Phenotypic signatures of genetic frontotemporal dementia

Frontotemporal dementia (FTD) is a clinically, pathologically and genetically heterogeneous disorder. Mutations in a number of genes are associated with FTD, although until recently only two [progranulin (GRN) and microtubule-associated protein tau (MAPT)] were known to be major causes of the disease. This review describes recent progress in identifying clinical and neuroanatomical phenotypes associated with autosomal-dominant FTD.

Around a third to a half of FTD patients have an autosomal dominant pattern of inheritance. Up to 10% of patients have a mutation in GRN and a similar proportion have a mutation in MAPT. Recently a group of patients have been shown to have a hexanucleotide repeat expansion in the noncoding region of chromosome 9 open reading frame 72 (C9ORF72). A further group of patients have an autosomal dominant family history but no mutations in any of the known genes including a group of patients who have the same pathology as GRN mutations (type A TDP-43 pathology) but are negative for GRN mutations. Clinical phenotypes vary across the different mutations. Neuroimaging studies show that GRN and MAPT mutations have distinct patterns of atrophy--asymmetric fronto-temporo-parietal atrophy with GRN versus relatively symmetric medial temporal and orbitofrontal lobe atrophy with MAPT mutations. Neuroimaging of patients with an expansion in C9ORF72 has yet to be studied in detail.

Genetic FTD is heterogeneous but certain phenotypic signatures of the major causative genes can be identified.