Milord E and Gragnoli C, Minerva Med. 2006 Oct;97(5):373-8.
AIM: Type 2 diabetes (T2D) is a complex polygenic disorder. Genetic predisposition may vary in different ethnic groups. A potential candidate gene for T2D is Neurogenin 3 (Ngn3, NEUROG3), which lies on chromosome 10 in a region with several potential linkage sig
nals to T2D in various population studies. The goal of this study was to establish whether NEUROG3 gene variants are contributing to T2D in an Italian T2D cohort. METHODS: We genotyped AFMa210xh1 macrosatellite marker in 202 Italian T2D families/sib-pairs. We performed two-point linkage analysis in the late- and early-onset dataset. For the case control study, we selected families with a positive logarithm of odds (LOD) score. Then, we screened NEUROG3 in the selected 61 single unrelated T2D patients and 101 Italian controls and performed association studies. RESULTS: Several variants were identified: a new 152ntC/G, and 44-45delCA, Gly167Arg, Ser 199Phe single nucleotide polymorphisms (SNPs) and 2 new 5'UTR variations (-nt498G/T and nt367C/T) and a new Gly167fsinsCAE Arg167X234 mutation. The variants 44-45delCA/ Ser199Phe and Gly167Arg/Ser199Phe show significant linkage disequilibrium. The haplotype CCCAGT/A/C shows association to T2D in our cohort, while the allele 167Arg, the haplotypes CCCAGT/A and A/C and the diplotype LL/GA/TC show a trend towards association to disease. The 5'UTR and frameshift variants are absent in the controls. Nonparametric linkage analysis within NEUROG3 variants in 9 early-onset T2D families shows a nonparametric LOD score=2.49 (P=0.006). CONCLUSIONS: The biological impact of NEUROG3 might be due to the presence of either CCCAGT at 44-45nt, 167Arg, 199Ser or by a haplotype combination of these 3 or 2 of them.
Insulinoma associated 1 (Insm1) plays an important role in regulating the development of cells in the central and peripheral nervous systems, olfactory epithelium and endocrine pancreas. To better define the role of Insm1 in pancreatic endocrine cell development we generated mice with an Insm1(GFPCr
e) reporter allele and used them to study Insm1-expressing and null populations. Endocrine progenitor cells lacking Insm1 were less differentiated and exhibited broad defects in hormone production, cell proliferation and cell migration. Embryos lacking Insm1 contained greater amounts of a non-coding Neurog3 mRNA splice variant and had fewer Neurog3/Insm1 co-expressing progenitor cells, suggesting that Insm1 positively regulates Neurog3. Moreover, endocrine progenitor cells that express either high or low levels of Pdx1, and thus may be biased towards the formation of specific cell lineages, exhibited cell type-specific differences in the genes regulated by Insm1. Analysis of the function of Ripply3, an Insm1-regulated gene enriched in the Pdx1-high cell population, revealed that it negatively regulates the proliferation of early endocrine cells. Taken together, these findings indicate that in developing pancreatic endocrine cells Insm1 promotes the transition from a ductal progenitor to a committed endocrine cell by repressing a progenitor cell program and activating genes essential for RNA splicing, cell migration, controlled cellular proliferation, vasculogenesis, extracellular matrix and hormone secretion.
The current model for endocrine cell specification in the pancreas invokes high-level production of the transcription factor Neurogenin 3 (Neurog3) in Sox9(+) bipotent epithelial cells as the trigger for endocrine commitment, cell cycle exit, and rapid delaminat
ion toward proto-islet clusters. This model posits a transient Neurog3 expression state and short epithelial residence period. We show, however, that a Neurog3(TA.LO) cell population, defined as Neurog3 transcriptionally active and Sox9(+) and often containing nonimmunodetectable Neurog3 protein, has a relatively high mitotic index and prolonged epithelial residency. We propose that this endocrine-biased mitotic progenitor state is functionally separated from a pro-ductal pool and endows them with long-term capacity to make endocrine fate-directed progeny. A novel BAC transgenic Neurog3 reporter detected two types of mitotic behavior in Sox9(+) Neurog3(TA.LO) progenitors, associated with progenitor pool maintenance or derivation of endocrine-committed Neurog3(HI) cells, respectively. Moreover, limiting Neurog3 expression dramatically increased the proportional representation of Sox9(+) Neurog3(TA.LO) progenitors, with a doubling of its mitotic index relative to normal Neurog3 expression, suggesting that low Neurog3 expression is a defining feature of this cycling endocrine-biased state. We propose that Sox9(+) Neurog3(TA.LO) endocrine-biased progenitors feed production of Neurog3(HI) endocrine-committed cells during pancreas organogenesis.
