I am always on the lookout for cool examples of human genetics informing drug adverse effects. Here is an interesting one I recently came across in this great review article 🧵
nature.com/articles/s41576-022-00572-8
Diacylglycerol acyltransferase 1 (DGAT1) inhibitor is one of the many drug candidates designed purely based on biochemistry knowledge and animal experiments but never saw the light of the day.
ahajournals.org/doi/10.1161/01.atv.0000151874.81059.ad
The final step of triglyceride synthesis is catalyzed by two structurally different but functionally similar enzymes: DGAT1 and DGAT2. The two enzymes account for nearly all the triglyceride synthesis in the body. The loss of one is likely compensated by the other.
So, drug developers found DGAT enzymes as an attractive target to treat obesity and type 2 diabetes. Scientists started exploring the consequence of partially or completely deleting DGAT1 or DGAT2 in animal models.
The two enzymes had interesting differences in their tissue distribution in humans. While DGAT1 is expressed predominantly in the intestine and fat tissues, DGAT2 is expressed highly in the liver, fat and skin.
Animal experiments revealed that DGAT2 deletion is lethal. The mice completely lacking DGAT2 died soon after birth due to lethal dehydration caused by rapid water loss through their defective skin. So, that ruled out DGAT2 as a good target.
On the other hand, the scientist found that the mice lacking DGAT1 were viable, metabolically healthy and lived longer than the wild-type mice and importantly, showed impressive resistance to diet-induced obesity and glucose intolerance.
This led to the development of DGAT1 inhibitors as a potential treatment for type 2 diabetes and obesity. For e.g. in 2012, AstraZeneca (AZ) scientists published the discovery of a small molecule AZD7687, a potent and selective inhibitor of DGAT1.
pubs.acs.org/doi/full/10.1021/jm301296t
In 2014, the AZ scientists reported the results from their first clinical trial. A single dose of AZD7687 resulted in dramatic reduction in postprandial triglycerides rise. But there was a problem: the participants had severe gastrointestinal side effects.
pubmed.ncbi.nlm.nih.gov/22950654/
The authors concluded
"The attenuating effect of AZD7687 on postprandial TAG excursion provides proof of mechanism with respect to gut DGAT1 inhibition. However, dose and diet-related gastrointestinal side effects may impact further development of DGAT1 inhibitors."
In 2015, they reported results from a follow up trial done mainly to evaluate the safety and tolerability of AZD7687. The drug was administered in multiple doses and the participants were evaluated if they tolerated GI side effects over time.
pubmed.ncbi.nlm.nih.gov/24118885/
Unfortunately, they didn't. With increasing doses, the diarrhea became worse and >50% of the participants discontinued treatment. The trial hammered the final nail in the coffin of the drug programme.
The authors concluded that "lack of therapeutic window owing to GI side effects of AZD7687, particularly diarrhea, makes the utility of DGAT1 inhibition as a novel treatment for diabetes and obesity questionable." and killed the drug programme.
The part of the DGAT1 story that interested me most is a case report on two children who were were knockouts for DGAT1 published in JCI in 2012.
jci.org/articles/view/64873
In this case report, the authors describe two pediatric cases of congenital diarrheal disorder (CDD) born in an Ashkenazi Jewish family and found to be full knockouts for DGAT1.
Case 1 was a girl child born normal but soon developed severe vomiting, colicky pain and watery diarrhea on the 3rd day after she was fed breast milk. She couldn't take anything orally and had to be put on total parenteral nutrition.
Puzzlingly, the child had elevated triglycerides and cholesterol blood levels (so did her mother who had partial DGAT1 deficiency) unlike what was seen in the DGAT deficient mice.
The poor child couldn't tolerate the parenteral nutrition and eventually died due to malnutrition and sepsis at 17 months of age.
Case 2 was a boy child, the girl's brother, also had a similar history: born normal but on 3rd day started having vomiting and diarrhea and had to be put under total parenteral nutrition, which he tolerated and recovered from malnutrition.
The boy too had elevated blood lipid levels and so had to be treated with cholestyramine which reduced his blood lipid levels. He made a full recovery and at 47 months of age, he was thriving on an unrestricted diet.
Exome sequencing of the family revealed that both the kids were homozygous for splice donor variant that skipped an entire exon resulting in complete loss of DGAT1 activity. The chances of observing this homozygous mutation in general population is 1 in 50-100 million.
It's amazing to see how a rare genetic disease informs about the adverse effect of a drug designed to treat common diseases. It's not clear though how DGAT1 loss (or inhibition) lead to diarrhea only in humans.
It seems, unlike in humans where only DGAT1 is expressed in intestine, in mice both DGAT1 and DGAT2 are expressed in intestine. So, it's possible that in mice DGAT2 enzyme in the intestine compensates for DGAT1 loss.
Anyways, DGAT1 adds to the list of genes where animal models failed to recapitulate human physiology and reminds us the fact that at the end of the day the ultimate animal model is human themselves.
twitter.com/doctorveera/status/1616283586797400065?s=20&t=sRw612pvsu_defrcURXFFg