Asymmetric pathology in primary progressive aphasia with progranulin mutations and TDP inclusions

GRN mutation spectrum and genotype-phenotype correlation in Chinese dementia patients: data from PUMCH dementia cohort

BACKGROUND

METHODS: The GRN mutations, especially of the loss of function type, are causative of frontotemporal dementia (FTD). However, several GRN variants can be found in other neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease. So far, there have been over 300 GRN mutations reported globally. However, the genetic spectrum and phenotypic characteristics have not been fully elucidated in Chinese population.The participants were from the dementia cohort of Peking Union Medical College Hospital (n=1945). They received history inquiry, cognitive evaluation, brain imaging and exome sequencing. The dementia subjects carrying the rare variants of the GRN were included in this study. Those with the pathogenic or likely pathogenic variants of other dementia-related genes were excluded.

RESULTS

14 subjects carried the rare variants of GRN. They were clinically diagnosed with behavioural variant of FTD (n=2), non-fluent/agrammatic variant primary progressive aphasia (PPA, n=3), semantic variant PPA (n=1), AD (n=6) and mixed dementia (n=2). 13 rare variants of GRN were found, including 6 novel variants (W49X, S226G, M152I, A91E, G79E and A303S). The most prevalent symptom was amnesia (85.7%, 12/14), followed by psychiatric and behavioural disorder (78.6%, 11/14). In terms of lobar atrophy, temporal atrophy/hypometabolism was the most common (85.7%, 12/14), followed by parietal atrophy/hypometabolism (78.6%, 11/14).

CONCLUSION

The novel GRN variants identified in this study contribute to enrich the GRN mutation repertoire. There is phenotypic similarity and diversity among Chinese patients with the GRN mutations.

Phenotypically concordant distribution of pick bodies in aphasic versus behavioral dementias

Pick's disease (PiD) is a subtype of the tauopathy form of frontotemporal lobar degeneration (FTLD-tau) characterized by intraneuronal 3R-tau inclusions. PiD can underly various dementia syndromes, including primary progressive aphasia (PPA), characterized by an isolated and progressive impairment of language and left-predominant atrophy, and behavioral variant frontotemporal dementia (bvFTD), characterized by progressive dysfunction in personality and bilateral frontotemporal atrophy. In this study, we investigated the neocortical and hippocampal distributions of Pick bodies in bvFTD and PPA to establish clinicopathologic concordance between PiD and the salience of the aphasic versus behavioral phenotype. Eighteen right-handed cases with PiD as the primary pathologic diagnosis were identified from the Northwestern University Alzheimer's Disease Research Center brain bank (bvFTD, N = 9; PPA, N = 9). Paraffin-embedded sections were stained immunohistochemically with AT8 to visualize Pick bodies, and unbiased stereological analysis was performed in up to six regions bilaterally [middle frontal gyrus (MFG), superior temporal gyrus (STG), inferior parietal lobule (IPL), anterior temporal lobe (ATL), dentate gyrus (DG) and CA1 of the hippocampus], and unilateral occipital cortex (OCC). In bvFTD, peak neocortical densities of Pick bodies were in the MFG, while the ATL was the most affected in PPA. Both the IPL and STG had greater leftward pathology in PPA, with the latter reaching significance (p < 0.01). In bvFTD, Pick body densities were significantly right-asymmetric in the STG (p < 0.05). Hippocampal burden was not clinicopathologically concordant, as both bvFTD and PPA cases demonstrated significant hippocampal pathology compared to neocortical densities (p < 0.0001). Inclusion-to-neuron analyses in a subset of PPA cases confirmed that neurons in the DG are disproportionately burdened with inclusions compared to neocortical areas. Overall, stereological quantitation suggests that the distribution of neocortical Pick body pathology is concordant with salient clinical features unique to PPA vs. bvFTD while raising intriguing questions about the selective vulnerability of the hippocampus to 3R-tauopathies.

TDP-43 and tau concurrence in the entorhinal subfields in primary age-related tauopathy and preclinical Alzheimer's disease

Phosphorylated tau (p-tau) pathology correlates strongly with cognitive decline and is a pathological hallmark of Alzheimer's Disease (AD). In recent years, phosphorylated transactive response DNA-binding protein (pTDP-43) has emerged as a common comorbidity, found in up to 70% of all AD cases (Josephs et al., Acta Neuropathol, 131(4), 571-585; Josephs, Whitwell, et al., Acta Neuropathol, 127(6), 811-824). Current staging schemes for pTDP-43 in AD and primary age-related tauopathy (PART) track its progression throughout the brain, but the distribution of pTDP-43 within the entorhinal cortex (EC) at the earliest stages has not been studied. Moreover, the exact nature of p-tau and pTDP-43 co-localization is debated. We investigated the selective vulnerability of the entorhinal subfields to phosphorylated pTDP-43 pathology in preclinical AD and PART postmortem tissue. Within the EC, posterior-lateral subfields showed the highest semi-quantitative pTDP-43 density scores, while the anterior-medial subfields had the lowest. On the rostrocaudal axis, pTDP-43 scores were higher posteriorly than anteriorly (p < 0.010), peaking at the posterior-most level (p < 0.050). Further, we showed the relationship between pTDP-43 and p-tau in these regions at pathology-positive but clinically silent stages. P-tau and pTDP-43 presented a similar pattern of affected subregions (p < 0.0001) but differed in density magnitude (p < 0.0001). P-tau burden was consistently higher than pTDP-43 at every anterior-posterior level and in most EC subfields. These findings highlight pTDP-43 burden heterogeneity within the EC and the posterior-lateral subfields as the most vulnerable regions within stage II of the current pTDP-43 staging schemes for AD and PART. The EC is a point of convergence for p-tau and pTDP-43 and identifying its most vulnerable neuronal populations will prove key for early diagnosis and disease intervention.

Brain total creatine differs between primary progressive aphasia (PPA) subtypes and correlates with disease severity

Primary progressive aphasia (PPA) is comprised of three subtypes: logopenic (lvPPA), non-fluent (nfvPPA), and semantic (svPPA). We used magnetic resonance spectroscopy (MRS) to measure tissue-corrected metabolite levels in the left inferior frontal gyrus (IFG) and right sensorimotor cortex (SMC) from 61 PPA patients. We aimed to: (1) characterize subtype differences in metabolites; and (2) test for metabolite associations with symptom severity. tCr differed by subtype across the left IFG and right SMC. tCr levels were lowest in lvPPA and highest in svPPA. tCr levels predicted lvPPA versus svPPA diagnosis. Higher IFG tCr and lower Glx correlated with greater disease severity. As tCr is involved in brain energy metabolism, svPPA pathology might involve changes in specific cellular energy processes. Perturbations to cellular energy homeostasis in language areas may contribute to symptoms. Reduced cortical excitatory capacity (i.e. lower Glx) in language regions may also contribute to symptoms. Thus, tCr may be useful for differentiating between PPA subtypes, and both tCr and Glx might have utility in understanding PPA mechanisms and tracking progression.

Temporopolar regions of the human brain

Following prolonged neglect during the formative decades of behavioural neurology, the temporopolar region has become a site of vibrant research on the neurobiology of cognition and conduct. This turnaround can be attributed to increasing recognition of neurodegenerative diseases that target temporopolar regions for peak destruction. The resultant syndromes include behavioural dementia, associative agnosia, semantic forms of primary progressive aphasia and semantic dementia. Clinicopathological correlations show that object naming and word comprehension are critically dependent on the language-dominant (usually left) temporopolar region, whereas behavioural control and non-verbal object recognition display a more bilateral representation with a rightward bias. Neuroanatomical experiments in macaques and neuroimaging in humans show that the temporoparietal region sits at the confluence of auditory, visual and limbic streams of processing at the downstream (deep) pole of the 'what' pathway. The functional neuroanatomy of this region revolves around three axes, an anterograde horizontal axis from unimodal to heteromodal and paralimbic cortex; a radial axis where visual (ventral), auditory (dorsal) and paralimbic (medial) territories encircle temporopolar cortex and display hemispheric asymmetry; and a vertical depth-of-processing axis for the associative elaboration of words, objects and interoceptive states. One function of this neural matrix is to support the transformation of object and word representations from unimodal percepts to multimodal concepts. The underlying process is likely to start at canonical gateways that successively lead to generic (superordinate), specific (basic) and unique levels of recognition. A first sign of left temporopolar dysfunction takes the form of taxonomic blurring where boundaries among categories are preserved but not boundaries among exemplars of a category. Semantic paraphasias and coordinate errors in word-picture verification tests are consequences of this phenomenon. Eventually, boundaries among categories are also blurred and comprehension impairments become more profound. The medial temporopolar region belongs to the amygdalocentric component of the limbic system and stands to integrate exteroceptive information with interoceptive states underlying social interactions. Review of the pertinent literature shows that word comprehension and conduct impairments caused by temporopolar strokes and temporal lobectomy are far less severe than those seen in temporopolar atrophies. One explanation for this unexpected discrepancy invokes the miswiring of residual temporopolar neurons during the many years of indolently progressive neurodegeneration. According to this hypothesis, the temporopolar regions become not only dysfunctional but also sources of aberrant outputs that interfere with the function of areas elsewhere in the language and paralimbic networks, a juxtaposition not seen in lobectomy or stroke.

Differential vulnerability of the dentate gyrus to tauopathies in dementias

The dentate gyrus (DG), a key hippocampal subregion in memory processing, generally resists phosphorylated tau accumulation in the amnestic dementia of the Alzheimer's type due to Alzheimer's disease (DAT-AD), but less is known about the susceptibility of the DG to other tauopathies. Here, we report stereologic densities of total DG neurons and tau inclusions in thirty-two brains of human participants with autopsy-confirmed tauopathies with distinct isoform profiles-3R Pick's disease (PiD, N = 8), 4R corticobasal degeneration (CBD, N = 8), 4R progressive supranuclear palsy (PSP, N = 8), and 3/4R AD (N = 8). All participants were diagnosed during life with primary progressive aphasia (PPA), an aphasic clinical dementia syndrome characterized by progressive deterioration of language abilities with spared non-language cognitive abilities in early stages, except for five patients with DAT-AD as a comparison group. 51% of total participants were female. All specimens were stained immunohistochemically with AT8 to visualize tau pathology, and PPA cases were stained for Nissl substance to visualize neurons. Unbiased stereological analysis was performed in granule and hilar DG cells, and inclusion-to-neuron ratios were calculated. In the PPA group, PiD had highest mean total (granule + hilar) densities of DG tau pathology (p < 0.001), followed by CBD, AD, then PSP. PPA-AD cases showed more inclusions in hilar cells compared to granule cells, while the opposite was true in PiD and CBD. Inclusion-to-neuron ratios revealed, on average, 33% of all DG neurons in PiD cases contained a tau inclusion, compared to ~ 7% in CBD, 2% in AD, and 0.4% in PSP. There was no significant difference between DAT-AD and PPA-AD pathologic tau burden, suggesting that differences in DG burden are not specific to clinical phenotype. We conclude that the DG is differentially vulnerable to pathologic tau accumulation, raising intriguing questions about the structural integrity and functional significance of hippocampal circuits in neurodegenerative dementias.

Resistance of Basal Forebrain Cholinergic Neurons to TDP-43 Proteinopathy in Primary Progressive Aphasia

Basal forebrain cholinergic neurons (BFCN) display accumulation of neurofibrillary tangles and degeneration in Alzheimer disease and are targets of therapeutic intervention. This study determined vulnerability of BFCN to accumulation of TDP-43 in primary progressive aphasia with TDP-43 proteinopathy (PPA-TDP). Brains from 16 PPA participants with pathologically confirmed TDP-43 proteinopathy, with available paraffin-embedded sections (Group 1), or systematically sampled frozen sections (Group 2), were studied. Immunohistochemistry was performed with an antibody against phosphorylated TDP-43. BFCN were identified by their magnocellular appearance in Nissl preparations. Presence of TDP-43 inclusions and preinclusions in BFCN was determined and quantitative analysis was performed in Group 2. In Group 1, BFCN were completely free of inclusions except for occasional dystrophic neurites. Sparse TDP-43 preinclusions with smooth or granular staining in BFCN were detected. In Group 2, extremely rare TDP-43 intranuclear inclusions were detected in 0.1% of BFCN per section, along with occasional dystrophic neurites. Although sparse, significantly more preinclusions (1.4% of BFCN) were present when compared with inclusions. No hemispheric differences were noted. Small neurons near BFCN contained more preinclusions compared with BFCN. Thus, BFCN in PPA-TDP are resistant to TDP-43 proteinopathy and degeneration, suggesting that cholinergic therapy is unlikely to be effective in this disorder.

Neuropathological fingerprints of survival, atrophy and language in primary progressive aphasia

Primary progressive aphasia is a neurodegenerative disease that selectively impairs language without equivalent impairment of speech, memory or comportment. In 118 consecutive autopsies on patients with primary progressive aphasia, primary diagnosis was Alzheimer's disease neuropathological changes (ADNC) in 42%, corticobasal degeneration or progressive supranuclear palsy neuropathology in 24%, Pick's disease neuropathology in 10%, transactive response DNA binding proteinopathy type A [TDP(A)] in 10%, TDP(C) in 11% and infrequent entities in 3%. Survival was longest in TDP(C) (13.2 ± 2.6 years) and shortest in TDP(A) (7.1 ± 2.4 years). A subset of 68 right-handed participants entered longitudinal investigations. They were classified as logopenic, agrammatic/non-fluent or semantic by quantitative algorithms. Each variant had a preferred but not invariant neuropathological correlate. Seventy-seven per cent of logopenics had ADNC, 56% of agrammatics had corticobasal degeneration/progressive supranuclear palsy or Pick's disease and 89% of semantics had TDP(C). Word comprehension impairments had strong predictive power for determining underlying neuropathology positively for TDP(C) and negatively for ADNC. Cortical atrophy was smallest in corticobasal degeneration/progressive supranuclear palsy and largest in TDP(A). Atrophy encompassed posterior frontal but not temporoparietal cortex in corticobasal degeneration/progressive supranuclear palsy, anterior temporal but not frontoparietal cortex in TDP(C), temporofrontal but not parietal cortex in Pick's disease and all three lobes with ADNC or TDP(A). There were individual deviations from these group patterns, accounting for less frequent clinicopathologic associations. The one common denominator was progressive asymmetric atrophy overwhelmingly favouring the left hemisphere language network. Comparisons of ADNC in typical amnestic versus atypical aphasic dementia and of TDP in type A versus type C revealed fundamental biological and clinical differences, suggesting that members of each pair may constitute distinct clinicopathologic entities despite identical downstream proteinopathies. Individual TDP(C) participants with unilateral left temporal atrophy displayed word comprehension impairments without additional object recognition deficits, helping to dissociate semantic primary progressive aphasia from semantic dementia. When common and uncommon associations were considered in the set of 68 participants, one neuropathology was found to cause multiple clinical subtypes, and one subtype of primary progressive aphasia to be caused by multiple neuropathologies, but with different probabilities. Occasionally, expected clinical manifestations of atrophy sites were absent, probably reflecting individual peculiarities of language organization. The hemispheric asymmetry of neurodegeneration and resultant language impairment in primary progressive aphasia reflect complex interactions among the cellular affinities of the degenerative disease, the constitutive biology of language cortex, familial or developmental vulnerabilities of this network and potential idiosyncrasies of functional anatomy in the affected individual.

Signature laminar distributions of pathology in frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) with either tau (FTLD-tau) or TDP-43 (FTLD-TDP) inclusions are distinct proteinopathies that frequently cause similar frontotemporal dementia (FTD) clinical syndromes. FTD syndromes often display macroscopic signatures of neurodegeneration at the level of regions and networks, but it is unclear if subregional laminar pathology display patterns unique to proteinopathy or clinical syndrome. We hypothesized that FTLD-tau and FTLD-TDP accumulate pathology in relatively distinct cortical layers independent of clinical syndrome, with greater involvement of lower layers in FTLD-tau. The current study examined 170 patients with either FTLD-tau (n = 73) or FTLD-TDP (n = 97) spanning dementia and motor phenotypes in the FTD spectrum. We digitally measured the percent area occupied by tau and TDP-43 pathology in upper layers (I-III), lower layers (IV-VI), and juxtacortical white matter (WM) from isocortical regions in both hemispheres where available. Linear mixed-effects models compared ratios of upper to lower layer pathology between FTLD groups and investigated relationships with regions, WM pathology, and global cognitive impairment while adjusting for demographics. We found lower ratios of layer pathology in FTLD-tau and higher ratios of layer pathology in FTLD-TDP, reflecting lower layer-predominant tau pathology and upper layer-predominant TDP-43 pathology, respectively (p < 0.001). FTLD-tau displayed lower ratios of layer pathology related to greater WM tau pathology (p = 0.002) and to earlier involved/severe pathology regions (p = 0.007). In contrast, FTLD-TDP displayed higher ratios of layer pathology not related to either WM pathology or regional severity. Greater cognitive impairment was associated with higher ratios of layer pathology in FTLD-tau (p = 0.018), but was not related to ratios of layer pathology in FTLD-TDP. Lower layer-predominant tau pathology and upper layer-predominant TDP-43 pathology are proteinopathy-specific, regardless of clinical syndromes or regional networks that define these syndromes. Thus, patterns of laminar change may provide a useful anatomical framework for investigating how degeneration of select cells and corresponding laminar circuits influence large-scale networks and clinical symptomology in FTLD.

The enigma and implications of brain hemispheric asymmetry in neurodegenerative diseases

The lateralization of the human brain may provide clues into the pathogenesis and progression of neurodegenerative diseases. Though differing in their presentation and underlying pathologies, neurodegenerative diseases are all devastating and share an intriguing theme of asymmetrical pathology and clinical symptoms. Parkinson’s disease, with its distinctive onset of motor symptoms on one side of the body, stands out in this regard, but a review of the literature reveals asymmetries in several other neurodegenerative diseases. Here, we review the lateralization of the structure and function of the healthy human brain and the common genetic and epigenetic patterns contributing to the development of asymmetry in health and disease. We specifically examine the role of asymmetry in Parkinson’s disease, Alzheimer’s disease, amyotrophic lateral sclerosis, and multiple sclerosis, and interrogate whether these imbalances may reveal meaningful clues about the origins of these diseases. We also propose several hypotheses for how lateralization may contribute to the distinctive and enigmatic features of asymmetry in neurodegenerative diseases, suggesting a role for asymmetry in the choroid plexus, neurochemistry, protein distribution, brain connectivity and the vagus nerve. Finally, we suggest how future studies may reveal novel insights into these diseases through the lens of asymmetry.

Identification of Phonology-Related Genes and Functional Characterization of Broca's and Wernicke's Regions in Language and Learning Disorders

Impaired phonological processing is a leading symptom of multifactorial language and learning disorders suggesting a common biological basis. Here we evaluated studies of dyslexia, dyscalculia, specific language impairment (SLI), and the logopenic variant of primary progressive aphasia (lvPPA) seeking for shared risk genes in Broca's and Wernicke's regions, being key for phonological processing within the complex language network. The identified "phonology-related genes" from literature were functionally characterized using Atlas-based expression mapping (JuGEx) and gene set enrichment. Out of 643 publications from the last decade until now, we extracted 21 candidate genes of which 13 overlapped with dyslexia and SLI, six with dyslexia and dyscalculia, and two with dyslexia, dyscalculia, and SLI. No overlap was observed between the childhood disorders and the late-onset lvPPA often showing symptoms of learning disorders earlier in life. Multiple genes were enriched in Gene Ontology terms of the topics learning (CNTNAP2, CYFIP1, DCDC2, DNAAF4, FOXP2) and neuronal development (CCDC136, CNTNAP2, CYFIP1, DCDC2, KIAA0319, RBFOX2, ROBO1). Twelve genes showed above-average expression across both regions indicating moderate-to-high gene activity in the investigated cortical part of the language network. Of these, three genes were differentially expressed suggesting potential regional specializations: ATP2C2 was upregulated in Broca's region, while DNAAF4 and FOXP2 were upregulated in Wernicke's region. ATP2C2 encodes a magnesium-dependent calcium transporter which fits with reports about disturbed calcium and magnesium levels for dyslexia and other communication disorders. DNAAF4 (formerly known as DYX1C1) is involved in neuronal migration supporting the hypothesis of disturbed migration in dyslexia. FOXP2 is a transcription factor that regulates a number of genes involved in development of speech and language. Overall, our interdisciplinary and multi-tiered approach provided evidence that genetic and transcriptional variation of ATP2C2, DNAAF4, and FOXP2 may play a role in physiological and pathological aspects of phonological processing.

The Combination of Metabolic Posterior Cingulate Cortical Abnormalities and Structural Asymmetries Improves the Differential Diagnosis Between Primary Progressive Aphasia and Alzheimer's Disease

Differential diagnosis between primary progressive aphasia (PPA) and Alzheimer's disease (AD) could be difficult if based on clinical grounds alone. We evaluated the combination of proton MR spectroscopy of posterior cingulate cortex (PCC) and quantitative structural imaging asymmetries to differentiate PPA from AD patients. A greater left-lateralized temporo-parietal atrophy (higher accuracy for the PCC, 81.4%) and metabolic neurodegenerative changes in PCC (accuracy 76.8%) was demonstrated in PPA versus AD. The combined multiparametric approach increased the accuracy to 94%in the differential diagnosis between these two neurodegenerative diseases.

Primary Progressive Aphasia Associated With GRN Mutations: New Insights Into the Nonamyloid Logopenic Variant

OBJECTIVE

To determine relative frequencies and linguistic profiles of primary progressive aphasia (PPA) variants associated with GRN (progranulin) mutations and to study their neuroanatomic correlates.

METHODS

Patients with PPA carrying GRN mutations (PPA-GRN) were selected among a national prospective research cohort of 1,696 patients with frontotemporal dementia, including 235 patients with PPA. All patients with amyloid-positive CSF biomarkers were excluded. In this cross-sectional study, speech/language and cognitive profiles were characterized with standardized evaluations, and gray matter (GM) atrophy patterns using voxel-based morphometry. Comparisons were performed with controls and patients with sporadic PPA.

RESULTS

Among the 235 patients with PPA, 45 (19%) carried GRN mutations, and we studied 32 of these. We showed that logopenic PPA (lvPPA) was the most frequent linguistic variant (n = 13, 41%), followed by nonfluent/agrammatic (nfvPPA; n = 9, 28%) and mixed forms (n = 8, 25%). Semantic variant was rather rare (n = 2, 6%). Patients with lvPPA, qualified as nonamyloid lvPPA, presented canonical logopenic deficit. Seven of 13 had a pure form; 6 showed subtle additional linguistic deficits not fitting criteria for mixed PPA and hence were labeled as logopenic-spectrum variant. GM atrophy involved primarily left posterior temporal gyrus, mirroring neuroanatomic changes of amyloid-positive-lvPPA. Patients with nfvPPA presented agrammatism (89%) rather than apraxia of speech (11%).

CONCLUSIONS

This study shows that the most frequent PPA variant associated with GRN mutations is nonamyloid lvPPA, preceding nfvPPA and mixed forms, and illustrates that the language network may be affected at different levels. GRN testing is indicated for patients with PPA, whether familial or sporadic. This finding is important for upcoming GRN gene-specific therapies.

Nosology of Primary Progressive Aphasia and the Neuropathology of Language

Primary progressive aphasia (PPA) is a dementia syndrome associated with several neuropathologic entities, including Alzheimer's disease (AD) and all major forms of frontotemporal lobar degeneration (FTLD). It is classified into subtypes defined by the nature of the language domain that is most impaired. The asymmetric neurodegeneration of the hemisphere dominant for language (usually left) is one consistent feature of all PPA variants. This feature offers unique opportunities for exploring mechanisms of selective vulnerability in neurodegenerative diseases and the neuroanatomy of language. This chapter reviews some of the current trends in PPA research as well as the challenges that remain to be addressed on the nosology, clinicopathologic correlations, and therapy of this syndrome.

Distribution of TDP-43 Pathology in Hippocampal Synaptic Relays Suggests Transsynaptic Propagation in Frontotemporal Lobar Degeneration

Hyperphosphorylation, nuclear depletion, and aggregation of TDP-43 in ubiquitinated inclusions is a hallmark of frontotemporal lobar degeneration (FTLD-TDP). Evidence of potential spread of TDP-43 along synaptic connections in the human is largely limited to qualitative and semiquantitative observations. We quantitatively investigated potential transsynaptic propagation of TDP-43 across the well-established chain of single synaptic connections of the hippocampus. Hippocampi from 5 participants with clinical diagnoses of primary progressive aphasia and 2 participants with behavioral variant frontotemporal dementia, all with postmortem diagnoses of FTLD-TDP, were examined. TDP-43-positive mature (darkly stained) and pre-inclusions (diffuse puncta or fibrillar staining) in the granule cell layer of dentate gyrus (DG) and pyramidal cell layers of Cornu Ammonis (CA)3, CA2, and CA1 were quantified using unbiased stereology. The density of mature TDP-43 inclusions was higher in the DG than in the CA fields (p < 0.05). There were no differences in inclusion densities across the CA fields. TDP-43 pre-inclusions densities were not different across the 4 subregions. There was significantly higher preinclusion density than mature inclusions in CA3, but not in other subregions. Analysis of normalized total counts in place of densities revealed virtually identical results. Our finding of greatest mature inclusion deposition in the DG, coupled with more preinclusions than mature inclusions at the next relay station (CA3), and reduced densities of both in CA2-CA1, provide evidence in support of a sequential transsynaptic propagation mechanism of TDP-43 aggregates.

Cerebrospinal Fluid YKL-40 and Chitotriosidase Levels in Frontotemporal Dementia Vary by Clinical, Genetic and Pathological Subtype

BACKGROUND

Chronic glial dysfunction may contribute to the pathogenesis of frontotemporal dementia (FTD). Cerebrospinal fluid (CSF) levels of glia-derived proteins YKL-40 and chitotriosidase are increased in Alzheimer's disease (AD) but have not been explored in detail across the spectrum of FTD.

METHODS

We investigated whether CSF YKL-40 and chitotriosidase levels differed between FTD patients and controls, across different clinical and genetic subtypes of FTD, and between individuals with a clinical FTD syndrome due to AD versus non-AD (frontotemporal lobar degeneration, FTLD) pathology (based on CSF neurodegenerative biomarkers). Eighteen healthy controls and 64 people with FTD (behavioural variant FTD, n = 20; primary progressive aphasia [PPA], n = 44: nfvPPA, n = 16, svPPA, n = 11, lvPPA, n = 14, PPA-NOS, n = 3) were included. 10/64 had familial FTD, with mutations in GRN(n = 3), MAPT(n = 4), or C9orf72 (n = 3). 15/64 had neurodegenerative biomarkers consistent with AD pathology. Levels were measured by immunoassay and compared using multiple linear regressions. We also examined relationships of YKL-40 and chitotriosidase with CSF total tau (T-tau), phosphorylated tau 181 (P-tau) and β-amyloid 1-42 (Aβ42), with each other, and with age and disease du-ration.

RESULTS

CSF YKL-40 and chitotriosidase levels were higher in FTD, particularly lvPPA (both) and nfvPPA (YKL-40), compared with controls. GRN mutation carriers had higher levels of both proteins than controls and C9orf72 expansion carriers, and YKL-40 was higher in MAPT mutation carriers than controls. Individuals with underlying AD pathology had higher YKL-40 and chitotriosidase levels than both controls and those with likely FTLD pathology. CSF YKL-40 and chitotriosidase levels were variably associated with levels of T-tau, P-tau and Aβ42, and with each other, depending on clinical syndrome and underlying pathology. CSF YKL-40 but not chitotriosidase was associated with age, but not disease duration.

CONCLUSION

CSF YKL-40 and chitotriosidase levels are increased in individuals with clinical FTD syndromes, particularly due to AD pathology. In a preliminary analysis of genetic groups, levels of both proteins are found to be highly elevated in FTD due to GRN mutations, while YKL-40 is increased in individuals with MAPT mutations. As glia-derived protein levels generally correlate with T-tau and P-tau levels, they may reflect the glial response to neurodegeneration in FTLD.

Neuropathology of Speech Network Distinguishes Bulbar From Nonbulbar Amyotrophic Lateral Sclerosis

Bulbar amyotrophic lateral sclerosis (ALS) is a debilitating neurodegenerative subtype affecting speech and swallowing motor functions as well as associated with the burden of cognitive deficits. The neuroanatomical underpinnings of bulbar ALS are not well understood. The aim of this study was to compare neuropathology of the speech network (SpN) between 3 cases of bulbar-onset ALS (bALS), 3 cases of spinal-onset ALS (sALS) with antemortem bulbar ALS (sALSwB) against 3 sALS without antemortem bulbar ALS (sALSnoB) and 3 controls. Regional distribution and severity of neuronal loss, TDP-43 (transactive response DNA-binding protein of 43 kDa), and tau proteinopathy were examined. All 3 bALS cases showed marked neuronal loss and severe proteinopathy across most SpN regions; sALSwB cases showed no neuronal loss but mild and variable TDP-43 pathology in focal regions; sALSnoB cases demonstrated an absence of pathology. Two bALS cases had coexisting tauopathy in SpN regions, which was not noted in any sALS cases. The findings suggested that bALS may have a distinct neuropathological signature characterized by marked neuronal loss and polypathology in the SpN. Milder TDP-43 pathology in the SpN for sALSwB cases suggested a link between severity of bulbar ALS and SpN damage. Findings support a clinicopathologic link between bulbar symptoms and pathology in the SpN.

Variability of clinical syndromes and cerebral glucose metabolism in symptomatic frontotemporal lobar degeneration associated with progranulin mutations

Objective: The aims of our study were to describe the clinical phenotype and to characterize the cerebral glucose metabolism patterns as measured with fluordesoxyglucose-positron emission tomography (FDG-PET) in symptomatic FTLD-patients with different GRN variants. Methods: For this study, data were included from all patients (n = 10) of a single-center FTLD registry study who had a pathogenic GRN variant and who had undergone a cerebral FDG-PET scan. Results: An overt variability of clinical phenotypes was identified with half of the cases being not unambiguously classifiable into one of the clinical FTLD subtypes. Furthermore, GRN + patients showed a considerable inter-individual variability of FDG uptake pattern. In half of the GRN + patients, metabolic changes expanded from frontal and temporal brain regions to parietal brain regions including the posterior cingulate cortex. Striking asymmetry without a preference for either hemisphere was overt in half of GRN + cases. Conclusion: We conclude that GRN mutations cause variable patterns of neurodegeneration that often exceed the anatomical boundaries of the frontotemporal brain regions and produce clinical syndromes that cannot clearly be classified into one of the subtypes as defined by the diagnostic criteria.

Morphology and Distribution of TDP-43 Pre-inclusions in Primary Progressive Aphasia

Diffusely stained phosphorylated 43-kDa TAR DNA-binding protein (TDP-43)-positive "pre-inclusions" have been described. This experiment investigated morphological subtypes of pre-inclusions and their relationship with TDP-43 inclusions in primary progressive aphasia (PPA), a dementia characterized by gradual dissolution of language. Brain sections from 5 PPA participants with postmortem diagnoses of frontotemporal lobar degeneration with TDP-43 pathology (FTLD-TDP) were immunohistochemically stained using an antibody to phosphorylated TDP-43 and quantitatively examined for regional and hemispheric distribution using unbiased stereology. Cortical TDP-43 pre-inclusions included smooth, granular/dot-like, or fibrillar staining with localization to the nucleus, cytoplasm, or both. Mature and pre-inclusions were quantified in a region with high and a region with low mature inclusion density, and contralateral homologs. Regions with lower mature inclusions were characterized by higher densities of pre-inclusions, while increasing burden of inclusions corresponded to lower densities of pre-inclusions (p < 0.05). Mature inclusions showed significant asymmetry that favored the language-dominant hemisphere (p < 0.01), while pre-inclusions displayed the opposite pattern (p < 0.01). Granular-type pre-inclusions were more abundant (p < 0.05) and drove the hemispheric and regional differences (p < 0.02). These results suggest that pre-inclusions are present in greater abundance prior to the formation of mature TDP-43 inclusions, and appear to develop through progressive stages into mature intracytoplasmic, or intranuclear aggregates.

Genetic forms of frontotemporal lobar degeneration: Current diagnostic approach and new directions in therapeutic strategies

Recent advances in the genetics of neurodegenerative diseases have substantially improved our knowledge about the genetic causes of frontotemporal lobar degeneration (FTLD). Three major genes, namely progranulin (GRN), C9orf72 and MAPT, as well as several less common genes, are responsible for the majority of familial cases and for a significant proportion of sporadic forms, including FTLD with or without associated amyotrophic lateral sclerosis and some rarer clinical presentations. Plasma progranulin dosage and next-generation sequencing are currently available tools which allow the detection of a genetic cause in a more rapid and efficient way. This has important consequences for clinical practice and genetic counseling for patients and families. The ongoing investigations on some therapeutic candidates targeting different biological pathways involved in the most frequent genetic forms of FTLD, as well as a better understanding of the early pathophysiological modifications occurring during the presymptomatic phase of the disease could hopefully contribute to develop effective disease-modifying therapies. The identification of a causal mutation in a family is of outmost importance indeed to propose to presymptomatic carriers their inclusion in clinical trials with the aim to prevent or delay the onset of disease.

Cerebral perfusion changes in presymptomatic genetic frontotemporal dementia: a GENFI study

Genetic forms of frontotemporal dementia are most commonly due to mutations in three genes, C9orf72, GRN or MAPT, with presymptomatic carriers from families representing those at risk. While cerebral blood flow shows differences between frontotemporal dementia and other forms of dementia, there is limited evidence of its utility in presymptomatic stages of frontotemporal dementia. This study aimed to delineate the cerebral blood flow signature of presymptomatic, genetic frontotemporal dementia using a voxel-based approach. In the multicentre GENetic Frontotemporal dementia Initiative (GENFI) study, we investigated cross-sectional differences in arterial spin labelling MRI-based cerebral blood flow between presymptomatic C9orf72, GRN or MAPT mutation carriers (n = 107) and non-carriers (n = 113), using general linear mixed-effects models and voxel-based analyses. Cerebral blood flow within regions of interest derived from this model was then explored to identify differences between individual gene carrier groups and to estimate a timeframe for the expression of these differences. The voxel-based analysis revealed a significant inverse association between cerebral blood flow and the expected age of symptom onset in carriers, but not non-carriers. Regions included the bilateral insulae/orbitofrontal cortices, anterior cingulate/paracingulate gyri, and inferior parietal cortices, as well as the left middle temporal gyrus. For all bilateral regions, associations were greater on the right side. After correction for partial volume effects in a region of interest analysis, the results were found to be largely driven by the C9orf72 genetic subgroup. These cerebral blood flow differences first appeared approximately 12.5 years before the expected symptom onset determined on an individual basis. Cerebral blood flow was lower in presymptomatic mutation carriers closer to and beyond their expected age of symptom onset in key frontotemporal dementia signature regions. These results suggest that arterial spin labelling MRI may be a promising non-invasive imaging biomarker for the presymptomatic stages of genetic frontotemporal dementia.

Genotype-phenotype links in frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) represents a group of neurodegenerative brain diseases with highly heterogeneous clinical, neuropathological and genetic characteristics. This high degree of heterogeneity results from the presence of several different underlying molecular disease processes; consequently, it is unlikely that all patients with FTLD will benefit from a single therapy. Therapeutic strategies for FTLD are currently being explored, and tools are urgently needed that enable the selection of patients who are the most likely to benefit from a particular therapy. Definition of the phenotypic characteristics in patients with different FTLD subtypes that share the same underlying disease processes would assist in the stratification of patients into homogeneous groups. The most common subtype of FTLD is characterized by TAR DNA-binding protein 43 (TDP43) pathology (FTLD-TDP). In this group, pathogenic mutations have been identified in four genes: C9orf72, GRN, TBK1 and VCP. Here, we provide a comprehensive overview of the phenotypic characteristics of patients with FTLD-TDP, highlighting shared features and differences among groups of patients who have a pathogenic mutation in one of these four genes.

Primary Progressive Aphasia and Stroke Aphasia

PURPOSE OF REVIEW

This article summarizes the clinical and anatomic features of the three named variants of primary progressive aphasia (PPA): semantic variant PPA, nonfluent/agrammatic variant PPA, and logopenic variant PPA. Three stroke aphasia syndromes that resemble the PPA variants (Broca aphasia, Wernicke aphasia, and conduction aphasia) are also presented.

RECENT FINDINGS

Semantic variant PPA and Wernicke aphasia are characterized by fluent speech with naming and comprehension difficulty; these syndromes are associated with disease in different portions of the left temporal lobe. Patients with nonfluent/agrammatic variant PPA or Broca aphasia have nonfluent speech with grammatical difficulty; these syndromes are associated with disease centered in the left inferior frontal lobe. Patients with logopenic variant PPA or conduction aphasia have difficulty with repetition and word finding in conversational speech; these syndromes are associated with disease in the left inferior parietal lobe. While PPA and stroke aphasias resemble one another, this article also presents their distinguishing features.

SUMMARY

Primary progressive and stroke aphasia syndromes interrupt the left perisylvian language network, resulting in identifiable aphasic syndromes.

Preferential Disruption of Auditory Word Representations in Primary Progressive Aphasia With the Neuropathology of FTLD-TDP Type A

Four patients with primary progressive aphasia displayed a greater deficit in understanding words they heard than words they read, and a further deficiency in naming objects orally rather than in writing. All four had frontotemporal lobar degeneration-transactive response DNA binding protein Type A neuropathology, three determined postmortem and one surmised on the basis of granulin gene (GRN) mutation. These features of language impairment are not characteristic of any currently recognized primary progressive aphasia variant. They can be operationalized as manifestations of dysfunction centered on a putative auditory word-form area located in the superior temporal gyrus of the left hemisphere. The small size of our sample makes the conclusions related to underlying pathology and auditory word-form area dysfunction tentative. Nonetheless, a deeper assessment of such patients may clarify the nature of pathways that link modality-specific word-form information to the associations that mediate their recognition as concepts. From a practical point of view, the identification of these features in patients with primary progressive aphasia should help in the design of therapeutic interventions where written communication modalities are promoted to circumvent some of the oral communication deficits.

Divergent patterns of TDP-43 and tau pathologies in primary progressive aphasia

OBJECTIVE

To measure postmortem burden of frontotemporal lobar degeneration (FTLD) with TDP-43 (FTLD-TDP) or tau (FTLD-Tau) proteinopathy across hemispheres in primary progressive aphasia (PPA) using digital histopathology and to identify clinicopathological correlates of these distinct proteinopathies.

METHODS

In an autopsy cohort of PPA (FTLD-TDP = 13, FTLD-Tau = 14), we analyzed laterality and regional distribution of postmortem pathology, quantified using a validated digital histopathological approach, in available brain tissue from up to 8 cortical regions bilaterally. We related digital pathology to antemortem structural neuroimaging and specific clinical language features.

RESULTS

Postmortem cortical pathology was left-lateralized in both FTLD-TDP (beta = -0.15, standard error [SE] = 0.05, p = 0.007) and FTLD-Tau (beta = -0.09, SE = 0.04, p = 0.015), but the degree of lateralization decreased with greater overall dementia severity before death (beta = -8.18, SE = 3.22, p = 0.015). Among 5 core pathology regions sampled, we found greatest pathology in left orbitofrontal cortex (OFC) in FTLD-TDP, which was greater than in FTLD-Tau (F = 47.07, df = 1,17, p < 0.001), and in left midfrontal cortex (MFC) in FTLD-Tau, which was greater than in FTLD-TDP (F = 19.34, df = 1,16, p < 0.001). Postmortem pathology was inversely associated with antemortem magnetic resonance imaging cortical thickness (beta = -0.04, SE = 0.01, p = 0.007) in regions matching autopsy sampling. Irrespective of PPA syndromic variant, single-word comprehension impairment was associated with greater left OFC pathology (t = -3.72, df = 10.72, p = 0.004) and nonfluent speech with greater left MFC pathology (t = -3.62, df = 12.00, p = 0.004) among the 5 core pathology regions.

INTERPRETATION

In PPA, FTLD-TDP and FTLD-Tau have divergent anatomic distributions of left-lateralized postmortem pathology that relate to antemortem structural imaging and distinct language deficits. Although other brain regions may be implicated in neural networks supporting these complex language measures, our observations may eventually help to improve antemortem diagnosis of neuropathology in PPA. Ann Neurol 2019;85:630-643.

Asymmetry of post-mortem neuropathology in behavioural-variant frontotemporal dementia

Antemortem behavioural and anatomic abnormalities have largely been associated with right hemisphere disease in behavioural-variant frontotemporal dementia, but post-mortem neuropathological examination of bilateral hemispheres remains to be defined. Here we measured the severity of post-mortem pathology in both grey and white matter using a validated digital image analysis method in four cortical regions sampled from each hemisphere in 26 patients with behavioural-variant frontotemporal dementia, including those with frontotemporal degeneration (i.e. tau = 9, TDP-43 = 14, or FUS = 1 proteinopathy) or Alzheimer's pathology (n = 2). We calculated an asymmetry index based on the difference in measured pathology from each left-right sample pair. Analysis of the absolute value of the asymmetry index (i.e. degree of asymmetry independent of direction) revealed asymmetric pathology for both grey and white matter in all four regions sampled in frontototemporal degeneration patients with tau or TDP-43 pathology (P ≤ 0.01). Direct interhemispheric comparisons of regional pathology measurements within-subjects in the combined tauopathy and TDP-43 proteinopathy group found higher pathology in the right orbitofrontal grey matter compared to the left (P < 0.01) and increased pathology in ventrolateral temporal lobe grey matter of the left hemisphere compared to the right (P < 0.02). Preliminary group-wise comparisons between tauopathy and TDP-43 proteinopathy groups found differences in patterns of interhemispheric burden of grey and white matter regional pathology, with greater relative white matter pathology in tauopathies. To test the association of pathology measurement with ante-mortem observations, we performed exploratory analyses in the subset of patients with imaging data (n = 15) and found a direct association for increasing pathologic burden with decreasing cortical thickness in frontotemporal regions on ante-mortem imaging in tauopathy (P = 0.001) and a trend for TDP-43 proteinopathy (P = 0.06). Exploratory clinicopathological correlations demonstrated an association of socially-inappropriate behaviours with asymmetric right orbitofrontal grey matter pathology, and reduced semantically-guided category naming fluency was associated asymmetric white matter pathology in the left ventrolateral temporal region. We conclude that pathologic disease burden is distributed asymmetrically in behavioural-variant frontotemporal dementia, although not universally in the right hemisphere, and this asymmetry contributes to the clinical heterogeneity of the disorder. The basis for this asymmetric profile is enigmatic but may reflect distinct species or strains of tau and TDP-43 pathologies with propensities to spread by distinct cell- and region-specific mechanisms. Patterns of region-specific pathology in the right hemisphere as well as the left hemisphere may play a role in antemortem clinical observations, and these observations may contribute to antemortem identification of molecular pathology in frontotemporal degeneration.

Cortical cholinergic denervation in primary progressive aphasia with Alzheimer pathology

OBJECTIVE

To investigate the status of the basal forebrain cholinergic system in primary progressive aphasia (PPA) as justification for cholinergic therapy.

METHODS

A cohort of 36 brains from PPA participants with the neuropathology of Alzheimer disease (PPA-AD, n = 14) or frontotemporal lobar degeneration (PPA-tau, n = 12; PPA-TDP, n = 10) were used for semiquantitative rating of degeneration and gliosis of basal forebrain cholinergic neurons (BFCN). A subpopulation of 5 PPA-AD and 7 control brains underwent detailed analysis of BFCN pathology and cortical cholinergic axonal loss employing immunohistochemical and histochemical methods and stereologic analysis.

RESULTS

Semiquantitatively, 11 (∼80%) PPA-AD participants were rated as having moderate/severe BFCN loss and gliosis, whereas none of the PPA-tau and only 1 (10%) PPA-TDP participant received such a rating. Detailed analysis in the subpopulation of PPA-AD participants revealed substantial tangle formation, loss of BFCN, and degeneration of cortical cholinergic axons. Compared to controls, loss of p75 low affinity neurotrophin receptor-positive BFCN was detected in the PPA-AD participants (p < 0.01). Acetylcholinesterase-positive cholinergic axons in all cortical areas studied displayed loss in PPA-AD (p < 0.005-0.0001). The loss was more severe in the language-dominant left hemisphere and, within the left hemisphere, in language-affiliated cortical areas.

CONCLUSIONS

Our results demonstrate prominent depletion of BFCN and cortical cholinergic axons in PPA-AD when compared with normal control or other neuropathologic variants of PPA. The demonstration of cholinergic denervation with an anatomy that fits the clinical picture suggests that cholinergic treatment is justified in patients with PPA who have positive AD biomarkers.

Atrophy and microglial distribution in primary progressive aphasia with transactive response DNA-binding protein-43 kDa

OBJECTIVE

To quantitatively determine the density and distribution of activated microglia across cortical regions and hemispheres in the brains of primary progressive aphasia (PPA) participants with pathological diagnoses of frontotemporal lobar degeneration with transactive response DNA-binding protein-43 (TDP-43) inclusions and to examine the relationships between microglial densities, patterns of focal atrophy, (TDP-43) inclusions, and clinical phenotype.

METHODS

Activated microglia and TDP-43 inclusions were visualized in whole-hemisphere brain sections using immunohistochemical methods from five participants with PPA-TDP. Unbiased stereology was used to bilaterally quantify human leuckocyte antigen/D related-positive activated microglia and TDP-43 inclusions across five language-related regions. Density and distribution of both markers were compared across cortical regions and hemispheres, and their relationships to patterns of focal atrophy and clinical phenotype were determined.

RESULTS

Activated microglia displayed asymmetric distribution favoring the language-dominant hemisphere, consistent with greater postmortem and/or in vivo atrophy in that hemisphere, in PPA-TDP. In one participant with no asymmetric atrophy, quantitative distribution of microglia also lacked asymmetry. Patterns of microglial activation also showed variation that favored areas of high atrophy in regions affiliated with language function, demonstrating concordance between patterns of microglial activation, atrophy, and clinical phenotype. TDP-43 also showed higher inclusion densities in areas of high atrophy than in regions with low atrophy, but no clear relationship with microglia density at a regional level.

INTERPRETATION

The initial activation of microglia is most likely a response to cortical abnormalities in PPA-TDP, which contribute to atrophy. The patterns of microglial activation, TDP-43 inclusion deposition, atrophy, and clinical phenotype suggest that activated microglia may make unique contributions to cortical thinning and TDP-43 inclusion formation. Ann Neurol 2018;83:1096-1104.

Prominent microglial activation in cortical white matter is selectively associated with cortical atrophy in primary progressive aphasia

AIMS

Primary progressive aphasia (PPA) is a clinical syndrome characterized by selective language impairments associated with focal cortical atrophy favouring the language dominant hemisphere. PPA is associated with Alzheimer's disease (AD), frontotemporal lobar degeneration (FTLD) and significant accumulation of activated microglia. Activated microglia can initiate an inflammatory cascade that may contribute to neurodegeneration, but their quantitative distribution in cortical white matter and their relationship with cortical atrophy remain unknown. We investigated white matter activated microglia and their association with grey matter atrophy in 10 PPA cases with either AD or FTLD-TDP pathology.

METHODS

Activated microglia were quantified with optical density measures of HLA-DR immunoreactivity in two regions with peak cortical atrophy, and one nonatrophied region within the language dominant hemisphere of each PPA case. Nonatrophied contralateral homologues of the language dominant regions were examined for hemispheric asymmetry.

RESULTS

Qualitatively, greater densities of activated microglia were observed in cortical white matter when compared to grey matter. Quantitative analyses revealed significantly greater densities of activated microglia in the white matter of atrophied regions compared to nonatrophied regions in the language dominant hemisphere (P < 0.05). Atrophied regions of the language dominant hemisphere also showed significantly more activated microglia compared to contralateral homologues (P < 0.05).

CONCLUSIONS

White matter activated microglia accumulate more in atrophied regions in the language dominant hemisphere of PPA. While microglial activation may constitute a response to neurodegenerative processes in white matter, the resultant inflammatory processes may also exacerbate disease progression and contribute to cortical atrophy.

Asymmetric TDP pathology in primary progressive aphasia with right hemisphere language dominance

OBJECTIVE

To quantitatively examine the regional densities and hemispheric distribution of the 43-kDa transactive response DNA-binding protein (TDP-43) inclusions, neurons, and activated microglia in a left-handed patient with right hemisphere language dominance and logopenic-variant primary progressive aphasia (PPA).

METHODS

Phosphorylated TDP-43 inclusions, neurons, and activated microglia were visualized with immunohistochemical and histologic methods. Markers were quantified bilaterally with unbiased stereology in language- and memory-related cortical regions.

RESULTS

Clinical MRI indicated cortical atrophy in the right hemisphere, mostly in the temporal lobe. Significantly higher densities of TDP-43 inclusions were present in right language-related temporal regions compared to the left or to other right hemisphere regions. The memory-related entorhinal cortex (ERC) and language regions without significant atrophy showed no asymmetry. Activated microglia displayed extensive asymmetry (R > L). A substantial density of neurons remained in all areas and showed no hemispheric asymmetry. However, perikaryal size was significantly smaller in the right hemisphere across all regions except the ERC. To demonstrate the specificity of this finding, sizes of residual neurons were measured in a right-handed case with PPA and were found to be smaller in the language-dominant left hemisphere.

CONCLUSIONS

The distribution of TDP-43 inclusions and microglial activation in right temporal language regions showed concordance with anatomic distribution of cortical atrophy and clinical presentation. The results revealed no direct relationship between density of TDP-43 inclusions and activated microglia. Reduced size of the remaining neurons is likely to contribute to cortical atrophy detected by MRI. These findings support the conclusion that there is no obligatory relationship between logopenic PPA and Alzheimer pathology.