataset (p < 9.9 x 10(-5)). Six SNPs located within gene regions had positive signals in at least one other independent dbGaP dataset: LOC100505836 (Chr3p24), LOC153328/SLC25A48 (Chr5q31.1), UNC13B (9p13.3), SLCO3A1(15q26.1), WNT3(17q21.3) and NSF (17q21.3). We also replicated published associations for the gene regions SNCA (Chr4q21; rs3775442, p = 0.037), PARK16 (Chr1q32.1; rs823114 (NUCKS1), p = 6.12 x 10(-4)), BST1 (Chr4p15; rs12502586, p = 0.027), STK39 (Chr2q24.3; rs3754775, p = 0.005), and LAMP3 (Chr3; rs12493050, p = 0.005) in addition to the two most common PD susceptibility genes in the AJ population LRRK2 (Chr12q12; rs34637584, p = 1.56 x 10(-4)) and GBA (Chr1q21; rs2990245, p = 0.015). CONCLUSIONS: We have demonstrated the utility of the AJ dataset in PD candidate gene and SNP discovery both by replication in dbGaP datasets with a larger sample size and by replicating association of previously identified PD susceptibility genes. Our GWAS study has identified candidate gene regions for PD that are implicated in neuronal signalling and the dopamine pathway.