Shady Rahayel

Background: Dementia with Lewy bodies shares clinical and pathological features with both Parkinson’s disease and Alzheimer’s disease, but the local biological factors that render specific cortical regions vulnerable to atrophy remain poorly defined. In particular, it is unclear whether cortical thinning in dementia with Lewy bodies reflects generic neurodegenerative mechanisms, processes shared with Parkinson’s disease and Alzheimer’s disease, or dementia with Lewy bodies-specific molecular and network susceptibilities. Methods: A total of 89 patients with dementia with Lewy bodies and 89 matched controls underwent T1-weighted brain MRI. Scans were processed to generate surface-based cortical thickness maps. Regional cortical thickness estimates, after slice-by-slice manual correction, were mapped to gene expression data from healthy postmortem human brains to identify transcriptomic signatures associated with decreased thickness in dementia with Lewy bodies. We assessed whether genes whose expression was increased with regional thinning converged onto established Parkinson’s disease- and Alzheimer’s disease-related pathways and identified genes uniquely implicated in dementia with Lewy bodies. Spatial annotation mapping was then used to test whether patterns of cortical thinning overlapped with in vivo neurotransmitter system distributions and whether the observed thickness pattern was constrained by large-scale structural connectivity, consistent with a network-based propagation process. Results: Cortical thinning predominated in regions that, in the healthy brain, show higher expression of genes involved in mitochondrial function and synaptic transmission. The transcriptomic profile associated with thinning significantly overlapped with genes belonging to Parkinson’s disease and Alzheimer’s disease pathways, supporting shared pathogenic mechanisms across Lewy body- and Alzheimer-type neurodegeneration. However, 90 genes associated with cortical thinning did not overlap with Parkinson’s disease or Alzheimer’s disease pathways and were enriched for GABAergic signalling. Spatial mapping analyses showed that regions with greatest thickness reductions colocalized with GABA _A , serotoninergic 5-HT _1A , 5-HT _1B , 5-HT4, and dopaminergic D2 receptor distributions, and that the thickness pattern followed structural connectivity. Conclusions: MRI-derived cortical thickness changes in dementia with Lewy bodies reflect selective molecular and network vulnerabilities rather than a non-specific degenerative process. Mitochondrial and synaptic genes, together with a distinct GABAergic association and connectivity constraints, delineate mechanisms explaining why some cortical territories are more affected in dementia with Lewy bodies.

BACKGROUND: Dementia with Lewy bodies shares clinical and pathological features with both Parkinson’s disease and Alzheimer’s disease, but the local biological factors that render specific cortical regions vulnerable to atrophy remain poorly defined. In particular, it is unclear whether cortical thinning in dementia with Lewy bodies reflects generic neurodegenerative mechanisms, processes shared with Parkinson’s disease and Alzheimer’s disease, or dementia with Lewy bodies-specific molecular and network susceptibilities. METHODS: A total of 89 patients with dementia with Lewy bodies and 89 matched controls underwent T1-weighted brain MRI. Scans were processed to generate surface-based cortical thickness maps. Regional cortical thickness estimates, after slice-by-slice manual correction, were mapped to gene expression data from healthy postmortem human brains to identify transcriptomic signatures associated with decreased thickness in dementia with Lewy bodies. We assessed whether genes whose expression was increased with regional thinning converged onto established Parkinson’s disease- and Alzheimer’s disease-related pathways and isolated genes uniquely implicated in dementia with Lewy bodies. Spatial annotation mapping was then used to test whether patterns of cortical thinning overlapped with in vivo neurotransmitter system distributions and whether the observed thickness pattern was constrained by large-scale structural connectivity, consistent with a network-based propagation process. RESULTS: Cortical thinning predominated in regions that, in the healthy brain, show higher expression of genes involved in mitochondrial function and synaptic transmission. The transcriptomic profile associated with thinning significantly overlapped with genes belonging to Parkinson’s disease and Alzheimer’s disease pathways, supporting shared pathogenic mechanisms across Lewy body and Alzheimer-type neurodegeneration. However, 90 genes associated with cortical thinning did not overlap with Parkinson’s disease or Alzheimer’s disease pathways and were enriched for GABAergic signalling. Spatial mapping analyses showed that regions with greatest thickness reductions colocalized with GABA _A , serotoninergic 5-HT _1A , 5-HT _1B , 5-HT4, and dopaminergic D2 receptor distributions, and that the thickness pattern followed structural connectivity. CONCLUSIONS: MRI-derived cortical thickness changes in dementia with Lewy bodies reflect selective molecular and network vulnerabilities rather than a non-specific degenerative process. Mitochondrial and synaptic genes, together with a distinct GABAergic association and connectivity constraints, delineate mechanisms explaining why some cortical territories are more affected in dementia with Lewy bodies.

Oligodendroglial pathology is a hallmark of multiple system atrophy (MSA), yet it remains unclear whether MRI-detected atrophy reflects underlying biological mechanisms. This study investigated whether regional atrophy aligns with gene expression and neurotransmitter systems. We recruited 65 MSA patients and derived brain atrophy measures from T1-weighted MRIs. Using postmortem data from the Allen Human Brain Atlas, partial least squares (PLS) regression identified gene expression components associated with brain atrophy. Gene enrichment analyses explored biological processes, and annotation mapping identified neurotransmitter systems matching atrophy patterns. Specificity was tested against 57 patients with Parkinson’s disease. Atrophy primarily affected the cerebellar white matter, pons, putamen, olive, and substantia nigra. PLS revealed two latent variables explaining 27.5% of the covariance. Atrophic regions overexpressed genes linked to mitochondrial function and oligodendrocytes, showing patterns distinct from Parkinson’s disease. These regions also exhibited lower serotonin and GABA levels, and higher acetylcholine and noradrenaline receptor densities. MRI-derived atrophy in MSA is biologically grounded and may inform future therapeutic studies.

Parkinson’s disease is associated with extensive structural brain changes. Recent work has proposed that the spatial pattern of disease pathology is shaped by both network spread and local vulnerability. However, few studies have assessed these biological frameworks in large patient samples across disease stages. Analysing the largest imaging cohort in Parkinson’s disease to date (n = 3096 patients), we investigated the roles of network architecture and local brain features by relating regional abnormality maps to normative profiles of connectivity, intrinsic networks, cytoarchitectonics, neurotransmitter receptor densities and gene expression. We found widespread cortical and subcortical atrophy in Parkinson’s disease to be associated with advancing disease stage, longer time since diagnosis and poorer global cognition. Structural brain connectivity best explained cortical atrophy patterns in Parkinson’s disease and across disease stages. These patterns were robust among individual patients. The precuneus, lateral temporal cortex and amygdala were identified as likely network-based epicentres, with high convergence across disease stages. Individual epicentres varied significantly among patients, yet they consistently localized to the default mode and limbic networks. Furthermore, we showed that regional overexpression of genes implicated in synaptic structure and signalling conferred increased susceptibility to brain atrophy in Parkinson’s disease. In summary, this study demonstrates in a well-powered sample that structural brain abnormalities in Parkinson’s disease across disease stages and within individual patients are influenced by both network spread and local vulnerability.

Isolated REM sleep behavior disorder, characterized by dream-enacting movements during REM sleep, is a male-predominant parasomnia and the strongest prodromal marker of synucleinopathies. Individuals with this disorder show cortical atrophy whose regional distribution covaries with gene expression patterns measured in the healthy human brain. However, the effect of sex on these brain changes remains unknown. The study objective is to comprehensively assess sex differences in cortical morphology and to characterize the healthy-brain gene expression correlates of brain abnormalities using the largest international multicentric MRI dataset of polysomnography-confirmed patients. Males have significantly more extensive and severe cortical thinning compared to females, despite similar age and clinical features. Imaging transcriptomics analyses indicate that regions affected in female patients map onto areas with higher expression of estrogen-related receptor genes, particularly ESRRG and ESRRA, in the healthy brain. These findings support potential sex-specific neuroprotection in the prodromal stages of synucleinopathies and may inform personalized and targeted therapeutic strategies.

Isolated REM sleep behavior disorder (iRBD) is the strongest prodromal marker of synucleinopathies, including Parkinson disease (PD) and dementia with Lewy bodies (DLB). Identifying brain biomarkers that predict progression and distinguish phenoconversion trajectories remains a challenge. The glymphatic system is involved in interstitial waste clearance, and its dysfunction has been associated with pathologic protein accumulation and neurodegeneration. Diffusion tensor imaging along the perivascular space (DTI-ALPS) has been proposed as a noninvasive proxy for glymphatic function. The aim of this study was to determine whether patients with iRBD show a reduced DTI-ALPS index compared with controls and whether a lower DTI-ALPS index predicts future phenoconversion to PD or DLB. We conducted a longitudinal, multicenter cohort study using brain MRI scans from patients with polysomnography-confirmed iRBD and healthy controls recruited across 5 international centers. All participants underwent T1-weighted and diffusion-weighted MRI. DTI-ALPS indices were computed from diffusivity along projection and associative fibers adjacent to the lateral ventricles. The primary outcome was time to phenoconversion to synucleinopathy. Linear models assessed baseline group differences and clinical correlates, and Cox proportional hazard models assessed the predictive value of DTI-ALPS for time to phenoconversion. A total of 250 patients with iRBD (mean age: 66.5 ± 6.8 years; 87% male) and 178 controls (65.7 ± 6.8 years; 81% male) were included. Patients with iRBD showed a lower left DTI-ALPS index compared with controls (mean difference = -0.034, 95% CI -0.067 to -0.001; p = 0.043). Of 224 patients with iRBD followed for a mean of 6.1 ± 3.5 years, 65 phenoconverted to a synucleinopathy. Converters had a lower left DTI-ALPS index than nonconverters (mean difference = -0.050, 95% CI -0.098 to -0.003; p = 0.038). Lower left DTI-ALPS index was associated with an increased risk of conversion to PD over time (hazard ratio = 2.43, 95% CI 1.13-5.25; p = 0.012). Other diffusion metrics inside periventricular masks, namely fractional anisotropy, diffusivity metrics, and free water, did not differ between groups. Patients with iRBD exhibit a reduced DTI-ALPS index, suggesting altered glymphatic function. This reduction was associated with future phenoconversion to PD, supporting the DTI-ALPS index as a potential prognostic MRI biomarker of progression in prodromal synucleinopathies.

INTRODUCTION: Most individuals with isolated rapid eye movement sleep behavior disorder (iRBD) develop dementia with Lewy bodies (DLB) or Parkinson's disease (PD). Brain biomarkers predicting specific phenoconversion trajectories are lacking. METHODS: In this multicenter diffusion magnetic resonance imaging study (261 iRBD, 177 controls), free water (FW) was measured in the nucleus basalis of Meynert (NBM) and posterior substantia nigra (SN). Among 230 iRBD patients with follow‐up, 64 converted (16 DLB, 38 PD). Time‐to‐event analyses were performed to assess differential phenoconversion. RESULTS: Phenoconverters had higher FW in the NBM and posterior SN. Only FW in the NBM predicted conversion to DLB over PD. NBM volume predicted DLB conversion, but only FW remained significant when both were modeled. FW in the NBM correlated with lower MoCA scores in iRBD. DISCUSSION: FW in the NBM is a sensitive biomarker of cognitive decline and DLB progression in iRBD, outperforming volume and supporting its use in early stratification. Highlights: FW in the NBM specifically identifies conversion to DLB. Increased FW in the NBM is associated with lower global cognition in iRBD. FW in the SN in iRBD does not relate more to DLB than PD. FW in the NBM is a biomarker of differential phenoconversion in iRBD.

INTRODUCTION: Isolated rapid eye movement (REM) sleep behavior disorder (iRBD), characterized by abnormal movements during REM sleep, is a prodromal stage of dementia with Lewy bodies (DLB) and Parkinson's disease (PD). While iRBD shows emerging brain changes, their impact on structural connectivity and network efficiency, and their predictive value, remain poorly characterized.METHODS: In this international prospective study, 198 polysomnography‐confirmed iRBD patients and 174 controls underwent diffusion magnetic resonance imaging and were analyzed. Cutting‐edge diffusion tractography and network‐based statistics were applied to reconstruct individual connectomes and assess network properties predicting DLB or PD.RESULTS: Structural architecture was already disrupted in iRBD, with both reduced and compensatory increased connections. Global efficiency was decreased. Local efficiency in motor regions was altered and associated with early clinical symptoms. Altered local efficiency in the supramarginal gyrus predicted DLB only.DISCUSSION: Early disruption of brain architecture in iRBD predicts progression to synucleinopathy‐related dementia, offering a novel potential prognostic biomarker.Highlights: Isolated rapid eye movement sleep behavior disorder (iRBD) patients show significant alterations in inter‐regional structural connectivity. Global efficiency is reduced in iRBD compared to controls. Areas with increased local efficiency contribute to decreased global efficiency. Altered network efficiency is associated with emerging Parkinsonian features. Higher supramarginal efficiency predicts dementia with Lewy bodies in iRBD.

Neurodegeneration observed in synucleinopathies, like Parkinson’s disease (PD), are hypothesized to be a consequence of the progressive accumulation and spread of misfolded alpha-synuclein (aSyn) throughout the brain. Here, we study the generalizability of this hypothesis across multiple biologically-relevant factors including genotype, aSyn species, and regional vulnerability (i.e.: does the introduction of aSyn into different brain regions have a similar impact) using a detailed longitudinal brain (using magnetic resonance imaging; [MRI]) and behavioural approaches. We first examined wild-type and M83 A53T hemizygous transgenic (engineered to over-express human aSyn) C57BL/6 x C3H mice receiving striatal inoculation with human or mouse preformed fibrils (PFFs) (n=89 mice at the last time point; n=687 MRI). Longitudinal analyses demonstrated a time-dependent increase in network-like atrophy and motor deficits, generalized across genotype and PFF species. We further derived latent dimensions relating brain-behaviour relationships revealing a pattern of sex- and genotype-relevant atrophy. Overall, atrophy was most prominent in M83 mice with mouse PFFs, while human PFFs or wild-type hosts showed attenuated effects. Changing the inoculation site to the hippocampus, a major connectivity hub, revealed differential regional vulnerability in the form of localized atrophy. Computational models previously validated in clinical PD further indicated regional vulnerability with increased predictive ability for atrophy patterns associated with striatal compared to hippocampal inoculation. Our findings reveal atrophy resulting from aSyn spreading is generalizable across genotype and PFF species, but not disease epicentres, emphasizing the role of regional vulnerability in disease progression.

Isolated REM sleep behavior disorder (iRBD), a prodromal synucleinopathy, generally precedes Parkinson’s disease (PD) or dementia with Lewy bodies (DLB). While disruptions in structural brain connectivity have been reported in these diseases, their presence in prodromal phases such as iRBD remains unclear. In this cross-sectional study, we analysed diffusion MRI in a large multicentric dataset (198 iRBD, 174 controls), mapping white matter pathways between 462 regions. Comparing groups, we found disrupted structural connectivity in iRBD. This included reduced density in multiple cortical areas alongside focal increases suggesting compensation in parietal, orbitofrontal and visual cortices. Global and local efficiency were altered in iRBD, notably in motor-related regions (putamen, thalamus, sensorimotor and parietal cortices), and was associated with emerging motor features. Importantly, increased local efficiency in the supramarginal gyrus predicted phenoconversion to DLB, but not PD. These findings highlight early structural connectivity disruptions in iRBD, offering a potential marker for progression to DLB.