Genetics of frontotemporal lobar degeneration: an up-date and diagnosis algorithm

The last decade marked a turning point in the knowledge of frontotemporal lobar degenerations (FTLD). Major discoveries were made with the identification of TDP-43 and FUS, two novel key players in FTLD. The growing number of FTLD genes has considerably changed our clinical practice. The high intrafamilial variability of phenotypes underlines the necessity of a careful interview concerning the family history, regarding FTLD diseases, but also other neurodegenerative and extra-neurological disorders. Knowledge of the different genetic forms of FTLD and their associated phenotypes become essential to propose appropriate genetic diagnosis to the patients, and deliver accurate genetic counseling to their families. We propose an algorithm based on four criteria to help to pinpoint the genetic cause of FTLD: Presence of ALS in the patient or family; age at onset of FTLD; progranulin plasma level; and other disorders present in the patient or family. Presence of ALS is strongly indicative of a C9ORF72 expansion; a very early age at onset (<50 years), parkinsonism and oculomotor dysfunction are indicative of MAPT mutations; whereas hallucinations, CBDS and PNFA are indicative of PGRN mutations. A C9ORF72 repeat expansion should be searched for therefore in patients with FTLD-ALS, followed by sequencing of exon 6 of TARDBP gene in negative cases. Since C9ORF72 expansions are as frequent as PGRN mutations in patients with pure FTLD, both should be investigated, except in early familial FTLD (<50) where MAPT mutations should be searched for first. VCP, SQSTM1 and hnRNPA2B1 gene-sequencing could be proposed in patients or families presenting 'multisystem proteinopathy'. The genes currently identified explain 50-60% of familial forms of FTLD. The identification of new FTLD genes involved remains a major challenge to gain further insight into the pathology and even better clarify the classification of FTLD in the future.