Ian R. Mackenzie

Inflammation and Alzheimer’s disease

Inflammation clearly occurs in pathologically vulnerable regions of the Alzheimers disease (AD) brain, and it does so with the full complexity of local peripheral inflammatory responses. In the periphery, degenerating tissue and the deposition of highly insoluble abnormal materials are classical stimulants of inflammation. Likewise, in the AD brain damaged neurons and neurites and highly insoluble amyloid beta peptide deposits and neurofibrillary tangles provide obvious stimuli for inflammation. Because these stimuli are discrete, microlocalized, and present from early preclinical to terminal stages of AD, local upregulation of complement, cytokines, acute phase reactants, and other inflammatory mediators is also discrete, microlocalized, and chronic. Cumulated over many years, direct and bystander damage from AD inflammatory mechanisms is likely to significantly exacerbate the very pathogenic processes that gave rise to it. Thus, animal models and clinical studies, although still in their infancy, strongly suggest that AD inflammation significantly contributes to AD pathogenesis. By better understanding AD inflammatory and immunoregulatory processes, it should be possible to develop anti-inflammatory approaches that may not cure AD but will likely help slow the progression or delay the onset of this devastating disorder.

Expanded GGGGCC Hexanucleotide Repeat in Noncoding Region of C9ORF72 Causes Chromosome 9p-Linked FTD and ALS

Several families have been reported with autosomal-dominant frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS), genetically linked to chromosome 9p21. Here, we report an expansion of a noncoding GGGGCC hexanucleotide repeat in the gene C9ORF72 that is strongly associated with disease in a large FTD/ALS kindred, previously reported to be conclusively linked to chromosome 9p. This same repeat expansion was identified in the majority of our families with a combined FTD/ALS phenotype and TDP-43-based pathology. Analysis of extended clinical series found the C9ORF72 repeat expansion to be the most common genetic abnormality in both familial FTD (11.7%) and familial ALS (23.5%). The repeat expansion leads to the loss of one alternatively spliced C9ORF72 transcript and to formation of nuclear RNA foci, suggesting multiple disease mechanisms. Our findings indicate that repeat expansion in C9ORF72 is a major cause of both FTD and ALS.

Mutations in progranulin cause tau-negative frontotemporal dementia linked to chromosome 17.

Frontotemporal dementia (FTD) is the second most common cause of dementia in people under the age of 65 years. A large proportion of FTD patients (35–50%) have a family history of dementia, consistent with a strong genetic component to the disease. In 1998, mutations in the gene encoding the microtubule-associated protein tau (MAPT) were shown to cause familial FTD with parkinsonism linked to chromosome 17q21 (FTDP-17). The neuropathology of patients with defined MAPT mutations is characterized by cytoplasmic neurofibrillary inclusions composed of hyperphosphorylated tau. However, in multiple FTD families with significant evidence for linkage to the same region on chromosome 17q21 (D17S1787–D17S806), mutations in MAPT have not been found and the patients consistently lack tau-immunoreactive inclusion pathology. In contrast, these patients have ubiquitin (ub)-immunoreactive neuronal cytoplasmic inclusions and characteristic lentiform ub-immunoreactive neuronal intranuclear inclusions. Here we demonstrate that in these families, FTD is caused by mutations in progranulin (PGRN) that are likely to create null alleles. PGRN is located 1.7 Mb centromeric of MAPT on chromosome 17q21.31 and encodes a 68.5-kDa secreted growth factor involved in the regulation of multiple processes including development, wound repair and inflammation. PGRN has also been strongly linked to tumorigenesis. Moreover, PGRN expression is increased in activated microglia in many neurodegenerative diseases including Creutzfeldt–Jakob disease, motor neuron disease and Alzheimers disease. Our results identify mutations in PGRN as a cause of neurodegenerative disease and indicate the importance of PGRN function for neuronal survival.

Neuropathologic diagnostic and nosologic criteria for frontotemporal lobar degeneration: consensus of the Consortium for Frontotemporal Lobar Degeneration

The aim of this study was to improve the neuropathologic recognition and provide criteria for the pathological diagnosis in the neurodegenerative diseases grouped as frontotemporal lobar degeneration (FTLD); revised criteria are proposed. Recent advances in molecular genetics, biochemistry, and neuropathology of FTLD prompted the Midwest Consortium for Frontotemporal Lobar Degeneration and experts at other centers to review and revise the existing neuropathologic diagnostic criteria for FTLD. The proposed criteria for FTLD are based on existing criteria, which include the tauopathies [FTLD with Pick bodies, corticobasal degeneration, progressive supranuclear palsy, sporadic multiple system tauopathy with dementia, argyrophilic grain disease, neurofibrillary tangle dementia, and FTD with microtubule-associated tau (MAPT) gene mutation, also called FTD with parkinsonism linked to chromosome 17 (FTDP-17)]. The proposed criteria take into account new disease entities and include the novel molecular pathology, TDP-43 proteinopathy, now recognized to be the most frequent histological finding in FTLD. TDP-43 is a major component of the pathologic inclusions of most sporadic and familial cases of FTLD with ubiquitin-positive, tau-negative inclusions (FTLD-U) with or without motor neuron disease (MND). Molecular genetic studies of familial cases of FTLD-U have shown that mutations in the progranulin (PGRN) gene are a major genetic cause of FTLD-U. Mutations in valosin-containing protein (VCP) gene are present in rare familial forms of FTD, and some families with FTD and/or MND have been linked to chromosome 9p, and both are types of FTLD-U. Thus, familial TDP-43 proteinopathy is associated with defects in multiple genes, and molecular genetics is required in these cases to correctly identify the causative gene defect. In addition to genetic heterogeneity amongst the TDP-43 proteinopathies, there is also neuropathologic heterogeneity and there is a close relationship between genotype and FTLD-U subtype. In addition to these recent significant advances in the neuropathology of FTLD-U, novel FTLD entities have been further characterized, including neuronal intermediate filament inclusion disease. The proposed criteria incorporate up-to-date neuropathology of FTLD in the light of recent immunohistochemical, biochemical, and genetic advances. These criteria will be of value to the practicing neuropathologist and provide a foundation for clinical, clinico-pathologic, mechanistic studies and in vivo models of pathogenesis of FTLD.

Correlation of Alzheimer Disease Neuropathologic Changes With Cognitive Status: A Review of the Literature

Abstract Clinicopathologic correlation studies are critically important for the field of Alzheimer disease (AD) research. Studies on human subjects with autopsy confirmation entail numerous potential biases that affect both their general applicability and the validity of the correlations. Many sources of data variability can weaken the apparent correlation between cognitive status and AD neuropathologic changes. Indeed, most persons in advanced old age have significant non-AD brain lesions that may alter cognition independently of AD. Worldwide research efforts have evaluated thousands of human subjects to assess the causes of cognitive impairment in the elderly, and these studies have been interpreted in different ways. We review the literature focusing on the correlation of AD neuropathologic changes (i.e. &bgr;-amyloid plaques and neurofibrillary tangles) with cognitive impairment. We discuss the various patterns of brain changes that have been observed in elderly individuals to provide a perspective forunderstanding AD clinicopathologic correlation and conclude that evidence from many independent research centers strongly supports the existence of a specific disease, as defined by the presence of A&bgr; plaques and neurofibrillary tangles. Although A&bgr; plaques may play a key role in AD pathogenesis, the severity of cognitive impairment correlates best with the burden of neocortical neurofibrillary tangles.

Pathological TDP‐43 distinguishes sporadic amyotrophic lateral sclerosis from amyotrophic lateral sclerosis with SOD1 mutations

Amyotrophic lateral sclerosis (ALS) is a common, fatal motor neuron disorder with no effective treatment. Approximately 10% of cases are familial ALS (FALS), and the most common genetic abnormality is superoxide dismutase‐1 (SOD1) mutations. Most ALS research in the past decade has focused on the neurotoxicity of mutant SOD1, and this knowledge has directed therapeutic strategies. We recently identified TDP‐43 as the major pathological protein in sporadic ALS. In this study, we investigated TDP‐43 in a larger series of ALS cases (n = 111), including familial cases with and without SOD1 mutations.

A harmonized classification system for FTLD-TDP pathology

In 2006, two papers were published, each describing pathological heterogeneity in cases of frontotemporal lobar degeneration (FTLD) with ubiquitin-positive, tau-negative inclusions (FTLD-U) [7, 11]. In both studies, large series of cases were evaluated and the investigators felt that they could recognize three distinct histological patterns, based on the morphology and anatomical distribution of ubiquitin immunoreactive neuronal inclusions. The findings of Sampathu et al. were further supported by differential labelling of the pathology, using a panel of novel monoclonal antibodies; whereas, Mackenzie et al. found relatively specific clinicopathological correlations. Most importantly, the pathological features that defined the subtypes in these two studies were almost identical, providing powerful validation of the results. However, because the studies were conducted simultaneously and independently, the numbering of the subtypes, used in the respective papers, did not match (Table 1). Table 1 Proposed new classification system for FTLD-TDP pathology, compared with existing systems Shortly thereafter, further work by one of the two groups led to the identification of the transactive response DNA-binding protein with Mr 43 kD (TDP-43) as the ubiquitinated pathological protein in most cases of FTLD-U as well as the majority of sporadic amyotrophic lateral sclerosis (ALS) and some familial ALS [10]. It was subsequently confirmed that most FTLD-U cases had TDP-43 pathology and that the same pathological patterns could be recognized based on the results of TDP-43 immunohistochemistry (IHC) [1, 2]. By this time, a fourth FTLD-U subtype had been described, specifically associated with the familial syndrome of inclusion body myopathy with Paget’s disease of bone and frontotemporal dementia (IBMPFD) caused by mutations in the valosin-containing protein (VCP) gene [4], and this was also shown to have TDP-43 pathology [9]. As a result, cases of FTLD with TDP-43 pathology are now designated as FTLD-TDP and the term FTLD-U is no longer recommended [8]. The two classification systems for FTLD-U/FTLD-TDP have now gained wide acceptance and have repeatedly been validated by the discovery of additional clinical, genetic and pathological correlations. However, the continued use of two discordant numbering systems proves to be an ongoing source of confusion within the field. Previous attempts, by other groups of authors, to promote one classification over the other have not been successful. To resolve this issue, the principal authors of the original two papers are now proposing a new classification for FTLD-TDP pathology, the sole purpose of which is to provide a single harmonized system that replaces the two currently in use. In developing this new classification, the following principles were adhered to: (1) different pathological subtypes are designated by letters to help distinguish this from the pre-existing number-based systems, (2) the order of subtypes should not exactly match either of the previous systems to avoid any apparent bias, and (3) the order of the subtypes should be based on their relative frequency, with “A” being the most common. The result is summarized in Table 1. Type A is equivalent to type 1 of Mackenzie et al. and type 3 of Sampathu et al., being characterized by numerous short dystrophic neurites (DN) and crescentic or oval neuronal cytoplasmic inclusions (NCI), concentrated primarily in neocortical layer 2. Moderate numbers of lentiform neuronal intranuclear inclusions (NII) are also a common but inconsistent feature of this subtype. Type B matches Mackenzie et al. type 3 and Sampathu et al. type 2, with moderate numbers of NCI, throughout all cortical layers, but very few DN. Type C is the same as Mackenzie et al. type 2 and Sampathu et al. type 1, having a predominance of elongated DN in upper cortical layers, with very few NCI. Finally, Type D refers to the pathology associated with IBMPFD caused by VCP mutations, characterized by numerous short DN and frequent lentiform NII. Based on the results of more recent studies, there are a number of other modifications that we could have considered incorporating into this new system. Additional pathological subtypes could be added; for instance, to describe the TDP-43 pathology that is found in the mesial temporal lobe in a high proportion of cases of Alzheimer’s disease and most other common neurodegenerative conditions [3]. The pathological criteria for each of the subtypes could be expanded to include characteristic findings in subcortical regions [5, 6]. The description of the pathological features could be modified to take into account the greater sensitivity and specificity of TDP-43 IHC, which may demonstrate additional findings, not recognized with the ubiquitin immunostaining techniques upon which the original classifications were based (such as neuronal “pre-inclusions”) [2]. Although these and other recent findings represent important advances in our understanding of FTLD-TDP, most have not yet been broadly replicated or completely defined. Therefore, in order to make the transition to a new classification as simple and widely acceptable as possible and, most importantly, to allow for direct translation with the currently existing systems, we are not proposing any other significant changes, beyond the coding of the subtypes. In summary, we believed that adoption of a single harmonized system for the classification of FTLD-TDP neuropathology would greatly improve communication within the rapidly advancing field of FTLD diagnosis and research. Future attempts to resolve any outstanding issues related to the practical implementation and interpretation of FTLD pathological classification should also benefit. As indicated by their inclusion as co-authors on this paper, this proposal has received the unanimous support of all of the neuropathologists involved in the original two studies [7, 11].

Primary age-related tauopathy (PART): a common pathology associated with human aging

We recommend a new term, “primary age-related tauopathy” (PART), to describe a pathology that is commonly observed in the brains of aged individuals. Many autopsy studies have reported brains with neurofibrillary tangles (NFTs) that are indistinguishable from those of Alzheimer’s disease (AD), in the absence of amyloid (Aβ) plaques. For these “NFT+/Aβ−” brains, for which formal criteria for AD neuropathologic changes are not met, the NFTs are mostly restricted to structures in the medial temporal lobe, basal forebrain, brainstem, and olfactory areas (bulb and cortex). Symptoms in persons with PART usually range from normal to amnestic cognitive changes, with only a minority exhibiting profound impairment. Because cognitive impairment is often mild, existing clinicopathologic designations, such as “tangle-only dementia” and “tangle-predominant senile dementia”, are imprecise and not appropriate for most subjects. PART is almost universally detectable at autopsy among elderly individuals, yet this pathological process cannot be specifically identified pre-mortem at the present time. Improved biomarkers and tau imaging may enable diagnosis of PART in clinical settings in the future. Indeed, recent studies have identified a common biomarker profile consisting of temporal lobe atrophy and tauopathy without evidence of Aβ accumulation. For both researchers and clinicians, a revised nomenclature will raise awareness of this extremely common pathologic change while providing a conceptual foundation for future studies. Prior reports that have elucidated features of the pathologic entity we refer to as PART are discussed, and working neuropathological diagnostic criteria are proposed.

Novel mutations in TARDBP (TDP-43) in patients with familial amyotrophic lateral sclerosis

The TAR DNA-binding protein 43 (TDP-43) has been identified as the major disease protein in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with ubiquitin inclusions (FTLD-U), defining a novel class of neurodegenerative conditions: the TDP-43 proteinopathies. The first pathogenic mutations in the gene encoding TDP-43 (TARDBP) were recently reported in familial and sporadic ALS patients, supporting a direct role for TDP-43 in neurodegeneration. In this study, we report the identification and functional analyses of two novel and one known mutation in TARDBP that we identified as a result of extensive mutation analyses in a cohort of 296 patients with variable neurodegenerative diseases associated with TDP-43 histopathology. Three different heterozygous missense mutations in exon 6 of TARDBP (p.M337V, p.N345K, and p.I383V) were identified in the analysis of 92 familial ALS patients (3.3%), while no mutations were detected in 24 patients with sporadic ALS or 180 patients with other TDP-43–positive neurodegenerative diseases. The presence of p.M337V, p.N345K, and p.I383V was excluded in 825 controls and 652 additional sporadic ALS patients. All three mutations affect highly conserved amino acid residues in the C-terminal part of TDP-43 known to be involved in protein-protein interactions. Biochemical analysis of TDP-43 in ALS patient cell lines revealed a substantial increase in caspase cleaved fragments, including the ∼25 kDa fragment, compared to control cell lines. Our findings support TARDBP mutations as a cause of ALS. Based on the specific C-terminal location of the mutations and the accumulation of a smaller C-terminal fragment, we speculate that TARDBP mutations may cause a toxic gain of function through novel protein interactions or intracellular accumulation of TDP-43 fragments leading to apoptosis.

Nomenclature for neuropathologic subtypes of frontotemporal lobar degeneration: Consensus recommendations

Nomenclature for neuropathologic subtypes of frontotemporal lobar degeneration : consensus recommendations