👤 Christophe Philippe

Bardet-Biedl syndrome (BBS) is a multisystemic disorder characterized by rod-cone dystrophy, truncal obesity, postaxial polydactyly, cognitive impairment, male hypogonadotropic hypogonadism, complex female genitourinary malformations, and renal abnormalities. There is a large clinical and also genetic heterogeneity in BBS. Here, we report a patient with polydactyly, hyperechogenic kidneys increased in size with normal corticomedullary differentiation, anal imperforation, and malformation of genitals with presence of a genital tubercle with ventral urethral meatus associated with two unfused lateral genital swelling and absent urethral folds, in the context of 46, XY karyotype. Karyotype and solo exome sequencing were performed to look for a genetic etiology for the features described in our patient. We identified a homozygous in-frame deletion of exons 4 to 6 in the BBS4 gene (NM-033028 (BBS4-i001): c.[(157-?)₍₄₀₅ +?)del] p.(Ala53-Trp135del), which is classified as pathogenic variant. This analysis allowed the molecular diagnosis of BBS type 4 in this patient. Complex genital malformations are only reported in female BBS6 patients yet, and genital abnormalities and anal imperforation are not reported in male BBS4 patients to date. We discuss the possible hypotheses for this phenotype, including the phenotypic overlap between ciliopathies. Show less

15q24 microdeletion and microduplication syndromes are genetic disorders caused by non-allelic homologous recombination between low-copy repeats (LCRs) in the 15q24 chromosome region. Individuals with 15q24 microdeletion and microduplication syndromes share a common 1.2 Mb critical interval, spanning from LCR15q24B to LCR15q24C. Patients with 15q24 microdeletion syndrome exhibit distinct dysmorphic features, microcephaly, variable developmental delay, multiples congenital anomalies while individuals with reciprocal 15q24 microduplication syndrome show mild developmental delay, facial dysmorphism associated with skeletal and genital abnormalities. We report the first case of a 10 year-old girl presenting mild developmental delay, psychomotor retardation, epilepsy, ventricular arrhythmia, overweight and idiopathic central precocious puberty. 180K array-CGH analysis identified a 1.38 Mb heterozygous interstitial 15q24.1 BP4-BP1 microdeletion including HCN4 combined with a concomitant 2.6 Mb heterozygous distal 15q24.2q24.3 microduplication. FISH analysis showed that both deletion and duplication occurred de novo in the proband. Of note, both copy number imbalances did not involve the 1.2 Mb minimal deletion/duplication critical interval of the 15q24.1q24.2 chromosome region (74.3-75.5 Mb). Sequencing of candidate genes for epilepsy and obesity showed that the proband was hemizygous for paternal A-at risk allele of BBS4 rs7178130 and NPTN rs7171755 predisposing to obesity, epilepsy and intellectual deficits. Our study highlights the complex interaction of functional polymorphisms and/or genetic variants leading to variable clinical manifestations in patients with submicroscopic chromosomal aberrations. Show less

The Forkhead box A transcription factors are major regulators of glucose homeostasis. They show both distinct and redundant roles during pancreas development and in adult mouse β-cells. In vivo ablation studies have revealed critical implications of Foxa1 on glucagon biosynthesis and requirement of Foxa2 in α-cell terminal differentiation. In order to examine the respective role of these factors in mature α-cells, we used small interfering RNA (siRNA) directed against Foxa1 and Foxa2 in rat primary pancreatic α-cells and rodent α-cell lines leading to marked decreases in Foxa1 and Foxa2 mRNA levels and proteins. Both Foxa1 and Foxa2 control glucagon gene expression specifically through the G2 element. Although we found that Foxa2 controls the expression of the glucagon, MafB, Pou3f4, Pcsk2, Nkx2.2, Kir6.2, and Sur1 genes, Foxa1 only regulates glucagon gene expression. Interestingly, the Isl1 and Gipr genes were not controlled by either Foxa1 or Foxa2 alone but by their combination. Foxa1 and Foxa2 directly activate and bind the promoter region the Nkx2.2, Kir6.2 and Sur1, Gipr, Isl1, and Pou3f4 genes. We also demonstrated that glucagon secretion is affected by the combined effects of Foxa1 and Foxa2 but not by either one alone. Our results indicate that Foxa1 and Foxa2 control glucagon biosynthesis and secretion as well as α-cell differentiation with both common and unique target genes. Show less

The Pax6 transcription factor is crucial for pancreatic α-cells. Indeed, Pax6-deficient mouse models are characterized by markedly altered α-cell differentiation. Our objective was to investigate the role of Pax6 in glucagon secretion process. We used a Pax6-deficient model in rat primary enriched-α cells with specific small interfering RNA leading to a 70% knockdown of Pax6 expression. We first showed that Pax6 knockdown decreases glucagon biosynthesis as well as glucagon release. Through physiological assays, we demonstrated that the decrease of Pax6 affects specifically acute glucagon secretion in primary α-cell in response to glucose, palmitate, and glucose-dependent insulinotropic peptide (GIP) but not the response to arginine and epinephrine. We identified in Pax6 knockdown model that genes involved in glucagon secretion such as the glucokinase (GCK), G protein-coupled receptor (GPR40), and GIP receptor (GIPR) as well as the corresponding proteins were significantly decreased whereas the insulin receptor (IR) Kir6.2/Sur1, and glucose transporter 1 genes were not affected. We demonstrated that Pax6 directly binds and activates specific elements on the promoter region of the GPR40, GCK, and GIPR genes. Finally, through site-directed mutagenesis experiments, we showed that disruption of Pax6 binding on the GCK, GPR40, and GIPR gene promoters led to specific decreases of their activities in the αTC1.9 glucagon-producing cell line. Hence our results indicate that Pax6 acts on the regulation of glucagon secretion at least through the transcriptional control of GCK, GPR40, and GIPR. We propose that Pax6 is not only critical for glucagon biosynthesis but also for glucagon secretion particularly in response to nutrients. Show less

The Pax6 transcription factor is crucial for endocrine cell differentiation and function. Indeed, mutations of Pax6 are associated with a diabetic phenotype and a drastic decrease of insulin-positive cell number. Our aim was to better define the β-cell Pax6 transcriptional network and thus provide further information concerning the role of Pax6 in β-cell function. We developed a Pax6-deficient model in rat primary β-cells with specific small interfering RNA leading to a 75% knockdown of Pax6 expression. Through candidate gene approach, we confirmed that Pax6 controls the mRNA levels of the insulin 1 and 2, Pdx1, MafA, GLUT2, and PC1/3 genes in β-cells. Importantly, we identified new Pax6 target genes coding for GK, Nkx6.1, cMaf, PC2, GLP-1R and GIPR which are all involved in β-cell function. Furthermore, we demonstrated that Pax6 directly binds and activates specific elements on the promoter region of these genes. We also demonstrated that Pax6 knockdown led to decreases in insulin cell content, in insulin processing, and a specific defect of glucose-induced insulin secretion as well as a significant reduction of GLP-1 action in primary β-cells. Our results strongly suggest that Pax6 is crucial for β-cells through transcriptional control of key genes coding for proteins that are involved in insulin biosynthesis and secretion as well as glucose and incretin actions on β-cells. We provide further evidence that Pax6 represents a key element of mature β-cell function. Show less

During vertebrate oogenesis and early embryogenesis, gene expression is governed mainly by translational control. The recruitment of Poly(A) Binding Protein (PABP) during poly(A) tail lengthening appears to be the key to translational activation during this period of development in Xenopus laevis. We showed that PABP1 and ePABP proteins are both present during oogenesis and early development. We selected ePABP as an eRF3 binding protein in a two-hybrid screening of a X. laevis cDNA library and demonstrated that this protein is associated with translational complexes. It can complement essential functions of the yeast homologue Pab1p. We discuss specific expression patterns of the finely tuned PABP1 and ePABP proteins. Show less

Hereditary multiple exostoses (HME), the most frequent of all skeletal dysplasias, is an autosomal dominant disorder characterized by the presence of multiple exostoses localized mainly at the end of long bones. HME is genetically heterogeneous, with at least three loci, on 8q24.1 (EXT1), 11p11-p13 (EXT2), and 19p (EXT3). Both the EXT1 and EXT2 genes have been cloned recently and define a new family of potential tumor suppressor genes. This is the first study in which mutation screening has been performed for both the EXT1 and EXT2 genes prior to any linkage analysis. We have screened 17 probands with the HME phenotype, for alterations in all translated exons and flanking intronic sequences, in the EXT1 and EXT2 genes, by conformation-sensitive gel electrophoresis. We found the disease-causing mutation in 12 families (70%), 7 (41%) of which have EXT1 mutations and 5 (29%) EXT2 mutations. Together with the previously described 1-bp deletion in exon 6, which is present in 2 of our families, we report five new mutations in EXT1. Two are missense mutations in exon 2 (G339D and R340C), and the other three alterations (a nonsense mutation, a frameshift, and a splicing mutation) are likely to result in truncated nonfunctional proteins. Four new mutations are described in EXT2. A missense mutation (D227N) was found in 2 different families; the other three alterations (two nonsense mutations and one frameshift mutation) lead directly or indirectly to premature stop codons. The missense mutations in EXT1 and EXT2 may pinpoint crucial domains in both proteins and therefore give clues for the understanding of the pathophysiology of this skeletal disorder. Show less