Mutations in GET4 disrupt the transmembrane domain recognition complex pathway

Abstract

The transmembrane domain recognition complex (TRC) targets cytoplasmic C‐terminal tail‐anchored (TA) proteins to their respective membranes in the endoplasmic reticulum (ER), Golgi, and mitochondria. It is composed of three proteins, GET4, BAG6, and GET5. We identified an individual with compound heterozygous missense variants (p.Arg122His, p.Ile279Met) in GET4 that reduced all three TRC proteins by 70% to 90% in his fibroblasts, suggesting a possible defect in TA protein targeting. He presented with global developmental delay, intellectual disabilities, seizures, facial dysmorphism, and delayed bone age. We found the TA protein, syntaxin 5, is poorly targeted to Golgi membranes compared to normal controls. Since GET4 regulates ER to Golgi transport, we hypothesized that such transport would be disrupted in his fibroblasts, and discovered that retrograde (but not anterograde) transport was significantly reduced. Despite reduction in the three TRC proteins, their mRNA levels were unchanged, suggesting increased degradation in patient fibroblasts. Treating fibroblasts with the FDA‐approved proteasome inhibitor, bortezomib (10 nM), restored syntaxin 5 localization and nearly normalized the levels of all three TRC proteins. Our study identifies the first individual with GET4 mutations.

CAMLG-CDG: a novel congenital disorder of glycosylation linked to defective membrane trafficking

“…In addition, we confirm that this novel group of disorders lead to a CDG, in this case a combined O-linked and type II N-linked glycosylation defect characterized mostly by undersialylation, indicating late Golgi disruption. Clinical features were similar to those in one of the two other described TRC pathway disorder (GET4 deficiency; ( 9 )) and were largely neurological, consisting of severe DD/ID, seizures, hypotonia, limb contractures and structural brain abnormalities.…”

“…Crude fractionation of affected CAMLG-CDG fibroblasts showed a significant increase in the amount of STX5 mislocalized to the cytoplasm. This was identified in both the long ( P < 0.05) and short ( P < 0.001) forms of STX5 and also found in parallel in GET4 deficient fibroblasts ( 9 ) ( Supplementary Material, Fig. 5A ).…”

“…Previously, it was shown that both the long and short forms of syntaxin-5 (STX) were mislocalized to the cytoplasm in fibroblasts from an individual with pathogenic compound heterozygous variants in GET4 ( 9 ). The same was also true for the present variant in CAMLG .…”

“…BET1L is thought to be more associated with the short form than the long form of STX5, and is required for retrograde trafficking within the Golgi ( 15 ). In fact, previous data have shown that in GET4 deficient fibroblasts, retrograde trafficking is disrupted ( 9 ). Furthermore, previous data have even shown that expression of BET1L without its transmembrane domain leads to mislocalization of STX5 ( 17 ), analogous to data presented here.…”

“…Recently, pathogenic compound heterozygous missense variants (c.365G>A; p.(Arg122His), c.837A>G; p.(Ile279Met)) in GET4 were reported in an individual with intellectual disability (ID), seizures, facial dysmorphism and some skeletal abnormalities ( 9 ). The affected individual had a mild type 2 serum transferrin isoform pattern, indicating a possible congenital disorder of glycosylation (CDG) caused by dysfunction of the TRC pathway.…”