Characterisation of premature cell senescence in Alzheimer's disease using single nuclear transcriptomics.

Aging is associated with cell senescence and is the major risk factor for AD. We characterized premature cell senescence in postmortem brains from non-diseased controls (NDC) and donors with Alzheimer's disease (AD) using imaging mass cytometry (IMC) and single nuclear RNA (snRNA) sequencing (> 200,000 nuclei). We found increases in numbers of glia immunostaining for galactosidase beta (> fourfold) and p16INK4A (up to twofold) with AD relative to NDC. Increased glial expression of genes related to senescence was associated with greater ß-amyloid load. Prematurely senescent microglia downregulated phagocytic pathways suggesting reduced capacity for ß-amyloid clearance. Gene set enrichment and pseudo-time trajectories described extensive DNA double-strand breaks (DSBs), mitochondrial dysfunction and ER stress associated with increased ß-amyloid leading to premature senescence in microglia. We replicated these observations with independent AD snRNA-seq datasets. Our results describe a burden of senescent glia with AD that is sufficiently high to contribute to disease progression. These findings support the hypothesis that microglia are a primary target for senolytic treatments in AD.

Diverse human astrocyte and microglial transcriptional responses to Alzheimer's pathology.

To better define roles that astrocytes and microglia play in Alzheimer's disease (AD), we used single-nuclei RNA-sequencing to comprehensively characterise transcriptomes in astrocyte and microglia nuclei selectively enriched during isolation post-mortem from neuropathologically defined AD and control brains with a range of amyloid-beta and phospho-tau (pTau) pathology. Significant differences in glial gene expression (including AD risk genes expressed in both the astrocytes [CLU, MEF2C, IQCK] and microglia [APOE, MS4A6A, PILRA]) were correlated with tissue amyloid or pTau expression. The differentially expressed genes were distinct between with the two cell types and pathologies, although common (but cell-type specific) gene sets were enriched with both pathologies in each cell type. Astrocytes showed enrichment for proteostatic, inflammatory and metal ion homeostasis pathways. Pathways for phagocytosis, inflammation and proteostasis were enriched in microglia and perivascular macrophages with greater tissue amyloid, but IL1-related pathway enrichment was found specifically in association with pTau. We also found distinguishable sub-clusters in the astrocytes and microglia characterised by transcriptional signatures related to either homeostatic functions or disease pathology. Gene co-expression analyses revealed potential functional associations of soluble biomarkers of AD in astrocytes (CLU) and microglia (GPNMB). Our work highlights responses of both astrocytes and microglia for pathological protein clearance and inflammation, as well as glial transcriptional diversity in AD.

Understanding medical students' transformative experiences of early preclinical international rural placement over a 20-year period.

INTRODUCTION: Rural placements are an important component of rural medical education programs seeking to develop rural practice pathways for medical students. These placements are usually domestic, but James Cook University in Australia developed an international rural placement program in the first half of the medical course that was funded through bursaries. This study explores how the international rural placement helped to shape the lives (personal development and learning) of the participants, using Transformational Learning Theory as a framework for identifying and describing the transformational elements, process and impact of the program. METHODS: Sixty-five students received a bursary for an international rural placement between 2001-2019. All were contacted by email and invited to participate in a short survey and a follow-up interview. Fifteen participants agreed and twelve were able to participate in individual semi-structured interviews which were recorded, transcribed and analysed using inductive thematic analysis. RESULTS: Participants reported that the bursary provided a "once in a lifetime opportunity" to "experience eye-opening and culturally rich difference". Nonetheless, some elements of the placement experience presented disorientating dilemmas that triggered deep reflections and shifts in perceptions. The bursary recipients realised that "being open-minded" allowed them "enjoy good company". They were also able to assume "outsider view which allowed reassessment of their own country" and the "isolation experiences gingered desire to right health wrongs". The triggers and mental shifts had significant impact on the bursary recipients and fostered the development of "inspirational new horizons" based on an appreciation of the "value of rural practice" and "role-modelling for life-long learning." These findings are consistent with Transformational Learning Theory. CONCLUSION: Participants in this study reported meaningful and strongly positive impacts from the experiences gained during an international rural clinical placement early in their course. They described transformative experiences which appear to contribute strongly to personal development. This finding supports maintaining opportunities for international experiences during rurally-oriented medical programs as these may impact longer term career choice.

Cross-platform transcriptional profiling identifies common and distinct molecular pathologies in Lewy body diseases.

Parkinson's disease (PD), Parkinson's disease with dementia (PDD) and dementia with Lewy bodies (DLB) are three clinically, genetically and neuropathologically overlapping neurodegenerative diseases collectively known as the Lewy body diseases (LBDs). A variety of molecular mechanisms have been implicated in PD pathogenesis, but the mechanisms underlying PDD and DLB remain largely unknown, a knowledge gap that presents an impediment to the discovery of disease-modifying therapies. Transcriptomic profiling can contribute to addressing this gap, but remains limited in the LBDs. Here, we applied paired bulk-tissue and single-nucleus RNA-sequencing to anterior cingulate cortex samples derived from 28 individuals, including healthy controls, PD, PDD and DLB cases (n = 7 per group), to transcriptomically profile the LBDs. Using this approach, we (i) found transcriptional alterations in multiple cell types across the LBDs; (ii) discovered evidence for widespread dysregulation of RNA splicing, particularly in PDD and DLB; (iii) identified potential splicing factors, with links to other dementia-related neurodegenerative diseases, coordinating this dysregulation; and (iv) identified transcriptomic commonalities and distinctions between the LBDs that inform understanding of the relationships between these three clinical disorders. Together, these findings have important implications for the design of RNA-targeted therapies for these diseases and highlight a potential molecular "window" of therapeutic opportunity between the initial onset of PD and subsequent development of Lewy body dementia.

Examiners' decision-making processes in observation-based clinical examinations.

BACKGROUND: Objective structured clinical examinations (OSCEs) are commonly used to assess the clinical skills of health professional students. Examiner judgement is one acknowledged source of variation in candidate marks. This paper reports an exploration of examiner decision making to better characterise the cognitive processes and workload associated with making judgements of clinical performance in exit-level OSCEs. METHODS: Fifty-five examiners for exit-level OSCEs at five Australian medical schools completed a NASA Task Load Index (TLX) measure of cognitive load and participated in focus group interviews immediately after the OSCE session. Discussions focused on how decisions were made for borderline and clear pass candidates. Interviews were transcribed, coded and thematically analysed. NASA TLX results were quantitatively analysed. RESULTS: Examiners self-reported higher cognitive workload levels when assessing a borderline candidate in comparison with a clear pass candidate. Further analysis revealed five major themes considered by examiners when marking candidate performance in an OSCE: (a) use of marking criteria as a source of reassurance; (b) difficulty adhering to the marking sheet under certain conditions; (c) demeanour of candidates; (d) patient safety, and (e) calibration using a mental construct of the 'mythical [prototypical] intern'. Examiners demonstrated particularly higher mental demand when assessing borderline compared to clear pass candidates. CONCLUSIONS: Examiners demonstrate that judging candidate performance is a complex, cognitively difficult task, particularly when performance is of borderline or lower standard. At programme exit level, examiners intuitively want to rate candidates against a construct of a prototypical graduate when marking criteria appear not to describe both what and how a passing candidate should demonstrate when completing clinical tasks. This construct should be shared, agreed upon and aligned with marking criteria to best guide examiner training and calibration. Achieving this integration may improve the accuracy and consistency of examiner judgements and reduce cognitive workload.

Acute stress enhances general-knowledge semantic memory.

Acute psychological stress consistently impairs episodic memory, which consists of memory for events that are associated with a specific context. However, researchers have not yet established how stress influences semantic memory, which consists of general knowledge that is devoid of context. In the present study, participants either underwent stress induction or a control task prior to taking a trivia test that was designed to measure semantic memory. In contrast to the wealth of prior research on episodic memory, we found that stress enhanced semantic-memory retrieval. Supporting this finding, higher cortisol reactivity to stress was associated with better performance on the trivia test. Together with the results from previous studies of episodic memory, our findings suggest that stress differentially influences memory retrieval, depending on the degree to which the retrieval of a given memory relies on medial-temporal, neocortical, and striatal brain regions.

Retrieval practice improves item memory but not source memory in the context of stress.

Smith, Floerke, and Thomas (2016) demonstrated that learning by repeated testing, or retrieval practice, reduced stress-related memory impairment when compared to learning by repeatedly studying material. In the present experiment, we tested whether, relative to study practice, retrieval practice would improve post-stress memory by increasing access to both item and source information. Participants learned two wordlists, which were temporally segregated to facilitate distinction between the two lists. Participants returned one week later for stress induction and two memory tests. Each test featured a recognition test that was given to assess item memory accessibility, and a list-discrimination task that was given to assess source memory. Relative to study practice, successful retrieval practice during learning reduced false alarms but did not improve source memory on the post-stress test. Results are discussed as they relate to current theories surrounding stress effects and retrieval practice effects.

Mitochondrial dysfunction and increased glycolysis in prodromal and early Parkinson's blood cells.

BACKGROUND: Although primarily a neurodegenerative process, there is increasing awareness of peripheral disease mechanisms in Parkinson's disease. To investigate disease processes in accessible patient cells, we studied peripheral blood mononuclear cells in recently diagnosed PD patients and rapid eye movement-sleep behavior disorder patients who have a greatly increased risk of developing PD. We hypothesized that peripheral blood mononuclear cells may recapitulate cellular pathology found in the PD brain and investigated these cells for mitochondrial dysfunction and oxidative stress. METHODS: Peripheral blood mononuclear cells were isolated and studied from PD patients, rapid eye movement-sleep behavior disorder patients and age- and sex-matched control individuals from the well-characterized Oxford Discovery cohort. All participants underwent thorough clinical assessment. RESULTS: Initial characterization showed that PD patients had elevated levels of CD14 + monocytes and monocytes expressing C-C motif chemokine receptor 2. Mitochondrial dysfunction and oxidative stress were increased in PD patient peripheral blood mononuclear cells, with elevated levels of mitochondrial reactive oxygen species specifically in patient monocytes. This was combined with reduced levels of the antioxidant superoxide dismutase in blood cells from PD patients and, importantly, also in rapid eye movement-sleep behavior disorder patients. This mitochondrial dysfunction was associated with a concomitant increase in glycolysis in both PD and rapid eye movement-sleep behavior disorder patient blood cells independent of glucose uptake or monocyte activation. CONCLUSIONS: This work demonstrates functional bioenergetic deficits in PD and rapid eye movement-sleep behavior disorder patient blood cells during the early stages of human disease. © 2018 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Verbal long-term memory is enhanced by retrieval practice but impaired by prefrontal direct current stimulation.

Retrieval practice involves repeatedly testing a student during the learning experience, reliably conferring learning advantages relative to repeated study. Transcranial direct current stimulation (tDCS) of the left dorsolateral prefrontal cortex (dlPFC) has also been shown to confer learning advantages for verbal memory, though research is equivocal. The present study examined the effects of retrieval versus study practice with or without left dlPFC tDCS on verbal episodic memory. Participants (N = 150) experienced either retrieval practice or study practice, and active anodal, active cathodal, or sham tDCS while encoding word lists, and then returned two days later for a final recall test. Three primary patterns emerged: first, during encoding, tDCS did not influence recall rates in the retrieval practice group. Second, during final recall, participants in the retrieval practice groups recalled more than those in the study practice groups. Finally, during final recall, anodal tDCS decreased recall relative to sham and cathodal stimulation, suggesting that it interfered with developing highly detailed memories that could be relied upon for subsequent recollection. Data support existing research demonstrating the effectiveness of retrieval practice as a learning strategy, but also suggest that anodal dlPFC stimulation can induce long-term negative impacts on verbal episodic memory retrieval.

Implementation of a Standardized Screening Process to Increase Palliative Care Referrals in Primary Care: An Evidence-Based Quality Approach.

Despite initiatives to increase palliative care awareness, referrals in primary care settings are still primarily based on provider judgment, causing a lack of appropriate referrals and disparities in access to palliative care resources. The purpose of this quality improvement project was to develop and implement an evidence-based, standardized palliative care referral protocol to increase the palliative care referral rate for eligible patients at a primary care clinic. The project used a preimplementation and postimplementation design with the use of the RE-AIM (Reach, Effectiveness, Adoption, Implementation, and Maintenance) framework to successfully implement and evaluate the standardized referral process. Over the 10-month project period, the palliative care referral rate increased from 2% (4/193) preimplementation to 11% (16/147) postimplementation of the standardized referral process, which is an increase of 9%. Taking into consideration the potential impact of multiple extraneous variables, there was an overall decrease of 69% in emergency room visits and 73% in hospitalizations for patients who received a palliative care referral. These outcomes support expansion of the standardized referral process throughout other primary care clinics to increase palliative care referrals and sustain a high level of quality patient care.

Algorithmic Spaced Retrieval Enhances Long-Term Memory in Alzheimer Disease: Case-Control Pilot Study.

BACKGROUND: Spaced retrieval is a learning technique that involves engaging in repeated memory testing after increasingly lengthy intervals of time. Spaced retrieval has been shown to improve long-term memory in Alzheimer disease (AD), but it has historically been difficult to implement in the everyday lives of individuals with AD. OBJECTIVE: This research aims to determine, in people with mild cognitive impairment (MCI) due to AD, the efficacy and feasibility of a mobile app that combines spaced retrieval with a machine learning algorithm to enhance memory retention. Specifically, the app prompts users to answer questions during brief daily sessions, and a machine learning algorithm tracks each user's rate of forgetting to determine the optimal spacing schedule to prevent anticipated forgetting. METHODS: In this pilot study, 61 participants (young adults: n=21, 34%; healthy older adults: n=20, 33%; people with MCI due to AD: n=20, 33%) used the app for 4 weeks to learn new facts and relearn forgotten name-face associations. Participation during the 4-week period was characterized by using the app once per day to answer 15 questions about the facts and names. After the 4-week learning phase, participants completed 2 recognition memory tests approximately 1 week apart, which tested memory for information they had studied using the app as well as information they had not studied. RESULTS: After using the mobile app for 1 month, every person with MCI due to AD demonstrated improvements in memory for new facts that they had studied via the app compared to baseline (P<.001). All but one person with MCI due to AD (19/20, 95%) showed improvements of more than 10 percentage points, comparable to the improvements shown by young adults and healthy older adults. Memory for name-face associations was similarly improved for all participant groups after using the app but to a lesser degree. Furthermore, for both new facts and name-face associations, we found no memory decay for any participant group after they took a break of approximately 1 week from using the app at the end of the study. Regarding usability, of the 20 people with MCI due to AD, 16 (80%) self-adhered to the app's automated practice schedule, and half of them (n=10, 50%) expressed an interest in continuing to use it. CONCLUSIONS: These results demonstrate early evidence that spaced retrieval mobile apps are both feasible for people with early-stage AD to use in their everyday lives and effective for supporting memory retention of recently learned facts and name-face associations.

A single nuclear transcriptomic characterisation of mechanisms responsible for impaired angiogenesis and blood-brain barrier function in Alzheimer's disease.

Brain perfusion and blood-brain barrier (BBB) integrity are reduced early in Alzheimer's disease (AD). We performed single nucleus RNA sequencing of vascular cells isolated from AD and non-diseased control brains to characterise pathological transcriptional signatures responsible for this. We show that endothelial cells (EC) are enriched for expression of genes associated with susceptibility to AD. Increased ß-amyloid is associated with BBB impairment and a dysfunctional angiogenic response related to a failure of increased pro-angiogenic HIF1A to increased VEGFA signalling to EC. This is associated with vascular inflammatory activation, EC senescence and apoptosis. Our genomic dissection of vascular cell risk gene enrichment provides evidence for a role of EC pathology in AD and suggests that reducing vascular inflammatory activation and restoring effective angiogenesis could reduce vascular dysfunction contributing to the genesis or progression of early AD.

The transcription factor PU.1 is critical for viability and function of human brain microglia.

Microglia are the predominant resident immune cells of the brain and can assume a range of phenotypes. They are critical for normal brain development and function but can also contribute to many disease processes. Although they are widely studied, the transcriptional control of microglial phenotype and activation requires further research. PU.1 is a key myeloid transcription factor expressed by peripheral macrophages and rodent microglia. In this article, we report the presence of PU.1 specifically in microglia of the adult human brain and we examine its functional role in primary human microglia. Using siRNA, we achieved substantial PU.1 protein knock-down in vitro. By assessing a range of characteristic microglial proteins we found decreased viability of adult human microglia with reduced PU.1 protein expression. This observation was confirmed with PU.1 antisense DNA oligonucleotides. An important function of microglia is to clear debris by phagocytosis. We assessed the impact of loss of PU.1 on microglial phagocytosis and show that PU.1 siRNA reduces the ability of adult human microglia to phagocytose amyloid-beta1-42 peptide. These results show that PU.1 controls human microglial viability and function and suggest PU.1 as a molecular target for manipulation of human microglial phenotype.

M-CSF increases proliferation and phagocytosis while modulating receptor and transcription factor expression in adult human microglia.

BACKGROUND: Microglia are the primary immune cells of the brain whose phenotype largely depends on their surrounding micro-environment. Microglia respond to a multitude of soluble molecules produced by a variety of brain cells. Macrophage colony-stimulating factor (M-CSF) is a cytokine found in the brain whose receptor is expressed by microglia. Previous studies suggest a critical role for M-CSF in brain development and normal functioning as well as in several disease processes involving neuroinflammation. METHODS: Using biopsy tissue from patients with intractable temporal epilepsy and autopsy tissue, we cultured primary adult human microglia to investigate their response to M-CSF. Mixed glial cultures were treated with 25 ng/ml M-CSF for 96 hours. Proliferation and phagocytosis assays, and high through-put immunocytochemistry, microscopy and image analysis were performed to investigate microglial phenotype and function. RESULTS: We found that the phenotype of primary adult human microglia was markedly changed following exposure to M-CSF. A greater number of microglia were present in the M-CSF- treated cultures as the percentage of proliferating (BrdU and Ki67-positive) microglia was greatly increased. A number of changes in protein expression occurred following M-CSF treatment, including increased transcription factors PU.1 and C/EBPß, increased DAP12 adaptor protein, increased M-CSF receptor (CSF-1R) and IGF-1 receptor, and reduced HLA-DP, DQ, DR antigen presentation protein. Furthermore, a distinct morphological change was observed with elongation of microglial processes. These changes in phenotype were accompanied by a functional increase in phagocytosis of Aß1-42 peptide. CONCLUSIONS: We show here that the cytokine M-CSF dramatically influences the phenotype of adult human microglia. These results pave the way for future investigation of M-CSF-related targets for human therapeutic benefit.

Translocator protein is a marker of activated microglia in rodent models but not human neurodegenerative diseases.

Microglial activation plays central roles in neuroinflammatory and neurodegenerative diseases. Positron emission tomography (PET) targeting 18 kDa Translocator Protein (TSPO) is widely used for localising inflammation in vivo, but its quantitative interpretation remains uncertain. We show that TSPO expression increases in activated microglia in mouse brain disease models but does not change in a non-human primate disease model or in common neurodegenerative and neuroinflammatory human diseases. We describe genetic divergence in the TSPO gene promoter, consistent with the hypothesis that the increase in TSPO expression in activated myeloid cells depends on the transcription factor AP1 and is unique to a subset of rodent species within the Muroidea superfamily. Finally, we identify LCP2 and TFEC as potential markers of microglial activation in humans. These data emphasise that TSPO expression in human myeloid cells is related to different phenomena than in mice, and that TSPO-PET signals in humans reflect the density of inflammatory cells rather than activation state.

Distinct characteristics of microglia from neurogenic and non-neurogenic regions of the human brain in patients with Mesial Temporal Lobe Epilepsy.

The study of microglia isolated from adult human brain tissue provides unique insight into the physiology of these brain immune cells and their role in adult human brain disorders. Reports of microglia in post-mortem adult human brain tissue show regional differences in microglial populations, however, these differences have not been fully explored in living microglia. In this study biopsy tissue was obtained from epileptic patients undergoing surgery and consisted of both cortical areas and neurogenic ventricular and hippocampal (Hp) areas. Microglia were concurrently isolated from both regions and compared by immunochemistry. Our initial observation was that a greater number of microglia resulted from isolation and culture of ventricular/Hp tissue than cortical tissue. This was found to be due to a greater proliferative capacity of microglia from ventricular/Hp regions compared to the cortex. Additionally, ventricular/Hp microglia had a greater proliferative response to the microglial mitogen Macrophage Colony-Stimulating Factor (M-CSF). This enhanced response was found to be associated with higher M-CSF receptor expression and higher expression of proteins involved in M-CSF signalling DAP12 and C/EBPß. Microglia from the ventricular/Hp region also displayed higher expression of the receptor for Insulin-like Growth Factor-1, a molecule with some functional similarity to M-CSF. Compared to microglia isolated from the cortex, ventricular/Hp microglia showed increased HLA-DP, DQ, DR antigen presentation protein expression and a rounded morphology. These findings show that microglia from adult human brain neurogenic regions are more proliferative than cortical microglia and have a distinct protein expression profile. The data present a case for differential microglial phenotype and function in different regions of the adult human brain and suggest that microglia in adult neurogenic regions are "primed" to an activated state by their unique tissue environment.

Reactive astrocytes acquire neuroprotective as well as deleterious signatures in response to Tau and Aß pathology.

Alzheimer's disease (AD) alters astrocytes, but the effect of Aß and Tau pathology is poorly understood. TRAP-seq translatome analysis of astrocytes in APP/PS1 ß-amyloidopathy and MAPTP301S tauopathy mice revealed that only Aß influenced expression of AD risk genes, but both pathologies precociously induced age-dependent changes, and had distinct but overlapping signatures found in human post-mortem AD astrocytes. Both Aß and Tau pathology induced an astrocyte signature involving repression of bioenergetic and translation machinery, and induction of inflammation pathways plus protein degradation/proteostasis genes, the latter enriched in targets of inflammatory mediator Spi1 and stress-activated cytoprotective Nrf2. Astrocyte-specific Nrf2 expression induced a reactive phenotype which recapitulated elements of this proteostasis signature, reduced Aß deposition and phospho-tau accumulation in their respective models, and rescued brain-wide transcriptional deregulation, cellular pathology, neurodegeneration and behavioural/cognitive deficits. Thus, Aß and Tau induce overlapping astrocyte profiles associated with both deleterious and adaptive-protective signals, the latter of which can slow patho-progression.

Valproic acid induces microglial dysfunction, not apoptosis, in human glial cultures.

Valproic acid (VPA) is widely used for the treatment of mood disorders and epilepsy, but its mechanism of action is unclear. In vivo and in vitro studies using rodent models have demonstrated that VPA has both neuroprotective and neurotrophic effects. These beneficial effects are, in part, through modulation of glial cell function. Recently, we and others have shown that VPA selectively induces caspase-3 mediated apoptosis in rodent microglial cells. However, the effect of VPA on human microglia has not been tested. In this study, using microglia derived from adult human brains, we demonstrate that VPA does not induce microglial apoptosis as determined by the absence of caspase-3 cleavage. However, VPA does partially decrease the expression of the microglial markers PU.1 and CD45, as well as dramatically reducing microglial phagocytosis. Due to the many roles of microglia in the brain, these VPA-induced alterations in microglial phenotype could potentially have major effects on physiological and pathological actions of these cells.

Understanding Older Adults' Memory Distortion in the Light of Stereotype Threat.

Numerous studies have documented the detrimental impact of age-based stereotype threat (ABST) on older adults' cognitive performance and especially on veridical memory. However, far fewer studies have investigated the impact of ABST on older adults' memory distortion. Here, we review the subset of research examining memory distortion and provide evidence for the role of stereotype threat as a powerful socio-emotional factor that impacts age-related susceptibility to memory distortion. In this review we define memory distortion as errors in memory that are associated with gist-based errors or source misattributions. Whereas, some of the reviewed experiments support the conclusion that ABST should be considered in the context of age-related differences in memory distortion, others reported little or no impact of stereotype threat. These discrepancies suggest that the role of ABST, and socio-emotional processes generally, in age-related changes in memory distortion are less clear. In this review, we argue that ABST does play an important role in age-related changes in memory distortion. We present evidence suggesting that discrepancies in the reviewed literature may be reconciled when evaluated in the context of the leading theories about stereotype threat: the Executive Resource Depletion hypothesis and the Regulatory Focus theory. We also discuss how differences in methodology and participant characteristics can account for a priori contradictory results in the literature. Finally, we propose some recommendations for researchers and practitioners when assessing memory in older adults.

Characterization of Clonal Evolution in Microsatellite Unstable Metastatic Cancers through Multiregional Tumor Sequencing.

Microsatellites are short, repetitive segments of DNA, which are dysregulated in mismatch repair-deficient (MMRd) tumors resulting in microsatellite instability (MSI). MSI has been identified in many human cancer types with varying incidence, and microsatellite instability-high (MSI-H) tumors often exhibit increased sensitivity to immune-enhancing therapies such as PD-1/PD-L1 inhibition. Next-generation sequencing (NGS) has permitted advancements in MSI detection, and recent computational advances have enabled characterization of tumor heterogeneity via NGS. However, the evolution and heterogeneity of microsatellite changes in MSI-positive tumors remains poorly described. We determined MSI status in 6 patients using our previously published algorithm, MANTIS, and inferred subclonal composition and phylogeny with Canopy and SuperFreq. We developed a simulated annealing-based method to characterize microsatellite length distributions in specific subclones and assessed the evolution of MSI in the context of tumor heterogeneity. We identified three to eight tumor subclones per patient, and each subclone exhibited MMRd-associated base substitution signatures. We noted that microsatellites tend to shorten over time, and that MMRd fosters heterogeneity by introducing novel mutations throughout the disease course. Some microsatellites are altered among all subclones in a patient, whereas other loci are only altered in particular subclones corresponding to subclonal phylogenetic relationships. Overall, our results indicate that MMRd is a substantial driver of heterogeneity, leading to both MSI and subclonal divergence. IMPLICATIONS: We leveraged subclonal inference to assess clonal evolution based on somatic mutations and microsatellites, which provides insight into MMRd as a dynamic mutagenic process in MSI-H malignancies.