VAPB and glycogen storage disease VI: Another contribution to mutant VAPB pathogenicity, possibly as a consequence of its physical interaction with wt-VAPB or its direct binding to activating transcription factor 6 (ATF6), could derive from its reported role in the UPR, since overexpression of P56S-VAPB has been reported both to attenuate UPR signaling [128] and to increase ER stress in animal disease models [129,138,155,156].