Monogenic causes of pancreatic agenesis
Pancreatic agenesis is an extremely rare condition resulting from impaired formation of the pancreas during embryonic development. Clinically, pancreatic agenesis is defined as insulin dependent neonatal diabetes diagnosed before 6 months of age and pancreatic exocrine insufficiency requiring enzyme replacement therapy. In the last few years advances in genetic techniques have allowed identification of the underlying genetic defect in over 80% of patients with pancreatic agenesis, with mutations in the pancreatic transcription factor genes GATA6 and PTF1A being the most common.
Diagnosis of pancreatic agenesis
Patients affected by pancreatic agenesis usually present with intrauterine growth retardation (IUGR) as a result of reduced insulin secretion in utero and are diagnosed with hyperglycaemia in the first days of life. All patients with pancreatic agenesis are insulin treated.
From a morphological point of view the pancreas can be totally absent or extremely reduced in size (pancreatic hypoplasia). A diagnosis of pancreatic agenesis can be made:
- by imaging (MRI, ultrasound) showing reduction or absence of pancreatic tissue;
- by measurement of fecal elastase (or fecal fat). Fecal elastase is extremely reduced (often undetectable) in patients with pancreatic agenesis as a result of exocrine dysfunction;
- clinically by the presence of insulin-dependent neonatal diabetes and exocrine insufficiency requiring enzyme replacement therapy.
Genetic causes of pancreatic agenesis
A genetic diagnosis is possible for over 80% of patients with pancreatic agenesis. All patients reported to date have defects in genes encoding for transcription factors that regulate early development of the pancreas. Over 80% of cases are due to mutations in GATA6 or PTF1A.
Pancreatic agenesis due to dominant mutations in GATA6
Molecular function of GATA6
GATA6 (GATA-binding factor 6) belongs to the GATA family of transcription factors (GATA1-6), which are involved in development of different tissues. All GATA proteins contain a highly conserved DNA-binding domain composed of two tandem zinc fingers essential for DNA-binding and protein-protein interaction. GATA6 is expressed in heart, pancreas, lung, liver and ovary. During mouse embryonic development, Gata6 is first needed for correct extra-embryonic tissues formation. Mice completely lacking Gata6 (Gata6 -/-) die early during embryonic development (embryonic day 6.5-7.5) whilst mice harbouring one functional copy of Gata6 have no visible phenotype.
Heterozygous mutations in GATA6
Mutations in GATA6 were first identified as a cause of pancreatic agenesis in two patients with neonatal diabetes, exocrine insufficiency and congenital heart defects . The authors sequenced the exome (coding regions of all genes) in two patients and their unaffected parents (trio exome strategy) and identified a single de novo mutation in GATA6 in each patient. Sequencing of GATA6 in 25 additional patients revealed 13 further heterozygous mutations in patients with pancreatic agenesis.
A later study  looked for mutations in GATA6 in patients with neonatal diabetes but without known pancreatic exocrine insufficiency and found that:
- 54% of patients with pancreatic agenesis had a heterozygous mutation in GATA6, thus confirming that it is the most common cause of this disease;
- mutations in GATA6 caused a broad phenotypic spectrum of diabetes: ranging from pancreatic agenesis to adult onset diabetes without exocrine insufficiency;
- all patients with mutations in GATA6 had congenital extra-pancreatic features. The most common were heart defects (requiring corrective surgery in most cases), hepato-biliary malformations, hypothyroidism, gut abnormalities and neurodevelopmental defects.
Pancreatic agenesis due to recessively inherited mutations in PTF1A
Molecular function of PTF1A
PTF1A (pancreatic transcription factor 1a) is a transcription factor essential for formation of the endocrine, exocrine and ductal cells during embryonic development of the pancreas. Studies in mouse embryos have shown that Ptf1a is expressed as early as embryonic day 8.5 in the region of the future pancreas and in neural cell progenitors.
Representation of the genomic region of PTF1A. The enhancer is located 25 kb downstream of the gene and mutations in this genomic element cause pancreatic agenesis (adapted from Weedon et al Nature genetics 2014; 46(1): 61-4).
Recessive mutations in PTF1A
Homozygous mutations in the coding region of the PTF1A gene were initially reported in two families with pancreatic agenesis, microcephaly, and cerebellar hypoplasia/agenesis . All affected individuals died within a few weeks of birth. This combination of features closely resembled the phenotype seen in Ptf1a -/- mice. Using a combination of homozygosity mapping and a candidate gene approach, Sellick et al identified mutations in the PTF1A gene in both families. The two mutations were predicted to result in loss of protein function. Mutations in the coding regions of PTF1A have since been reported in 2 further patients with pancreatic and cerebellar agenesis. Mutations in the coding regions of PTF1A are therefore a rare cause of pancreatic agenesis.
Approximately 25% of patients with pancreatic agenesis have homozygous mutations in a distal regulatory element (an enhancer) of PTF1A . This region is located 25 kb downstream of PTF1A and recent studies have shown that it is selectively active during development of the pancreas. Patients with mutations in this region have a milder phenotype compared to patients with mutations in the coding region of PTF1A: they have pancreatic agenesis but not cerebellar agenesis. The absence of a neurological phenotype in these individuals is thought to be due to tissue specificity of the enhancer region. Recessively inherited mutations of the PTF1A enhancer are the most common cause of pancreatic agenesis among patients born to consanguineous parents.
Rares causes of pancreatic agenesis
Pancreatic agenesis due to recessively inherited mutations in PDX1
Molecular function of PDX1
PDX1 (Pancreatic and duodenal homeobox 1, also known as IPF1 Insulin promoter factor 1) is a pancreatic transcription factor essential for both development of the pancreas and maintenance of β cell function.
In mouse embryos, expression of Pdx1 is first detected in the developing pancreas at embryonic day 8.5. In adult β cells Pdx1 acts by binding and activating the insulin promoter. Neonatal mice carrying a null mutation of Pdx1 have complete pancreatic agenesis. Pdx1 +/- mice don’t show any relevant phenotype at birth, but they develop impaired glucose tolerance later in life.
Recessive mutations in PDX1
Recessively inherited mutations in PDX1 were the first genetic cause of pancreatic agenesis to be identified . In an infant with complete pancreatic agenesis Stoffers and colleagues identified a homozygous mutation in PDX1 which was predicted to result in premature termination of the protein. Two further cases of pancreatic agenesis due to mutations in PDX1 were subsequently reported. All these patients had isolated pancreatic agenesis.
More recently two studies have shown that mutations in PDX1 can cause isolated neonatal diabetes without clinical signs of exocrine insufficiency with 6 cases reported.
Pancreatic agenesis due to dominant mutations in GATA4
Molecular function of GATA4
GATA4 (GATA-binding factor 4) is a zinc finger transcription factor closely related to GATA6. It plays important roles in the development of multiple organs. Mouse studies have suggested a role for Gata4 in pancreatic development. Gata4 -/- mice die prior to pancreas formation because of defective extra-embryonic tissue development, but investigation of Gata4 -/- mice rescued from embryonic lethality showed agenesis of the pancreas. Gata4 +/- mice are phenotypically normal.
Heterozygous mutations in GATA4
Two studies  have reported 3 cases with heterozygous mutations in GATA4 and pancreatic agenesis: two of these patients also had congenital heart defects. One of the studies reported two additional patients with deletions over GATA4 who had diabetes diagnosed in early infancy but normal exocrine function. This suggests that mutations in GATA4 are associated with a more variable phenotype than GATA6 and are a much rarer cause of pancreatic agenesis in humans.
Pancreatic agenesis due to dominant mutations in HNF1B
Molecular function of HNF1B
The transcription factor HNF1B (Hepatocyte nuclear factor 1B) plays an essential role in development of kidneys, liver, Mullerian duct and embryonic pancreas. Studies in mice have showed that embryos completely deficient in Hnf1b have a significantly reduced pancreatic size. Similar to what is seen with mouse models of Gata6 and Gata4, heterozygous Hnf1b +/- mice were asymptomatic.
Heterozygous mutations in HNF1B
In most cases mutations/deletions of HNF1B cause adult onset diabetes and renal cysts (RCAD syndrome). The majority of these cases also have reduced size of the pancreas and subclinical exocrine insufficiency. Two cases of complete pancreatic agenesis in foetuses with severe renal malformations have been reported . This suggests that in some rare cases HNF1B mutations can cause a more severe phenotype that includes complete pancreatic agenesis.
^ Lango Allen H, Flanagan SE, Shaw-Smith C, et al. GATA6 haploinsufficiency causes pancreatic agenesis in humans. Nature genetics 2012; 44(1): 20-2.
^ De Franco E, Shaw-Smith C, Flanagan SE, Shepherd MH, Hattersley AT, Ellard S. GATA6 mutations cause a broad phenotypic spectrum of diabetes from pancreatic agenesis to adult-onset diabetes without exocrine insufficiency. Diabetes 2013; 62(3): 993-7.
^ Sellick GS, Barker KT, Stolte-Dijkstra I, et al. Mutations in PTF1A cause pancreatic and cerebellar agenesis. Nature genetics 2004; 36(12): 1301-5.
^ Weedon MN, Cebola I, Patch AM, et al. Recessive mutations in a distal PTF1A enhancer cause isolated pancreatic agenesis. Nature genetics 2014; 46(1): 61-4.
^ Stoffers DA, Zinkin NT, Stanojevic V, Clarke WL, Habener JF. Pancreatic agenesis attributable to a single nucleotide deletion in the human IPF1 gene coding sequence. Nature genetics 1997; 15(1): 106-10.
^ D'Amato E, Giacopelli F, Giannattasio A, et al. Genetic investigation in an Italian child with an unusual association of atrial septal defect, attributable to a new familial GATA4 gene mutation, and neonatal diabetes due to pancreatic agenesis. Diabetic medicine : a journal of the British Diabetic Association 2010; 27(10): 1195-200.
^ Shaw-Smith C, De Franco E, Allen HL, et al. GATA4 mutations are a cause of neonatal and childhood-onset diabetes. Diabetes 2014.
^ Clissold RL, Hamilton AJ, Hattersley AT, Ellard S, Bingham C. HNF1B-associated renal and extra-renal disease-an expanding clinical spectrum. Nature reviews Nephrology 2015; 11(2): 102-12.