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A pathologic expansion of a noncoding GGGGCC hexanucleotide repeat of the C9orf72 gene has been strongly associated with familial amyotrophic lateral sclerosis (ALS) and frontotemporal degeneration (FTD) cases predominantly in Caucasian populations. In the last decade, scientific interest had been drawn to this gene and many studies conducted have shown a possible correlation with other neurodegenerative diseases as well. We performed an extensive literature search for C9orf72 mutation and its frequency in various neurological and psychiatric diseases. In addition, we performed a meta-analysis of the data related to ALS and familial ALS. An online cloud-based database and an interactive map were developed. The overall mutation frequency of C9orf72 is 20% for familial FTD, 16% for familial ALS and around 6%–8% for sporadic ALS and FTD. The updated meta-analysis that we performed showed that the pooled frequency of C9orf72 repeat expansion in patients with familial ALS was 23% (CI: 18%–28%) and in patients with sporadic ALS 3% (CI: 3%–4%). The subgroup analysis regarding the origin of the population revealed significant differences between Caucasian and Asian patients. Our analysis supports the direct causal relation of the C9orf72 expansion in ALS and FTD. On the contrary, the role of C9orf72 in other neurodegenerative disorders remains controversial. The system that we developed—the online database and the interactive map—is hopefully a stepping stone for an ever-growing platform that will aid scientists from all over the world in contributing to the meta-analysis of C9orf72-related publications.

Nasu-hakola disease (NHD) is a rare disease characterized by bone cysts and fractures, frontal lobe syndrome, and progressive presenile dementia. NHD may be the prototype of primary microglial disorders of the CNS or, as they have been coined, "microgliopathies". Mutations in TREM2 and TYROBP genes are known to cause NHD. Interestingly, recent evidence-associated rare genetic variants of TREM2 gene with increased risk of Alzheimer's disease, frontotemporal dementia, amyotrophic lateral sclerosis, and Parkinson's disease. Here, we report a 33-year-old Greek female with phenotype suggestive of NHD. Full gene sequencing of the TREM2 and TYROBP genes revealed a novel mutation in exon 2 of TREM2 gene, namely c.244G>T (p.W50C) and heterozygosity in the parents and her brother. This report extends the range of TREM2 mutations that cause NHD phenotype. In addition, we provide a comprehensive review of all reported in the literature TREM2 gene mutations and the respective wide spectrum of clinical manifestations that highlights the importance of considering TREM2 gene mutations in a variety of neurodegenerative phenotypes.

Genome-wide association studies (GWAS) have been shown to be a powerful approach to identify risk loci for neurodegenerative diseases. Recent GWAS in Parkinson's disease (PD) have been successful in identifying numerous risk variants pointing to novel pathways potentially implicated in the pathogenesis of PD. Contributing to these GWAS efforts, we performed genotyping of previously identified risk alleles in PD patients and control subjects from Greece. We showed that previously published risk profiles for Northern European and American populations are also applicable to the Greek population. In addition, although our study was largely underpowered to detect individual associations, we replicated 5 of 32 previously published risk variants with nominal p values

Leucine-rich repeat kinase 2 (LRRK2) mutation is the most common cause of genetic-related parkinsonism and is usually associated with Lewy body pathology; however, tau, α-synuclein, and ubiquitin pathologies have also been reported. We report the case of a patient carrying the LRRK2 G2019S mutation and a novel heterozygous variant c.370C>G, p.Q124E in exon 4 of the microtubule-associated protein tau (MAPT). The patient developed parkinsonism with good levodopa response in her 70s. Neuropathological analysis revealed nigral degeneration and Alzheimer-type tau pathology without Lewy bodies. Immunohistochemical staining using phospho-TDP-43 antibodies identified occasional TDP-43 pathology in the hippocampus, temporal neocortex, striatum, and substantia nigra. However, TDP-43 pathology was not identified in another 4 archival LRRK2 G2019S cases with Lewy body pathology available in the Queen Square Brain Bank. Among other published cases of patients carrying LRRK2 G2019S mutation, only 3 were reportedly evaluated for TDP-43 pathology, and the results were negative. The role of the MAPT variant in the clinical and pathological manifestation in LRRK2 cases remains to be determined.

Microtubule-associated protein tau (MAPT) mutations have been shown to underlie frontotemporal dementia and a variety of additional sporadic tauopathies. We identified a rare p.A152T variant in MAPT exon 7 in two (of eight) patients with clinical presentation of parkinsonism and postmortem finding of neurofibrillary tangle pathology. Two siblings of one patient also carried the p.A152T variant, and both have progressive cognitive impairment. Further screening identified the variant in two other cases: one with pathologically confirmed corticobasal degeneration and another with the diagnosis of Parkinson's disease with dementia. The balance of evidence suggests this variant is associated with disease, but the very varied phenotype of the cases with the mutation is not consistent with it being a fully penetrant pathogenic mutation. Interestingly, this variation results in the creation of a new phosphorylation site that could cause reduced microtubule binding. We suggest that the A152T variant is a risk factor associated with the development of atypical neurodegenerative conditions with abnormal tau accumulation.