Achromatopsia mutations target sequential steps of ATF6 activation

Achromatopsia is an autosomal recessive disorder characterized by cone photoreceptor dysfunction. We recently identified activating transcription factor 6 (ATF6) as a genetic cause of achromatopsia. ATF6 is a key regulator of the unfolded protein response. In response to endoplasmic reticulum (ER) stress, ATF6 migrates from the ER to Golgi to undergo regulated intramembrane proteolysis to release a cytosolic domain containing a basic leucine zipper (bZIP) transcriptional activator. The cleaved ATF6 fragment migrates to the nucleus to transcriptionally up-regulate protein-folding enzymes and chaperones. ATF6 mutations in patients with achromatopsia include missense, nonsense, splice site, and single-nucleotide deletion or duplication changes found across the entire gene. Here, we comprehensively tested the function of achromatopsia-associated ATF6 mutations and found that they group into three distinct molecular pathomechanisms: class 1 ATF6 mutants show impaired ER-to-Golgi trafficking and diminished regulated intramembrane proteolysis and transcriptional activity; class 2 ATF6 mutants bear the entire ATF6 cytosolic domain with fully intact transcriptional activity and constitutive induction of downstream target genes, even in the absence of ER stress; and class 3 ATF6 mutants have complete loss of transcriptional activity because of absent or defective bZIP domains. Primary fibroblasts from patients with class 1 or class 3 ATF6 mutations show increased cell death in response to ER stress. Our findings reveal that human ATF6 mutations interrupt distinct sequential steps of the ATF6 activation mechanism. We suggest that increased susceptibility to ER stress-induced damage during retinal development underlies the pathology of achromatopsia in patients with ATF6 mutations.