GANAB

GANAB encodes GIIα, the catalytic α-subunit of the endoplasmic reticulum glucosidase II complex, where it functions partnered with the GIIβ subunit and is required for N-glycan processing that drives maturation of the polycystins (PMID:27259053, PMID:33097077). Loss of GIIα activity causes complete failure of polycystin-1 (PC1) and polycystin-2 (PC2) to reach the cell surface and primary cilium, and only wild-type — not disease-mutant — GIIα rescues PC1 surface localization, placing GANAB upstream of PC1/PC2 maturation and identifying defective PC1 maturation as the driver of cystogenesis in polycystic kidney and liver disease (PMID:27259053). Pathogenic GANAB variants act through several routes: coding frameshift, nonsense, and splice variants disrupt the GIIα/GIIβ subunit interaction (PMID:33097077), a noncoding intronic deletion near the exon 24 splice donor causes exon skipping and polycystic liver disease (PMID:29243290), and homozygous loss is embryonic lethal in mice while heterozygous loss preserves PC1, PC2, and ciliary tubulin and produces no cysts — indicating haploinsufficiency alone is insufficient for disease (PMID:32550232). GANAB expression is itself controlled by upstream regulatory inputs, being suppressed through a YTHDC1–PIK3R1–STAT3 axis in ovarian cancer cells (PMID:37781028).

1. 2016 High

   Established that GANAB/GIIα is required for polycystin maturation, defining the molecular link between glucosidase II and cystogenesis.

   Evidence GANAB-null cell lines with reciprocal rescue by wild-type vs. disease-mutant GIIα, surface and ciliary localization assays, western blot for mature PC1

   PMID:27259053

   Open questions at the time

   • Direct enzymatic glucan-trimming activity of GIIα on polycystin glycans not measured biochemically
   • Threshold of GIIα loss required for PC1 maturation failure not quantified
2. 2018 Medium

   Showed that noncoding GANAB variants can be pathogenic by disrupting splicing, broadening the mutational spectrum beyond coding changes.

   Evidence Minigene splicing assay in cell lines and primary human cholangiocytes

   PMID:29243290

   Open questions at the time

   • Functional consequence of exon 24 skipping on GIIα protein and enzymatic activity not assessed
   • Patient-tissue confirmation of aberrant transcript not shown
3. 2020 Medium

   Defined the in vivo dosage requirement for GANAB, distinguishing essential developmental function from disease-causing haploinsufficiency.

   Evidence CRISPR/Cas9 Ganab knockout mice with genotyping, western blot, and kidney/liver histology

   PMID:32550232

   Open questions at the time

   • Mechanism by which homozygous loss causes embryonic lethality not defined
   • Whether a second-hit cooperates with heterozygous loss to produce cysts not tested
   • Single laboratory model
4. 2020 Low

   Proposed disrupted GIIα/GIIβ subunit assembly as a pathogenic mechanism for several GANAB variants.

   Evidence Cell-line expression of variant proteins, immunofluorescence colocalization of GIIα and GIIβ, in silico active-site modeling

   PMID:33097077

   Open questions at the time

   • No direct measurement of glucosidase II enzymatic activity for the variants
   • Colocalization is a proxy and was not validated by co-immunoprecipitation
   • Effect on PC1 maturation not tested for these variants
5. 2023 Low

   Implicated GANAB in viral envelope-triggered ER stress, linking glucosidase II level to the unfolded protein response.

   Evidence In vitro ERVW-1 overexpression, ELISA, western blot, confocal ER morphology imaging

   PMID:37376599

   Open questions at the time

   • Causality of GANAB reduction in the UPR phenotype not established by rescue
   • Direct mechanism connecting GANAB to ATF6/XBP1 not defined
6. 2023 Medium

   Identified an upstream regulatory axis controlling GANAB transcription in cancer cells.

   Evidence RNA-seq, RIP-seq, m6A-RIP, mRNA stability assay, ChIP, western blot in vitro and in vivo

   PMID:37781028

   Open questions at the time

   • Functional consequence of GANAB suppression on N-glycan biosynthesis in this context not measured
   • Generality of the YTHDC1-STAT3-GANAB axis beyond ovarian cancer unknown

• How partial GANAB loss cooperates with additional genetic or environmental insults to cross the threshold for cystogenesis remains unresolved.
• No second-hit model defining the trigger for cyst formation under heterozygous loss
• No direct biochemical reconstitution of GIIα trimming of polycystin N-glycans