GRN and primary progressive aphasia: However, PPA has also been reported indominantly inherited forms of FTLD caused by mutations in genes that encode tau(MAPT), progranulin (GRN) or the Chromosome 9open reading frame 72 protein (C9orf72).47-49 In thegroup of dominantly inherited FTLD kindreds, the PPA phenotype has been describedmost frequently in families with GRN mutations.