Pancreatic exocrine insufficiency

The pancreas as the central organ of digestion

The pancreas is an exocrine and endocrine organ that breaks down food components and regulates blood glucose levels. The pancreas secretes an enzyme- and electrolyte-containing fluid whose secretion rate and composition changes depending on the food intake. The high bicarbonate content ensures an alkaline pH of around eight and, by neutralizing the acidic stomach contents, ensures optimal activity of the digestive enzymes in the intestine. Around 90% of the proteins in pancreatic secretions are digestive enzymes. These are mainly produced as inactive precursors (zymogens) in the acinar cells of the pancreas.

Diseases of the exocrine pancreas primarily manifest themselves as inflammation (pancreatitis) or as pancreatic insufficiency, which reduces the volume of secretions and the concentrations of bicarbonate and digestive enzymes and thus leads to failure to thrive and fatty stools.

The importance of transcription factors for organ function

The genes responsible for digestive enzymes production must be translated into protein molecules (proteins). Transcription factors (TF) are of central importance for this process. Transcription factors are proteins that control the conversion of genetic material (DNA) into the messenger molecule mRNA and thus ensure that a protein is produced in the required quantity. Transcription factors bind to specific DNA motifs and control whether and how much mRNA is synthesized (transcription). They thus regulate the expression of a gene so that the proteins are produced in the correct quantity and at the right time. The factors can activate (so-called inducer) or inhibit (so-called repressor) gene transcription. Mutations in the base sequence of TF genes can cause altered transcription rates, as the TFs no longer dock correctly.

GATA factors as central regulators of pancreatic function

In addition to PTF1 (consisting of PTF1A, RBPJL and basic helix-loop-helix TF), the central transcription factors for regulating the production of digestive enzymes in the pancreas are the so-called GATA transcription factors, particularly GATA4 and GATA6. GATA molecules bind to a specific base sequence in the DNA: (A/T)GATA(A/G).

GATA6 mutations cause a complete absence (aplasia) or underdevelopment (hypoplasia) of the pancreas in humans. It manifests itself with severe failure to thrive in utero, exocrine pancreatic insufficiency and diabetes. Mouse models in which GATA6 was destroyed (GATA6 knock-out mice) showed a similar clinical picture.

Furthermore, essential transcription factors of the acinar cell, which is responsible for digestive enzyme synthesis, such as PTF1A and RBPJL, were significantly reduced in these mice. As a result, almost all digestive enzymes were significantly reduced. Furthermore, the inactivation of GATA6 led to a severe remodelling of the acinar tissue: In addition to a massive loss of acinar cells, increased cell death (apoptosis), fatty degeneration of the organ (lipomatosis) and a transformation of acinar cells into duct cells were observed.

Altered GATA binding as a cause of pancreatitis

Pancreatitis is a recurrent or persistent inflammation of the pancreas. One of the most common causes in childhood is a genetic alteration in SPINK1, N34S. SPINK1 inhibits the activation of digestive enzymes in the pancreas. It thus prevents self-digestion of the organ, which manifests clinically as pancreatitis. The N34S mutation leads to reduced inhibitor production due to impaired gene regulation. However, the exact mechanism still needs to be clarified. We could decipher the mechanism using bioinformatic analyses and modern proteomics methods: The N34S mutation is in complete linkage with a variant located several thousand bases upstream of the gene. This linked variant creates a binding site for GATA factors, which inhibits SPINK1 expression. We were able to confirm the GATA binding in several functional analyses.

Reduced GATA binding in exocrine pancreatic insufficiency

We recently identified a family with exocrine pancreatic insufficiency in which half of the family members are affected over four generations, suggesting an autosomal dominant inheritance. Some affected individuals also have fatty degeneration (lipomatosis) of the pancreas and hypothyroidism. We carried out genome sequencing in this family. In the genome-wide analysis, a mutation not described in the databases (>120,000 people) was found in all affected individuals but not in healthy family members. This mutation is located in a regulatory region of an important transcription factor gene, essential for producing almost all digestive enzymes and is destroying a GATA binding site. These findings further support that GATA factors play a key role in organ development and the development of pancreatitis and exocrine pancreatic insufficiency.

• Prof. Giriraj Chandak (CSIR-Centre for Cellular and Molecular Biology, Hyderabad/Indien)
• Dr. Stefanie Hauck (HMGU, Neuherberg)
• Prof. Matthias Löhr (Karolinska Institutet, Stockholm/Schweden)
• Prof. Jonas Rosendahl (Universität Halle)
• Dr. Martin Seifert (Genomatix, München)