To read the full abstract: Am J Hum Genet. 2019 May 2;104(5):985989.
This study reports the identification of a novel gene that is involved in the regulation of the pancreatic development.
Understanding the molecular mechanisms of pancreatic development is important, for example to guide the progress of beta-cell replacement therapy for patients with Type 1 diabetes mellitus. Prior to this study six different genetic causes (PDX1, PTF1A, GATA4, GATA6, HNF1B and RFX6) of pancreatic agenesis/hypoplasia were known. This study identified heterozygous missense mutations in a novel gene (CNOT1) in 3 patients with pancreatic agenesis and holoprosencephaly. The CNOT1 protein has not previously been suggested to have a role in pancreatic development (it is known to act both as scaffold of the CCR4-NOT complex and as an independent factor). It is thought to mediate transcriptional repression and is expressed extremely early during embryonic development (E3.5 in the inner cell mass in mice).
In vitro studies have suggested that CNOT1 plays a critical role in maintaining human and mice embryonic stem cells in a pluripotent state by inhibiting primitive endoderm factors. CNOT1 expression peaks in undifferentiated human induced pluripotent stem (iPS) cells compared to subsequent stages of in vitro differentiation toward pancreatic endocrine cells, supporting its fundamental role in stem cells. Using CRISPR, CNOT1 knockout mice were generated to try and understand the molecular basis of the pancreatic agenesis observed in these patients. Heterozygous mice were born at a lower than expected frequency, but without an obvious phenotype. However, the homozygous mice were embryonically lethal. At E14.5, embryos were still alive and present at expected Mendelian ratios and their phenotype was studied further.
The pancreas was smaller in these embryos due to dorsal pancreatic agenesis. RNA analysis showed increased expression of Sonic hedgehog (Shh) and decreased expression of Pdx1, Ins, Hnf1b, and Ptf1a and no difference in GATA6 or Rxra. The CNOT1 mutation seems to increase the expression of Shh which prevents embryonic stem cells from differentiating into brain and pancreatic tissue. Shh is a key developmental factor that is known to be crucial for pancreatic and brain development. Heterozygous loss-of-function mutations in Shh cause holoprosencephaly and studies in both mouse and human embryos have shown that Shh expression needs to be repressed in the dorsal foregut endoderm for successful differentiation toward dorsal pancreas.
This is the 7th gene to be described that leads to pancreatic agenesis when mutated and further expands the knowledge on developmental biology of the pancreas. This has important implications for developing stem cell regenerative therapies for type 1 diabetes.