Annemieke J.M. Rozemuller

Aims The loss of von Economo neurons (VENs) and GABA receptor subunit theta (GABRQ) containing neurons is linked to early changes in social–emotional cognition and is seen in frontotemporal dementia (FTD) due to C9orf72 repeat expansion. We investigate the vulnerability of VENs and GABRQ‐expressing neurons in sporadic and genetic forms of FTD with different underlying molecular pathology and their association with the presence and severity of behavioural symptoms. Methods We quantified VENs and GABRQ‐immunopositive neurons in the anterior cingulate cortex (ACC) in FTD with underlying TDP43 (FTLD‐TDP) ( n = 34), tau (FTLD‐tau) ( n = 24) or FUS (FTLD‐FUS) ( n = 8) pathology, neurologically healthy controls ( n = 12) and Alzheimer's disease (AD) ( n = 7). Second, we quantified VENs and the GABRQ‐expressing population in relation to presence of behavioural symptoms in the first years of disease onset. Results The number of VENs and GABRQ‐expressing neurons and the ratio of VENs and GABRQ‐expressing neurons over total Layer 5 neuronal population decreased in FTLD‐TDP and FTLD‐FUS, but not in FTLD‐tau, compared to control and AD. The severity of early behavioural symptoms in all donors correlated with a lower VEN and GABRQ neuronal count. Conclusion We show that in FTD, a loss of VENs together with GABRQ‐expressing pyramidal neurons is associated with TDP43 and FUS pathology. No significant loss was found in donors with FTLD‐tau pathology; however, this could be due to the specific MAPT mutation studied and small sporadic FTLD‐tau sample size. Overall, we show the GABRQ‐expressing population correlates with behavioural changes and suggest they are key in modulating behaviour in FTD.