A novel PTF1A mutation in a patient with severe pancreatic and cerebellar involvement

Exocrine Pancreatic Insufficiency in Children - Challenges in Management

Cystic fibrosis (CF) is the leading etiology for exocrine pancreatic insufficiency (EPI) in children, followed by chronic pancreatitis, Shwachman-Diamond syndrome, and other genetic disorders. Management of EPI in children poses several unique challenges such as difficulties in early recognition, lack of widespread availability of diagnostic tests and limited number of pediatric-specific pancreatic centers. Pancreatic enzyme replacement therapy is the cornerstone of EPI management and in young children difficulties in administering pancreatic enzymes are frequently encountered. Patients with EPI also should be screened for fat-soluble vitamin deficiencies and receive appropriate supplementation. Among disorders with EPI in children, CF is the relatively well-studied condition, and most management recommendations for EPI in children come from expert consensus and conventional practice guidelines. The impact of EPI can be greater in children given their high metabolic demands and rapid growth. Early diagnosis and aggressive management of EPI prevent consequences of complications such as malnutrition, fat-soluble vitamin deficiencies, and poor bone health and improve outcomes. Management by multi-disciplinary team is the key to success.

Isolated Pancreatic Agenesis Secondary to PTF1A Gene Mutation: A Case Series and Literature Review

Background Neonatal diabetes mellitus is a rare form of monogenic diabetes which is diagnosed in the first six months of life. It is often related to genetic mutations; hence, genetic testing is warranted. Here, we present six cases of pancreatic agenesis resulting in neonatal diabetes with PTF1A gene mutation. Methodology This retrospective case series study included six pediatric cases of neonatal diabetes mellitus who are currently following at pediatric endocrinology clinics at King Faisal Specialist Hospital and Research Center, Riyadh, Saudi Arabia. Results The study reported six patients with a mean age of eight years who presented with pancreatic agenesis resulting in neonatal diabetes with PTF1A gene mutation. In four patients, there was no evidence of cerebellar agenesis. Conclusions Neonatal diabetes is a challenging disease that must be diagnosed early to prevent subsequent metabolic complications. Genetic testing is recommended in neonates who present with prolonged duration of hyperglycemia. Insulin replacement is the treatment of choice.

Congenital etiologies of exocrine pancreatic insufficiency

Congenital exocrine pancreatic insufficiency is a rare condition. In a vast majority of patients, exocrine dysfunction occurs as part of a multisystemic disease, the most prevalent being cystic fibrosis and Shwachman-Bodian-Diamond syndrome. Recent fundamental studies have increased our understanding of the pathophysiology of these diseases. Exocrine pancreatic dysfunction should be considered in children with failure to thrive and fatty stools. Treatment is mainly supportive and consists of pancreatic enzyme replacement and liposoluble vitamins supplementation.

Neonatal diabetes caused by disrupted pancreatic and β-cell development

Neonatal diabetes is diagnosed before the age of 6 months and is usually caused by single-gene mutations. More than 30 genetic causes of neonatal diabetes have been described to date, resulting in severely reduced β-cell number or function. Seven of these genes are known to cause neonatal diabetes through disrupted development of the whole pancreas, resulting in diabetes and exocrine pancreatic insufficiency. Pathogenic variants in five transcription factors essential for β-cell development cause neonatal diabetes without other pancreatic phenotypes. However, additional extra-pancreatic features are common. This review will focus on the genes causing neonatal diabetes through disrupted β-cell development, discussing what is currently known about the genetic and phenotypic features of these genetic conditions, and what discoveries may come in the future.

Interactions Between Purkinje Cells and Granule Cells Coordinate the Development of Functional Cerebellar Circuits

Cerebellar development has a remarkably protracted morphogenetic timeline that is coordinated by multiple cell types. Here, we discuss the intriguing cellular consequences of interactions between inhibitory Purkinje cells and excitatory granule cells during embryonic and postnatal development. Purkinje cells are central to all cerebellar circuits, they are the first cerebellar cortical neurons to be born, and based on their cellular and molecular signaling, they are considered the master regulators of cerebellar development. Although rudimentary Purkinje cell circuits are already present at birth, their connectivity is morphologically and functionally distinct from their mature counterparts. The establishment of the Purkinje cell circuit with its mature firing properties has a temporal dependence on cues provided by granule cells. Granule cells are the latest born, yet most populous, neuronal type in the cerebellar cortex. They provide a combination of mechanical, molecular and activity-based cues that shape the maturation of Purkinje cell structure, connectivity and function. We propose that the wiring of Purkinje cells for function falls into two developmental phases: an initial phase that is guided by intrinsic mechanisms and a later phase that is guided by dynamically-acting cues, some of which are provided by granule cells. In this review, we highlight the mechanisms that granule cells use to help establish the unique properties of Purkinje cell firing.

Molecular mechanisms of β-cell dysfunction and death in monogenic forms of diabetes

Monogenetic forms of diabetes represent 1%-5% of all diabetes cases and are caused by mutations in a single gene. These mutations, that affect genes involved in pancreatic β-cell development, function and survival, or insulin regulation, may be dominant or recessive, inherited or de novo. Most patients with monogenic diabetes are very commonly misdiagnosed as having type 1 or type 2 diabetes. The severity of their symptoms depends on the nature of the mutation, the function of the affected gene and, in some cases, the influence of additional genetic or environmental factors that modulate severity and penetrance. In some patients, diabetes is accompanied by other syndromic features such as deafness, blindness, microcephaly, liver and intestinal defects, among others. The age of diabetes onset may also vary from neonatal until early adulthood manifestations. Since the different mutations result in diverse clinical presentations, patients usually need different treatments that range from just diet and exercise, to the requirement of exogenous insulin or other hypoglycemic drugs, e.g., sulfonylureas or glucagon-like peptide 1 analogs to control their glycemia. As a consequence, awareness and correct diagnosis are crucial for the proper management and treatment of monogenic diabetes patients. In this chapter, we describe mutations causing different monogenic forms of diabetes associated with inadequate pancreas development or impaired β-cell function and survival, and discuss the molecular mechanisms involved in β-cell demise.

Clinical Characteristics and Long-term Follow-up of Patients with Diabetes Due To PTF1A Enhancer Mutations

CONTEXT

Biallelic mutations in the PTF1A enhancer are the commonest cause of isolated pancreatic agenesis. These patients do not have severe neurological features associated with loss-of-function PTF1A mutations. Their clinical phenotype and disease progression have not been well characterized.

OBJECTIVE

To evaluate phenotype and genotype characteristics and long-term follow-up of patients with PTF1A enhancer mutations.

SETTING

Twelve tertiary pediatric endocrine referral centers.

PATIENTS

Thirty patients with diabetes caused by PTF1A enhancer mutations. Median follow-up duration was 4 years.

MAIN OUTCOME MEASURES

Presenting and follow-up clinical (birthweight, gestational age, symptoms, auxology) and biochemical (pancreatic endocrine and exocrine functions, liver function, glycated hemoglobin) characteristics, pancreas imaging, and genetic analysis.

RESULTS

Five different homozygous mutations affecting conserved nucleotides in the PTF1A distal enhancer were identified. The commonest was the Chr10:g.23508437A>G mutation (n = 18). Two patients were homozygous for the novel Chr10:g.23508336A>G mutation. Birthweight was often low (median SDS = -3.4). The majority of patients presented with diabetes soon after birth (median age of diagnosis: 5 days). Only 2/30 presented after 6 months of age. All patients had exocrine pancreatic insufficiency. Five had developmental delay (4 mild) on long-term follow-up. Previously undescribed common features in our cohort were transiently elevated ferritin level (n = 12/12 tested), anemia (19/25), and cholestasis (14/24). Postnatal growth was impaired (median height SDS: -2.35, median BMI SDS: -0.52 SDS) with 20/29 (69%) cases having growth retardation.

CONCLUSION

We report the largest series of patients with diabetes caused by PTF1A enhancer mutations. Our results expand the disease phenotype, identifying recurrent extrapancreatic features which likely reflect long-term intestinal malabsorption.

Emerging technologies in pediatrics: the paradigm of neonatal diabetes mellitus

In the era of precision medicine, the tremendous progress in next-generation sequencing technologies has allowed the identification of an ever-increasing number of genes associated with known Mendelian disorders. Neonatal diabetes mellitus is a rare, genetically heterogeneous endocrine disorder diagnosed before 6 months of age. It may occur alone or in the context of genetic syndromes. Neonatal diabetes mellitus has been linked with genetic defects in at least 26 genes to date. Novel mutations in these disease-causing genes are being reported, giving us a better knowledge of the molecular events that occur upon insulin biosynthesis and secretion from the pancreatic β-cell. Of great importance, some of the identified genes encode proteins that can be therapeutically targeted by drugs per os, leading to transitioning from insulin to sulfonylureas. In this review, we provide an overview of pancreatic β-cell physiology, present the clinical manifestations and the genetic causes of the different forms of neonatal diabetes, and discuss the application of next-generation sequencing methods in the diagnosis and therapeutic management of neonatal diabetes and on research in this area.

Clinical Characteristics, Molecular Features, and Long-Term Follow-Up of 15 Patients with Neonatal Diabetes: A Single-Centre Experience

BACKGROUND

Diabetes diagnosed within the first 6 months of life is defined as neonatal diabetes mellitus (NDM). Mutations in the KCNJ11, ABCC8, and INS genes are the most common cause of permanent NDM. In populations with a high rate of consanguinity, Wolcott-Rallison syndrome caused by biallelic EIF2AK3 mutations is common.

METHODS

We studied the clinical characteristics and underlying genetic cause of disease in 15 individuals with diabetes onset before 6 months of age as defined by sustained hyperglycaemia requiring insulin treatment. Patients who had a remission of the diabetes, defined by a normal blood glucose and HbA1c value without insulin or sulphonylurea (SU) treatment, within the first 18 months of life were classified as having transient NDM (TNDM).

RESULTS

We report 15 patients with NDM from 14 unrelated families, including 10 with reported parental consanguinity. 1/15 patients had a remission of diabetes, leading to a diagnosis of TNDM. Mutations were detected in 80% (n = 12/15) of the cohort (ABCC8 [n = 4], PTF1A-distal enhancer [n = 3], KCNJ11 [n = 2], EIF2AK3 [n = 1], INS [n = 1], and SLC19A2 [n = 1]). All cases were initially treated with multiple dose insulin injections. One patient with an ABCC8 mutation transitioned from insulin to SU resulting in improved metabolic control at the age of 20 years.

CONCLUSION

Although the number of individuals born to consanguineous parents was considerably high in this cohort, KATP channel mutations (ABCC8/KCNJ11) were more common than EIF2AK3 mutations (n = 6 vs. n = 1). Genetic analyses should be performed in all NDM cases due to the potential impact on treatment and prognosis.

From Biology to Genes and Back Again: Gene Discovery for Monogenic Forms of Beta-Cell Dysfunction in Diabetes

This review focuses on gene discovery strategies used to identify monogenic forms of diabetes caused by reduced pancreatic beta-cell number (due to destruction or defective development) or impaired beta-cell function. Gene discovery efforts in monogenic diabetes have identified 36 genes so far. These genetic causes have been identified using four main approaches: linkage analysis, candidate gene sequencing and most recently, exome and genome sequencing. The advent of next-generation sequencing has allowed researchers to move away from linkage analysis (relying on large pedigrees and/or multiple families with the same genetic condition) and candidate gene (relying on previous knowledge on the gene's role) strategies to use a gene agnostic approach, utilizing genetic evidence (such as variant frequency, predicted variant effect on protein function, and predicted mode of inheritance) to identify the causative mutation. This approach led to the identification of seven novel genetic causes of monogenic diabetes, six by exome sequencing and one by genome sequencing. In many of these cases, the disease-causing gene was not known to be important for beta-cell function prior to the gene discovery study. These novel findings highlight a new role for gene discovery studies in furthering our understanding of beta-cell function and dysfunction in diabetes. While many gene discovery studies in the past were led by knowledge in the field (through the candidate gene strategy), now they often lead the scientific advances in the field by identifying new important biological players to be further characterized by in vitro and in vivo studies.

The clinical and genetic characteristics of permanent neonatal diabetes (PNDM) in the state of Qatar

Background

Neonatal diabetes mellitus (NDM) is a rare condition that occurs within the first six months of life. Permanent NDM (PNDM) is caused by mutations in specific genes that are known for their expression at early and/or late stages of pancreatic beta‐ cell development, and are either involved in beta‐cell survival, insulin processing, regulation, and release. The native population in Qatar continues to practice consanguineous marriages that lead to a high level of homozygosity. To our knowledge, there is no previous report on the genomics of NDM among the Qatari population. The aims of the current study are to identify patients with NDM diagnosed between 2001 and 2016, and examine their clinical and genetic characteristics.

Methods

To calculate the incidence of PNDM, all patients with PNDM diagnosed between 2001 and 2016 were compared to the total number of live births over the 16‐year‐period. Whole Genome Sequencing (WGS) was used to investigate the genetic etiology in the PNDM cohort.

Results

PNDM was diagnosed in nine (n = 9) patients with an estimated incidence rate of 1:22,938 live births among the indigenous Qatari. Seven different mutations in six genes (PTF1A, GCK, SLC2A2, EIF2AK3, INS, and HNF1B) were identified. In the majority of cases, the genetic etiology was part of a previously identified autosomal recessive disorder. Two novel de novo mutations were identified in INS and HNF1B.

Conclusion

Qatar has the second highest reported incidence of PNDM worldwide. A majority of PNDM cases present as rare familial autosomal recessive disorders. Pancreas associated transcription factor 1a (PTF1A) enhancer deletions are the most common cause of PNDM in Qatar, with only a few previous cases reported in the literature.

Transcription factor Ptf1a in development, diseases and reprogramming

The transcription factor Ptf1a is a crucial helix-loop-helix (bHLH) protein selectively expressed in the pancreas, retina, spinal cord, brain, and enteric nervous system. Ptf1a is preferably assembled into a transcription trimeric complex PTF1 with an E protein and Rbpj (or Rbpjl). In pancreatic development, Ptf1a is indispensable in controlling the expansion of multipotent progenitor cells as well as the specification and maintenance of the acinar cells. In neural tissues, Ptf1a is transiently expressed in the post-mitotic cells and specifies the inhibitory neuronal cell fates, mostly mediated by downstream genes such as Tfap2a/b and Prdm13. Mutations in the coding and non-coding regulatory sequences resulting in Ptf1a gain- or loss-of-function are associated with genetic diseases such as pancreatic and cerebellar agenesis in the rodent and human. Surprisingly, Ptf1a alone is sufficient to reprogram mouse or human fibroblasts into tripotential neural stem cells. Its pleiotropic functions in many biological processes remain to be deciphered in the future.

Prenatal diagnosis of posterior fossa anomalies: Additional value of chromosomal microarray analysis in fetuses with cerebellar hypoplasia

OBJECTIVE

To elucidate the relationship between copy number variations (CNVs) detected by high-resolution chromosomal microarray analysis (CMA) and the type of prenatal posterior fossa anomalies (PFAs), especially cerebellar hypoplasia (CH).

METHODS

This study involved 77 pregnancies with PFAs who underwent CMA.

RESULTS

Chromosomal aberrations including pathogenic CNVs and variants of unknown significance were detected in 31.2% (24/77) of all cases by CMA and in 18.5% (12/65) in fetuses with normal karyotypes. The high detection rate of clinically significant CNVs was evident in fetuses with cerebellar hypoplasia (54.6%, 6/11), vermis hypoplasia (33.3%, 1/3), and Dandy-Walker malformation (25.0%, 3/12). Compare with fetuses without other anomalies, cases with CH and additional malformations had the higher CMA detection rate (33.3% vs 88.9%). Three cases of isolated unilateral CH with intact vermis and normal CMA result had normal outcomes. The deletion of 5p15, 6q terminal deletion, and X chromosome aberrations were the most frequent genetic defects associated with cerebellar hypoplasia.

CONCLUSION

Among fetuses with PFA, those with cerebellar hypoplasia, vermis hypoplasia, or Dandy-Walker malformation are at the highest risk of clinically significant CNVs. Chromosomal microarray analysis revealed the most frequent chromosomal aberrations associated with CH.

Pancreatic Agenesis due to Compound Heterozygosity for a Novel Enhancer and Truncating Mutation in the PTF1A Gene

Neonatal diabetes, defined as the onset of diabetes within the first six months of life, is very rarely caused by pancreatic agenesis. Homozygous truncating mutations in the PTF1A gene, which encodes a transcriptional factor, have been reported in patients with pancreatic and cerebellar agenesis, whilst mutations located in a distal pancreatic-specific enhancer cause isolated pancreatic agenesis. We report an infant, born to healthy non-consanguineous parents, with neonatal diabetes due to pancreatic agenesis. Initial genetic investigation included sequencing of KCNJ11, ABCC8 and INS genes, but no mutations were found. Following this, 22 neonatal diabetes associated genes were analyzed by a next generation sequencing assay. We found compound heterozygous mutations in the PTF1A gene: A frameshift mutation in exon 1 (c.437_462 del, p.Ala146Glyfs*116) and a mutation affecting a highly conserved nucleotide within the distal pancreatic enhancer (g.23508442A>G). Both mutations were confirmed by Sanger sequencing. Isolated pancreatic agenesis resulting from compound heterozygosity for truncating and enhancer mutations in the PTF1A gene has not been previously reported. This report broadens the spectrum of mutations causing pancreatic agenesis.

A novel mechanism for variable phenotypic expressivity in Mendelian diseases uncovered by an AU-rich element (ARE)-creating mutation

Background

Variable expressivity is a well-known phenomenon in which patients with mutations in one gene display varying degrees of clinical severity, potentially displaying only subsets of the clinical manifestations associated with the multisystem disorder linked to the gene. This remains an incompletely understood phenomenon with proposed mechanisms ranging from allele-specific to stochastic.

Results

We report three consanguineous families in which an isolated ocular phenotype is linked to a novel 3′ UTR mutation in SLC4A4, a gene known to be mutated in a syndromic form of intellectual disability with renal and ocular involvement. Although SLC4A4 is normally devoid of AU-rich elements (AREs), a 3′ UTR motif that mediates post-transcriptional control of a subset of genes, the mutation we describe creates a functional ARE. We observe a marked reduction in the transcript level of SLC4A4 in patient cells. Experimental confirmation of the ARE-creating mutation is shown using a post-transcriptional reporter system that reveals consistent reduction in the mRNA-half life and reporter activity. Moreover, the neo-ARE binds and responds to the zinc finger protein ZFP36/TTP, an ARE-mRNA decay-promoting protein.

Conclusions

This novel mutational mechanism for a Mendelian disease expands the potential mechanisms that underlie variable phenotypic expressivity in humans to also include 3′ UTR mutations with tissue-specific pathology.

Isolated Pancreatic Aplasia Due to a Hypomorphic PTF1A Mutation

Homozygous truncating mutations in the helix-loop-helix transcription factor PTF1A are a rare cause of pancreatic and cerebellar agenesis. The correlation of Ptf1a dosage with pancreatic phenotype in a mouse model suggested the possibility of finding hypomorphic PTF1A mutations in patients with pancreatic agenesis or neonatal diabetes but no cerebellar phenotype. Genome-wide single nucleotide polymorphism typing in two siblings with neonatal diabetes from a consanguineous pedigree revealed a large shared homozygous region (31 Mb) spanning PTF1A Sanger sequencing of PTF1A identified a novel missense mutation, p.P191T. Testing of 259 additional patients using a targeted next-generation sequencing assay for 23 neonatal diabetes genes detected one additional proband and an affected sibling with the same homozygous mutation. All four patients were diagnosed with diabetes at birth and were treated with insulin. Two of the four patients had exocrine pancreatic insufficiency requiring replacement therapy but none of the affected individuals had neurodevelopmental delay. Transient transfection assays of the mutant protein demonstrated a 75% reduction in transactivation activity. This study shows that the functional severity of a homozygous mutation impacts the severity of clinical features found in patients.

Variable Phenotype of Diabetes Mellitus in Siblings with a Homozygous PTF1A Enhancer Mutation

Neonatal diabetes is a rare form of diabetes, characterized by onset in the first 6 months of life. A number of cases are due to pancreas agenesis. Recently, PTF1A enhancer mutations have been shown to cause neonatal diabetes associated with pancreatic agenesis. Herein, we report the clinical features of two siblings with PTF1A enhancer mutations, one of whom had neonatal diabetes, whereas the elder sister had a milder form of the disease with onset of diabetes at 9 years of age.

Mutations in the noncoding genome

PURPOSE OF REVIEW

Clinical diagnostic sequencing currently focuses on identifying causal mutations in the exome, wherein most disease-causing mutations are known to occur. The rest of the genome is mostly comprised of regulatory elements that control gene expression, but these have remained largely unexplored in clinical diagnostics due to the high cost of whole genome sequencing and interpretive challenges. The purpose of this review is to illustrate examples of diseases caused by mutations in regulatory elements and introduce the diagnostic potential for whole genome sequencing. Different classes of functional elements and chromatin structure are described to provide the clinician with a foundation for understanding the basis of these mutations.

RECENT FINDINGS

The utilization of whole-genome sequence data, epigenomic maps and induced pluripotent stem (IPS) cell technologies facilitated the discovery that mutations in the pancreas-specific transcription factor 1a enhancer can cause isolated pancreatic agenesis. High resolution array comparative genomic hybridisation (CGH), whole-genome sequencing, maps of 3-D chromatin architecture, and mouse models generated using clustered regularly interspaced short palindromic repeats (CRISPR)/Cas were used to show that disruption of topological-associated domain boundary elements cause limb defects. Structural variants that reposition enhancers in somatic cells have also been described in cancer.

SUMMARY

Although not ready for diagnostics, new technologies, epigenomic maps, and improved knowledge of chromatin architecture will soon enable a better understanding and diagnostic solutions for currently unexplained genetic disorders.

Genome, Exome, and Targeted Next-Generation Sequencing in Neonatal Diabetes

The use of targeted gene panels now allows the analysis of all the genes known to cause a disease in a single test. For neonatal diabetes, this has resulted in a paradigm shift with patients receiving a genetic diagnosis early and the genetic results guiding their clinical management. Exome and genome sequencing are powerful tools to identify novel genetic causes of known diseases. For neonatal diabetes, the use of these technologies has resulted in the identification of 2 novel disease genes (GATA6 and STAT3) and a novel regulatory element of PTF1A, in which mutations cause pancreatic agenesis.

Mutations in PTF1A are not a common cause for human VATER/VACTERL association or neural tube defects mirroring Danforth's short tail mouse

Danforth's short tail (Sd) mutant mice exhibit defects of the neural tube and other abnormalities, which are similar to the human vertebral anomalies, anal atresia, cardiac defects, tracheosophageal fistula and/or esophageal atresia, renal and radial abnormalities, and limb defects (VATER/VACTERL) association, including defects of the hindgut. Sd has been shown to underlie ectopic gene expression of murine Ptf1a, which encodes pancreas-specific transcription factor 1A, due to the insertion of a retrotansposon in its 5' regulatory domain. In order to investigate the possible involvement of this gene in human VATER/VACTERL association and human neural tube defects (NTDs), a sequence analysis was performed. DNA samples from 103 patients with VATER/VACTERL and VATER/VACTERL‑like association, all presenting with anorectal malformations, and 72 fetuses with NTDs, where termination of pregnancy had been performed, were included in the current study. The complete PTF1A coding region, splice sites and 1.5 kb of the 5' flanking promotor region was sequenced. However, no pathogenic alterations were detected. The results of the present study do not support the hypothesis that high penetrant mutations in these regions of PTF1A are involved in the development of human VATER/VACTERL association or NTDs, although rare mutations may be detectable in larger patient samples.

The E3 ubiquitin ligase thyroid hormone receptor-interacting protein 12 targets pancreas transcription factor 1a for proteasomal degradation

Pancreas transcription factor 1a (PTF1a) plays a crucial role in the early development of the pancreas and in the maintenance of the acinar cell phenotype. Several transcriptional mechanisms regulating expression of PTF1a have been identified. However, regulation of PTF1a protein stability and degradation is still unexplored. Here, we report that inhibition of proteasome leads to elevated levels of PTF1a and to the existence of polyubiquitinated forms of PTF1a. We used the Sos recruitment system, an alternative two-hybrid system method to detect protein-protein interactions in the cytoplasm and to map the interactome of PTF1a. We identified TRIP12 (thyroid hormone receptor-interacting protein 12), an E3 ubiquitin-protein ligase as a new partner of PTF1a. We confirmed PTF1a/TRIP12 interaction in acinar cell lines and in co-transfected HEK-293T cells. The protein stability of PTF1a is significantly increased upon decreased expression of TRIP12. It is reduced upon overexpression of TRIP12 but not a catalytically inactive TRIP12-C1959A mutant. We identified a region of TRIP12 required for interaction and identified lysine 312 of PTF1a as essential for proteasomal degradation. We also demonstrate that TRIP12 down-regulates PTF1a transcriptional and antiproliferative activities. Our data suggest that an increase in TRIP12 expression can play a part in PTF1a down-regulation and indicate that PTF1a/TRIP12 functional interaction may regulate pancreatic epithelial cell homeostasis.

The pancreatic β cell: recent insights from human genetics

Diabetes mellitus is a metabolic disease characterised by relative or absolute pancreatic β cell dysfunction. Genetic variants implicated in disease risk can be identified by studying affected individuals. To understand the mechanisms driving genetic associations, variants must be translated through causative transcripts to biological insights. Studies into the genetic basis of Mendelian forms of diabetes have successfully identified genes involved in both β cell function and pancreatic development. For type 2 diabetes (T2D), genome-wide association studies (GWASs) are uncovering an ever-increasing number of susceptibility variants that exert their effect through β cell dysfunction, but translation to mechanistic understanding has in most cases been slow. Improved annotations of the islet genome and advances in whole-genome and -exome sequencing (WHS and WES) have facilitated recent progress.

Transformation of the cerebellum into more ventral brainstem fates causes cerebellar agenesis in the absence of Ptf1a function

Model organism studies have demonstrated that cell fate specification decisions play an important role in normal brain development. Their role in human neurodevelopmental disorders, however, is poorly understood, with very few examples described. The cerebellum is an excellent system to study mechanisms of cell fate specification. Although signals from the isthmic organizer are known to specify cerebellar territory along the anterior-posterior axis of the neural tube, the mechanisms establishing the cerebellar anlage along the dorsal-ventral axis are unknown. Here we show that the gene encoding pancreatic transcription factor PTF1A, which is inactivated in human patients with cerebellar agenesis, is required to segregate the cerebellum from more ventral extracerebellar fates. Using genetic fate mapping in mice, we show that in the absence of Ptf1a, cells originating in the cerebellar ventricular zone initiate a more ventral brainstem expression program, including LIM homeobox transcription factor 1 beta and T-cell leukemia homeobox 3. Misspecified cells exit the cerebellar anlage and contribute to the adjacent brainstem or die, leading to cerebellar agenesis in Ptf1a mutants. Our data identify Ptf1a as the first gene involved in the segregation of the cerebellum from the more ventral brainstem. Further, we propose that cerebellar agenesis represents a new, dorsal-to-ventral, cell fate misspecification phenotype in humans.

Many faces of monogenic diabetes

Monogenic diabetes represents a heterogeneous group of disorders resulting from defects in single genes. Defects are categorized primarily into two groups: disruption of β-cell function or a reduction in the number of β-cells. A complex network of transcription factors control pancreas formation, and a dysfunction of regulators high in the hierarchy leads to pancreatic agenesis. Dysfunction among factors further downstream might cause organ hypoplasia, absence of islets of Langerhans or a reduction in the number of β-cells. Many transcription factors have pleiotropic effects, explaining the association of diabetes with other congenital malformations, including cerebellar agenesis and pituitary agenesis. Monogenic diabetes variants are classified conventionally according to age of onset, with neonatal diabetes occurring before the age of 6 months and maturity onset diabetes of the young (MODY) manifesting before the age of 25 years. Recently, certain familial genetic defects were shown to manifest as neonatal diabetes, MODY or even adult onset diabetes. Patients with neonatal diabetes require a thorough genetic work-up in any case, and because extensive phenotypic overlap exists between monogenic, type 2, and type 1 diabetes, genetic analysis will also help improve diagnosis in these cases. Next generation sequencing will facilitate rapid screening, leading to the discovery of digenic and oligogenic diabetes variants, and helping to improve our understanding of the genetics underlying other types of diabetes. An accurate diagnosis remains important, because it might lead to a change in the treatment of affected subjects and influence long-term complications.

Ectopic cerebellar cell migration causes maldevelopment of Purkinje cells and abnormal motor behaviour in Cxcr4 null mice

SDF-1/CXCR4 signalling plays an important role in neuronal cell migration and brain development. However, the impact of CXCR4 deficiency in the postnatal mouse brain is still poorly understood. Here, we demonstrate the importance of CXCR4 on cerebellar development and motor behaviour by conditional inactivation of Cxcr4 in the central nervous system. We found CXCR4 plays a key role in cerebellar development. Its loss leads to defects in Purkinje cell dentritogenesis and axonal projection in vivo but not in cell culture. Transcriptome analysis revealed the most significantly affected pathways in the Cxcr4 deficient developing cerebellum are involved in extra cellular matrix receptor interactions and focal adhesion. Consistent with functional impairment of the cerebellum, Cxcr4 knockout mice have poor coordination and balance performance in skilled motor tests. Together, these results suggest ectopic the migration of granule cells impairs development of Purkinje cells, causes gross cerebellar anatomical disruption and leads to behavioural motor defects in Cxcr4 null mice.

Analysis of transcription factors key for mouse pancreatic development establishes NKX2-2 and MNX1 mutations as causes of neonatal diabetes in man

Understanding transcriptional regulation of pancreatic development is required to advance current efforts in developing beta cell replacement therapies for patients with diabetes. Current knowledge of key transcriptional regulators has predominantly come from mouse studies, with rare, naturally occurring mutations establishing their relevance in man. This study used a combination of homozygosity analysis and Sanger sequencing in 37 consanguineous patients with permanent neonatal diabetes to search for homozygous mutations in 29 transcription factor genes important for murine pancreatic development. We identified homozygous mutations in 7 different genes in 11 unrelated patients and show that NKX2-2 and MNX1 are etiological genes for neonatal diabetes, thus confirming their key role in development of the human pancreas. The similar phenotype of the patients with recessive mutations and mice with inactivation of a transcription factor gene support there being common steps critical for pancreatic development and validate the use of rodent models for beta cell development.

Two novel GATA6 mutations cause childhood-onset diabetes mellitus, pancreas malformation and congenital heart disease

BACKGROUND

GATA6 mutations are the most frequent cause of pancreatic agenesis and diabetes in human sporadic cases. In families, dominantly inherited mutations show a variable phenotype also in terms of endocrine and exocrine pancreatic disease. We report two novel GATA6 mutations in an independent cohort of 8 children with pancreas aplasia or hypoplasia and diabetes.

METHODS

We sequenced GATA6 in 8 children with diabetes and inborn pancreas abnormalities, i.e. hypoplasia or aplasia in which other known candidate genes causing monogenic diabetes and pancreatic defects had been excluded.

RESULTS

We found two novel heterozygous GATA6 mutations (c.951_954dup and c.754_904del) in 2 patients with sporadic pancreas hypoplasia, diabetes and severe cardiac defects (common truncus arteriosus and tetralogy of Fallot), but not in the remaining 6 patients. GATA6 mutations in carriers exhibited hypoplastic pancreas with absent head in 1 patient and with increased echogenicity and decreasing exocrine function in the other patient. Additionally, hepatobiliary malformations and brain atrophy were found in 1 patient.

CONCLUSION

Our 2 cases with novel GATA6 mutations add more phenotype characteristics of GATA6 haploinsufficiency. In agreement with an increasing number of published cases, the wide phenotypic spectrum of GATA6 diabetes syndrome should draw the attention of both pediatric endocrinologists and geneticists.

A novel GATA6 mutation in a child with congenital heart malformation and neonatal diabetes

KEY CLINICAL MESSAGE

Diabetes in neonates is a monogenetic disease and genetic analysis is warranted to allow best treatment, prognosis, and genetic counseling. Transcription factor mutations may have a variable expression and different organs may be involved.

Recessive mutations in a distal PTF1A enhancer cause isolated pancreatic agenesis

The contribution of cis-regulatory mutations to human disease remains poorly understood. Whole genome sequencing can identify all non-coding variants, yet discrimination of causal regulatory mutations represents a formidable challenge. We used epigenomic annotation in hESC-derived embryonic pancreatic progenitor cells to guide the interpretation of whole genome sequences from patients with isolated pancreatic agenesis. This uncovered six different recessive mutations in a previously uncharacterized ~400bp sequence located 25kb downstream of PTF1A (pancreas-specific transcription factor 1a) in ten families with pancreatic agenesis. We show that this region acts as a developmental enhancer of PTF1A and that the mutations abolish enhancer activity. These mutations are the most common cause of isolated pancreatic agenesis. Integrating genome sequencing and epigenomic annotation in a disease-relevant cell type can uncover novel non-coding elements underlying human development and disease.