Heritability in frontotemporal dementia: more missing pieces?

Frontotemporal dementia (FTD) is reportedly highly heritable, even though a recognized genetic cause is often absent. To explain this contradiction, we explored the "strength" of family history in FTD, Alzheimer's disease (AD), and controls. Clinical syndromes associated with heritability of FTD and AD were also examined. FTD and AD patients were recruited from an FTD-specific research clinic, and patients were further sub-classified into FTD or AD phenotypes. The strength of family history was graded using the Goldman score (GS), and GS of 1-3 was regarded as a "strong" family history. A subset of FTD patients underwent screening for the main genetic causes of FTD. In total, 307 participants were included (122 FTD, 98 AD, and 87 controls). Although reported positive family history did not differ between groups, a strong family history was more common in FTD (FTD 17.2 %, AD 5.1 %, controls 2.3 %, P < 0.001). The bvFTD and FTD-ALS groups drove heritability, but 12.2 % of atypical AD patients also had a strong family history. A pathogenic mutation was identified in 16 FTD patients (10 C9ORF72 repeat expansion, 5 GRN, 1 MAPT), but more than half of FTD patients with a strong family history had no mutation detected. FTD is a highly heritable disease, even more than AD, and patients with bvFTD and FTD-ALS drive this heritability. Atypical AD also appears to be more heritable than typical AD. These results suggest that further genetic influences await discovery in FTD.