The definition of precision medicine in neurodegenerative disorders and the one disease-many diseases tension

Precision medicine is a patient-centered approach that aims to translate new knowledge to optimize the type and timing of interventions for the greatest benefit to individual patients. There is considerable interest in applying this approach to treatments designed to slow or halt the progression of neurodegenerative diseases. Indeed, effective disease-modifying treatment (DMT) remains the greatest unmet therapeutic need in this field. In contrast to the enormous progress in oncology, precision medicine in the field of neurodegeneration faces multiple challenges. These are related to major limitations in our understanding of many aspects of the diseases. A critical barrier to advances in this field is the question of whether the common sporadic neurodegenerative diseases (of the elderly) are single uniform disorders (particularly related to their pathogenesis) or whether they represent a collection of related but still very distinct disease states. In this chapter, we briefly touch on lessons from other fields of medicine that might be applied to the development of precision medicine for DMT in neurodegenerative diseases. We discuss why DMT trials may have failed to date, and particularly the importance of appreciating the multifaceted nature of disease heterogeneity and how this has and will impact on these efforts. We conclude with comments on how we can move from this complex disease heterogeneity to the successful application of precision medicine principles in DMT for neurodegenerative diseases.

Embracing Monogenic Parkinson's Disease: The MJFF Global Genetic PD Cohort

BACKGROUND

As gene-targeted therapies are increasingly being developed for Parkinson's disease (PD), identifying and characterizing carriers of specific genetic pathogenic variants is imperative. Only a small fraction of the estimated number of subjects with monogenic PD worldwide are currently represented in the literature and availability of clinical data and clinical trial-ready cohorts is limited.

OBJECTIVE

The objectives are to (1) establish an international cohort of affected and unaffected individuals with PD-linked variants; (2) provide harmonized and quality-controlled clinical characterization data for each included individual; and (3) further promote collaboration of researchers in the field of monogenic PD.

METHODS

We conducted a worldwide, systematic online survey to collect individual-level data on individuals with PD-linked variants in SNCA, LRRK2, VPS35, PRKN, PINK1, DJ-1, as well as selected pathogenic and risk variants in GBA and corresponding demographic, clinical, and genetic data. All registered cases underwent thorough quality checks, and pathogenicity scoring of the variants and genotype-phenotype relationships were analyzed.

RESULTS

We collected 3888 variant carriers for our analyses, reported by 92 centers (42 countries) worldwide. Of the included individuals, 3185 had a diagnosis of PD (ie, 1306 LRRK2, 115 SNCA, 23 VPS35, 429 PRKN, 75 PINK1, 13 DJ-1, and 1224 GBA) and 703 were unaffected (ie, 328 LRRK2, 32 SNCA, 3 VPS35, 1 PRKN, 1 PINK1, and 338 GBA). In total, we identified 269 different pathogenic variants; 1322 individuals in our cohort (34%) were indicated as not previously published.

CONCLUSIONS

Within the MJFF Global Genetic PD Study Group, we (1) established the largest international cohort of affected and unaffected individuals carrying PD-linked variants; (2) provide harmonized and quality-controlled clinical and genetic data for each included individual; (3) promote collaboration in the field of genetic PD with a view toward clinical and genetic stratification of patients for gene-targeted clinical trials. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Initiating dopamine agonists rather than levodopa in early Parkinson's disease does not delay the need for deep brain stimulation

BACKGROUND

While levodopa is the most effective symptomatic treatment for Parkinson's disease (PD), its use is associated with an increased risk of motor complications (MC) in the first five years of treatment compared to dopamine agonist (DA) first therapy. It is not known whether this translates into true benefit later in the disease. We aimed to determine whether there is a difference in the time between initial levodopa vs DA treatment and the development of disabling MC prompting DBS consideration.

METHODS

This was a retrospective cohort study of PD patients attending the DBS Clinic at Toronto Western Hospital, Canada between 03/2004-02/2022 who underwent globus pallidus interna (GPi) or subthalamic nucleus (STN) DBS in 2005 or later for disabling MC.

RESULTS

Of 438 patients included, 352 patients underwent STN DBS, 86 GPi. The median disease duration was 9 years (2-30). The majority (n=312) received levodopa first and 126 a DA. There was no significant difference in the disease duration, or amantadine use between the two groups. The duration first treatment-assessment for DBS (L-dopa median 8, IQR 4; DA median 9, IQR 4) or DBS surgery (levodopa median 10, IQR 5; DA median 10, IQR 5), did not differ.

CONCLUSION

To our knowledge this is the only study to date to evaluate the duration between L-dopa/DA first treatment and the development of MC of sufficient severity to warrant consideration of DBS. No association was found. The results suggest that the development of disabling MC warranting DBS is independent of the type of first dopaminergic treatment.

Cathepsin B p.Gly284Val Variant in Parkinson’s Disease Pathogenesis

Parkinson’s disease (PD) is generally considered a sporadic disorder, but a strong genetic background is often found. The aim of this study was to identify the underlying genetic cause of PD in two affected siblings and to subsequently assess the role of mutations in Cathepsin B (CTSB) in susceptibility to PD. A typical PD family was identified and whole-exome sequencing was performed in two affected siblings. Variants of interest were validated using Sanger sequencing. CTSB p.Gly284Val was genotyped in 2077 PD patients and 615 unrelated healthy controls from the Czech Republic, Ireland, Poland, Ukraine, and the USA. The gene burden analysis was conducted for the CTSB gene in an additional 769 PD probands from Mayo Clinic Florida familial PD cohort. CTSB expression and activity in patient-derived fibroblasts and controls were evaluated by qRT-PCR, western blot, immunocytochemistry, and enzymatic assay. The CTSB p.Gly284Val candidate variant was only identified in affected family members. Functional analysis of CTSB patient-derived fibroblasts under basal conditions did not reveal overt changes in endogenous expression, subcellular localization, or enzymatic activity in the heterozygous carrier of the CTSB variant. The identification of the CTSB p.Gly284Val may support the hypothesis that the CTSB locus harbors variants with differing penetrance that can determine the disease risk.

Neuroimaging Pearls from the MDS Congress Video Challenge. Part 1: Genetic Disorders

We selected several "imaging pearls" presented during the Movement Disorder Society (MDS) Video Challenge for this review. While the event, as implicated by its name, was video-centered, we would like to emphasize the important role of imaging in making the correct diagnosis. We divided this anthology into two parts: genetic and acquired disorders. Genetic cases described herein were organized by the inheritance pattern and the focus was put on the imaging findings and differential diagnoses. Despite the overlapping phenotypes, certain described disorders have pathognomonic MRI brain findings that would provide either the "spot" diagnosis or result in further investigations leading to the diagnosis. Despite this, the diagnosis is often challenging with a broad differential diagnosis, and hallmark findings may be present for only a limited time.

Ondine's Curse in Frontotemporal Dementia with Parkinsonism Linked to Chromosome 17 Caused by MAPT Variants

Background

"Ondine's curse" or central hypoventilation, induces an apparently spontaneous failure of automatic respiratory drive, henceforth necessitating a conscious effort to breathe and sleep induced hypoventilation. It is typically seen in congenital central hypoventilation syndrome, but may be acquired.

Objectives

To highlight Ondine's curse as part of frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17) secondary to microtubule associated protein tau (MAPT) variants.

Methods

We describe the clinical and neuropathological findings in two patients with fatal Ondine's curse associated with FTDP-17 and secondary to MAPT variants (FTDP-17t). We discuss neuroanatomical correlates. We review two prior reports of central hypoventilation associated with MAPT variants suggesting that Ondine's curse occurs uncommonly in FTDP-17t.

Results

Despite variants affecting different regions of MAPT and a degree of heterogeneity in pathological findings, the patients reviewed all experienced central hypoventilation during their disease course.

Conclusion

Tauopathy should be considered in patients with adult-onset Ondine's curse.

A clinical, molecular genetics and pathological study of a FTDP-17 family with a heterozygous splicing variant c.823-10G>T at the intron 9/exon 10 of the MAPT gene

We report the first clinical-radiological-genetic-molecular-pathological study of a kindred with c.823-10G>T MAPT intronic variant (rs63749974) associated with frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). We describe the clinical spectrum within this family and emphasize the association between MAPT gene variants and motor neuron disease. This report of a second family with FTDP-17 associated with c.823-10G>T MAPT variant strongly supports pathogenicity of the variant and confirms it is a 4-repeat (4R) tauopathy. This intronic point mutation, probably strengthens the polypyrimidine tract and alters the splicing of exon 10 (10 nucleotides into intron 9) close to the 3' splice site.

Association Between Glucocerebrosidase Mutations and Parkinson's Disease in Ireland

Multiple studies implicate heterozygous GBA mutations as a major genetic risk factor for Parkinson's disease (PD); however, the frequency of mutations has never been examined in PD patients from the Irish population. We prospectively recruited 314 unrelated Irish PD patients (UK Brain Bank Criteria) and 96 Irish healthy controls (without any signs or family history of parkinsonism) attending. The Dublin Neurological Institute (DNI). Complete exon GBA Sanger sequencing analysis with flanking intronic regions was performed. The GBA carrier frequency was 8.3% in PD and 3.1% in controls. We identified a number of potentially pathogenic mutations including a p.G195E substitution and a p.G377C variant, previously described in a case study of Gaucher's disease in Ireland. On genotype–phenotype assessment hallucinations, dyskinesia, and dystonia were more prevalent in GBA-PD. The genetic etiology of PD in Ireland differs from the continental Europe as seen with the lower LRRK2 and higher than in most European countries GBA mutation frequency. Determining genetic risk factors in different ethnicities will be critical for future personalized therapeutic approach.

Extending the Phenotypic Spectrum Associated with STUB1 Mutations: A Case of Dystonia

BACKGROUND

Mutations in the STIP1 homology and U-box containing protein 1 gene were first described in 2013 and lead to disorders with symptoms including ataxia and dysarthria, such as spinocerebellar autosomal-recessive ataxia type 16 (SCAR16), Gordon-Holmes syndrome, and spinocerebellar ataxia type 48. There have been 15 families described to date with SCAR16.

CASES

We describe a 45-year-old right-handed woman with dysarthria, ataxia, and cervical dystonia with SCAR16 with 2 compound heterozygous variants in the STIP1 homology and U-box containing protein 1 gene, and a family history significant for her 47-year-old sister with dysarthria and cognitive problems.

CONCLUSION

We present a comprehensive overview of the phenotypic data of all 15 families with SCAR16 and expand the phenotype by describing a third patient with SCAR16 and dystonia reported to date in the literature.

The benefits of a Neurogenetics clinic in an adult Academic Teaching Hospital

Genetics is the backbone of Neurology, where a number of disorders have a genetic aetiology and are complex, requiring a dedicated Neurogenetics clinic. Genetics in the Republic of Ireland is under-resourced, with the lowest number of consultants per million of population in Europe. In November 2014, we established the monthly adult Neurogenetics clinic in Ireland, staffed by 2 consultants and 2 registrars from each speciality. We see patients with complex rare neurological conditions that may potentially have an underlying genetic basis, in the presence or absence of a family history. We performed a retrospective cohort analysis, reviewing symptoms and work-up data. Twenty-seven patients attended a pilot clinic over 12 months. Conditions encountered included Parkin-related PD, leucodystrophy, ataxia, fronto-temporal lobar degeneration, spinocerebellar ataxia type 6 (SCA6) and ataxia-telangiectasia. Identification of pathogenic mutations directed screening, treatment and facilitated onward genetic counselling (n = 10, 33%). A number of novel mutations were identified in MAPT gene ("missing tau mutation" McCarthy et al., Brain, 2015), SLCA1 gene and GRN (progranulin). Phenotypic features not previously reported were seen; e.g. writer's cramp in SCA6; paroxysmal myoclonus in the glucose transporter protein type 1 (GLUT1) deficiency. Breast cancer screening for ATM mutations carriers and referral to international experts in two undiagnosed patients were arranged. The establishment of a Neurogenetics clinic has addressed a gap in service and allowed identification of rare and atypical diagnoses.

Loss of visual feedback revealing motor impairment - an early symptom of Parkinson's disease in two Irish farmers

Background

In the absence of visual feedback, humans depend upon proprioceptive information for reaching movements and coordination. Use of sensory information in order to assist movement is impaired in patients with early Parkinson’s disease (PD). It has been postulated that patients with PD compensate for this kinaesthetic deficit by relying on visual information.

Case presentation

We report two farmers who first noticed symptoms of PD when working on the farm in situations requiring processing of the proprioceptive/kinaesthetic information in order to execute motor output in the absence of visual cues. A 68-year-old right-handed farmer had a 5-year history of left hand awkwardness. He first noticed the problem while performing artificial insemination in cattle using the recto-vaginal technique. He was diagnosed with PD 15 months after his initial symptoms and responded well to a combination of carbidopa-levodopa and ropinirole but has not returned to performing artificial insemination since. Clinical examination revealed asymmetric parkinsonism with normal sensation on gross neurological examination (including proprioception). A 60-year-old right-handed farmer had a 6-year history of difficulty manoeuvring his right hand whilst turning the calf during calving only when he did not have direct sight of his hand. 18 months later he developed right hand tremor and bradykinesia. He was diagnosed with PD 2 years following these initial symptoms. He had a good response to a combination of carbidopa-levodopa, rasagiline and ropinirole. He switched to using his left hand during calf delivery but is not as dextrous as previously. Clinical examination revealed parkinsonism, more marked in the right hand and normal sensation in all modalities.

Conclusions

Although task-specific motor impairment could explain the symptoms of these patients, it is noteworthy that loss of visual feedback was central to both of these presentations. Given that early kinaesthetic deficits are known to be present in patients with PD, we postulate that removing visual feedback can expose such deficits in early PD, even when not detectable on routine examination. These two patients suggest that sensorimotor control can be impaired early in PD and may be the first symptom. Once the visual compensatory mechanism is not available, it becomes difficult or impossible to perform complex hand movements.

Novel gene (TMEM230) linked to Parkinson's disease

Mutations in six genes are known to cause Parkinson’s disease (PD) (autosomal dominant: alpha-synuclein, LRRK2, VPS35 and autosomal recessive: Parkin, PINK1 and DJ1) and number of other genes are implicated. In a recent article Deng and colleagues studied a large four generation American family of European descent and linked mutations in a novel gene, transmembrane-protein 230 gene (TMEM230) with lewy body confirmed PD. The authors demonstrated that pathogenic TMEM230 variants in primary mouse neurons affected movement of synaptic vesicles suggesting that TMEM230 may slow vesicular transport. Further experiments in HEK293 cells (carrying the pathogenic TMEM230 variants) showed increased alpha-synuclein levels. This study indicated that the impaired vesicular trafficking may contribute to the pathogenesis of PD. Understanding the various cellular mechanisms leading to PD may lead to the development of novel, much needed therapeutic options. These mechanisms could include: enhanced clearance of damaged mitochondria, development of kinase inhibitors, VPS35/retromer function enhancers or now the possibility of vesicular transport modification.

Closing the tau loop: the missing tau mutation

Frontotemporal lobar degeneration comprises a group of disorders characterized by behavioural, executive, language impairment and sometimes features of parkinsonism and motor neuron disease. In 1994 we described an Irish-American family with frontotemporal dementia linked to chromosome 17 associated with extensive tau pathology. We named this disinhibition-dementia-parkinsonism-amyotrophy complex. We subsequently identified mutations in the MAPT gene. Eleven MAPT gene splice site stem loop mutations were identified over time except for 5' splice site of exon 10. We recently identified another Irish family with autosomal dominant early amnesia and behavioural change or parkinsonism associated with the 'missing' +15 mutation at the intronic boundary of exon 10. We performed a clinical, neuropsychological and neuroimaging study on the proband and four siblings, including two affected siblings. We sequenced MAPT and performed segregation analysis. We looked for a biological effect of the tau variant by performing real-time polymerase chain reaction analysis of RNA extracted from human embryonic kidney cells transfected with exon trapping constructs. We found a c.915+15A>C exon 10/intron 10 stem loop mutation in all affected subjects but not in the unaffected. The c.915+15A>C variant caused a shift in tau splicing pattern to a predominantly exon 10+ pattern presumably resulting in predominant 4 repeat tau and little 3 repeat tau. This strongly suggests that the c.915+15A>C variant is a mutation and that it causes frontotemporal dementia linked to chromosome 17 in this pedigree by shifting tau transcription and translation to +4 repeat tau. Tau (MAPT) screening should be considered in families where amnesia or atypical parkinsonism coexists with behavioural disturbance early in the disease process. We describe the final missing stem loop tau mutation predicted 15 years ago. Mutations have now been identified at all predicted sites within the 'stem' when the stem-loop model was first proposed and no mutations have been found within the 'loop' region as expected. Therefore we 'close the tau loop' having 'opened the loop' 21 years ago.