Two genes that have been linked separately to rare intestinal diseases appear to share a functional relationship. The genes have independently been linked to osteo-oto-hepato-enteric (O2HE) syndrome and microvillus inclusion disease (MVID), which are characterized by congenital diarrhea and, in some patients, intrahepatic cholestasis.
It appears that one gene, UNC45A, is directly responsible for the proper function of the protein encoded by the other gene, called MYO5B, according to investigators, who published their findings in Cellular and Molecular Gastroenterology and Hepatology. UNC45A is a chaperone protein that helps proteins fold properly. It has been linked to O2HE patients experiencing congenital diarrhea and intrahepatic cholestasis. The mutation has been identified in four patients from three different families with O2HE, which can also present with sensorineural hearing loss and bone fragility. Cellular analyses have shown that the mutation leads to reduction in protein expression by 70%-90%.
Intestinal symptoms similar to those in O2HE have also been described in diseases caused by mutations in genes that encode the myosin motor proteins that are involved in cellular protein trafficking. This group of disorders includes MVID. The researchers hypothesized that the UNC45A mutation in O2HE might lead to similar symptoms as MVID and others through the altered protein’s failure to assist in the folding of myosin proteins, although to date only the myosin IIa protein has been shown to be a target of UNC45A.
To investigate the possibility, they examined in more detail the relationship between UNC45A and intestinal symptoms. There are various known mutations in myosin proteins. Some have been linked to deafness, but these do not appear to contribute to intestinal symptoms since patients with myelin-related inherited deafness don’t typically have diarrhea. Bone fragility, also sometimes caused by myosin mutations, also appears to be unrelated to intestinal symptoms.
Previous experiments in yeast suggest that the related gene UNC45 may serve as a chaperone for type V myosin: Loss of a yeast version of UNC45 caused a type V myosin called MYO4P to be mislocalized in yeast. In zebrafish, reduction in intestinal levels of the UNC45A gene or the fish’s version of MYO5B interfered with development of intestinal folds.
The researchers used CRISPR-Cas9 gene editing and site-directed mutagenesis in intestinal epithelial and liver cell lines to investigate the relationships between UNC45A and MYO5B mutants. UNC45A depletion or introduction of the UNC45A mutation found in patients led to lower MYO5B expression. Within epithelial cells, loss of UNC45A led to changes in MYO5B–linked processes that are known to play a role in MVID pathogenesis. These included alteration of microvilli development and interference with the location of rat sarcoma–associated binding protein (RAB) 11A–positive recycling endosomes. When normal UNC45A was reintroduced to these cells, MYO5B expression returned. Reintroduction of either UNC45A or MYO5B repaired the alterations to recycling endosome position and microvilli development.
Loss of UNC45A did not appear to affect transcription of the MYO5B gen, which suggests a suggesting a functional interaction between the two at a protein level.
UNC45A has been shown to destabilize microtubules. Exposure of a kidney epithelial cell line to the microtubule-stabilizing drug taxol also led to displacement of RAB11A-positve recycling endosomes, though the specific changes were different than what is seen in MYO5B mutants. The researchers were unable to validate the findings in tissue derived from O2HE patients because of insufficient material, but they maintain that the cell lines used have proven to be highly predictive for the cellular characteristics of MVID.
Overall, the study suggests that reductions in MYO5B and subsequent changes to the cellular processes that depend on it may underlie the intestinal symptoms in O2HE.
The researchers noted that O2HE patients have different phenotypes. Of the four patients they studied, three had severe chronic diarrhea and required parenteral nutrition. One patient later had the diarrhea resolve and her sister did not have diarrhea at all. This heterogeneity in severity and duration of clinical symptoms may be driven by differences in the molecular effects of patient-specific mutations. The two siblings had mutations in a different region of the UNC45A gene than the other two participants.
“Taken together, this study revealed a functional relationship between UNC45A and MYO5B protein expression, thereby connecting two rare congenital diseases with overlapping intestinal symptoms at the molecular level,” the authors wrote.
The authors reported that they had no conflicts of interest.
This article was updated 7/13/22.