Neuroimaging features in C9orf72 and TARDBP double mutation with FTD phenotype

Alzheimer's Disease and Frontotemporal Dementia: The Current State of Genetics and Genetic Testing Since the Advent of Next-Generation Sequencing

The advent of next-generation sequencing has changed genetic diagnostics, allowing clinicians to test concurrently for phenotypically overlapping conditions such as Alzheimer's disease (AD) and frontotemporal dementia (FTD). However, to interpret genetic results, clinicians require an understanding of the benefits and limitations of different genetic technologies, such as the inability to detect large repeat expansions in such diseases as C9orf72-associated FTD and amyotrophic lateral sclerosis. Other types of mutations such as large deletions or duplications and triple repeat expansions may also go undetected. Additionally, the concurrent testing of multiple genes or the whole exome increases the likelihood of discovering variants of unknown significance. Our goal here is to review the current knowledge about the genetics of AD and FTD and suggest up-to-date guidelines for genetic testing for these dementias. Despite the improvements in diagnosis due to biomarkers testing, AD and FTD can have overlapping symptoms. When used appropriately, genetic testing can elucidate the diagnosis and specific etiology of the disease, as well as provide information for the family and determine eligibility for clinical trials. Prior to ordering genetic testing, clinicians must determine the appropriate genes to test, the types of mutations that occur in these genes, and the best type of genetic test to use. Without this analysis, interpretation of genetic results will be difficult. Patients should be counseled about the benefits and limitations of different types of genetic tests so they can make an informed decision about testing.

Heterogeneous brain FDG-PET metabolic patterns in patients with C9orf72 mutation

OBJECTIVE

The hexanucleotide repeat expansion in C9orf72 is an associated genetic cause in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). In the "ALS/FTD" spectrum prevails clinical heterogeneity and an in vivo knowledge of the underling brain dysfunction in patients carrying C9orf72 mutation remain limited and only described at group level. The study aimed to assess the brain metabolic alterations characterizing patients with C9orf72 mutation using FDG-PET in single individuals.

METHODS

We applied a validated statistical parametric mapping (SPM) voxel-based procedure for FDG-PET data to obtain maps of brain relative hypometabolism and hypermetabolism at single-subject level in six FTD/ALS patients carrying the C9orf72 mutation.

RESULTS

Clinical diagnoses classified the patients as right semantic variant of frontotemporal dementia (one case, C9svFTD), behavioral variant of frontotemporal dementia (two cases, C9bvFTD), and bulbar amyotrophic lateral sclerosis (three cases, C9bALS). The FDG-PET SPM revealed a prevalent frontal hypometabolism in C9bvFTD cases, and right temporal polar and lateral involvement in C9svFTD, consistent with the clinical diagnosis. There was a quite comparable occipital and cerebellar hypermetabolism in these cases. The three C9bALS patients showed variable patterns of hypo- and hypermetabolism.

CONCLUSIONS

The present work is the first in vivo FDG-PET study showing the heterogeneous patterns of brain regional hypo- and hypermetabolism in single patients sharing C9orf72 mutation. Brain hypometabolism was consistent with the clinical phenotypes, supporting the diagnostic importance of neuroimaging functional biomarkers to capture at single-subject level specific brain dysfunction.

OPTN p.Met468Arg and ATXN2 intermediate length polyQ extension in families with C9orf72 mediated amyotrophic lateral sclerosis and frontotemporal dementia

We have ascertained two families affected with familial amyotrophic lateral sclerosis (ALS) in which they both carry a hexanucleotide repeat expansion in the C9orf72 gene, specifically in individuals who also presented with frontotemporal dementia (FTD) or behavioral variant FTD (bvFTD). While some reports attribute this phenotypic heterogeneity to the C9orf72 expansion alone, we screened for additional genetic variation in known ALS-FTD genes that may also contribute to or modify the phenotypes. We performed genetic testing consisting of C9orf72 hexanucleotide expansion, ATXN2 polyglutamine (polyQ) expansion, and targeted next generation sequencing using the ONDRISeq, a gene panel consisting of 80 genes known to be associated with neurodegenerative diseases such as ALS, FTD, Alzheimer's disease, Parkinson's disease, and vascular cognitive impairment. In addition to the C9orf72 expansion, we observed an ATXN2 polyQ intermediate length expansion, and OPTN p.Met468Arg in patients who exhibited ALS and FTD or bvFTD. We conclude that the C9orf72 expansion likely explains much of the ALS-FTD phenotype; however, inheritance of these additional variants likely modifies the disease course and may provide further evidence for biologically relevant oligogenic inheritance in ALS.

Defining the spectrum of frontotemporal dementias associated with TARDBP mutations

OBJECTIVES

We describe the largest series of patients with TARDBP mutations presenting with frontotemporal dementia (FTD) and review the cases in the literature to precisely characterize FTD diseases associated with this genotype.

METHODS

The phenotypic characteristics of 29 TARDBP patients, including 10 new French and Dutch cases and 19 reviewed from the literature, were evaluated.

RESULTS

The most frequent phenotype was a behavioral variant frontotemporal dementia (bvFTD), but a significant proportion (40%) of our patients had semantic (svFTD) or nonfluent variants (nfvFTD) at onset; and svFTD was significantly more frequent in TARDBP carriers than in other FTD genotypes (p < 0.001). Remarkably, only a minority (40%) of our patients secondarily developed amyotrophic lateral sclerosis (ALS). Two patients carried a homozygous mutation but strikingly different phenotypes (bvFTD and ALS) indicating that homozygosity does not result in a specific phenotype. Earlier age at onset in children than parent's generations, mimicking an apparent "anticipation" (21.8 ± 9.3 years, p = 0.001), and possible reduced penetrance were present in most families.

CONCLUSIONS

This study enlarges the phenotypic spectrum of TARDBP and will have important clinical implications: (1) FTD can be the only clinical manifestation of TARDBP mutations; (2) Initial language or semantic disorders might be indicative of a specific genotype; (3) Mutations should be searched in all FTD phenotypes after exclusion of major genes, even in the absence of ALS in the proband or in family history; (4) reduced penetrance and clinical variability should be considered to deliver appropriate genetic counseling.