C620R mutation of the murine ret proto-oncogene: loss of function effect in homozygotes and possible gain of function effect in heterozygotes

Kidney malformations and Hirschsprung's disease in carriers of cysteine mutations in exon 10 of the RET proto-oncogene

PURPOSE

This investigation aimed at characterizing penetrance and interrelation of the loss-of-function phenotypes, Hirschsprung's disease and kidney malformations, in carriers of exon 10 REarranged during Transfection (RET) mutations.

METHODS

Comparative analysis of penetrance of and interdependencies between Hirschsprung's disease and kidney malformations among carriers of mutations in RET codons 609, 611, 618, and 620.

RESULTS

Hirschsprung's disease and kidney malformations (kidney agenesis, ureteropelvicalyceal dilatation, or polycystic kidney disease) each affected 3.6% (4 patients) of 112 carriers of RET mutations in exon 10. This percentage increased to 13% (4 patients) and 9% (3 patients) of 32 p.Cys620 carriers and 22% (4 patients) and 17% (3 patients) of 18 p.Cys620Arg carriers, respectively. Overall, Hirschsprung's disease (17%, 4 of 24 carriers; P = 0.002) and kidney malformations (13%, 3 of 24 carriers; P = 0.030) were associated with arginine-for-cysteine substitutions. Two of the six patients, originating from different p.Cys620Arg families, harbored both Hirschsprung's disease and kidney malformations.

CONCLUSION

Hirschsprung's disease and kidney malformations are the more penetrant, the closer the cysteine mutations are located to the transmembrane domain (codon 636-657) of the RET kinase receptor.

Increased RET Activity Coupled with a Reduction in the RET Gene Dosage Causes Intestinal Aganglionosis in Mice

Mutations of the gene encoding the RET tyrosine kinase causes Hirschsprung's disease (HSCR) and medullary thyroid carcinoma (MTC). Current consensus holds that HSCR and MTC are induced by inactivating and activating RET mutations, respectively. However, it remains unknown whether activating mutations in the RET gene have adverse effects on ENS development in vivo We addressed this issue by examining mice engineered to express RET51(C618F), an activating mutation identified in MTC patients. Although Ret^{51(C618F)/51(C618F)} mice displayed hyperganglionosis of the ENS, Ret^51(C618F)/- mice exhibited severe intestinal aganglionosis because of premature neuronal differentiation. Reduced levels of glial cell-derived neurotrophic factor (GDNF), a RET-activating neurotrophic factor, ameliorated the ENS phenotype of Ret^51(C618F)/- mice, demonstrating that GDNF-mediated activation of RET51(C618F) is responsible for severe aganglionic phenotype. The RET51(C618F) allele showed genetic interaction with Ednrb gene, one of modifier genes for HSCR. These data reveal that proliferation and differentiation of ENS precursors are exquisitely controlled by both the activation levels and total dose of RET. Increased RET activity coupled with a decreased gene dosage can cause intestinal aganglionosis, a finding that provides novel insight into HSCR pathogenesis.

Mice conditionally expressing RET(C618F) mutation display C cell hyperplasia and hyperganglionosis of the enteric nervous system

Medullary thyroid carcinoma (MTC) develops from hyperplasia of thyroid C cells and represents one of the major causes of thyroid cancer mortality. Mutations in the cysteine-rich domain (CRD) of the RET gene are the most prevalent genetic cause of MTC. The current consensus holds that such cysteine mutations cause ligand-independent dimerization and constitutive activation of RET. However, given the number of the CRD mutations left uncharacterized, our understanding of the pathogenetic mechanisms by which CRD mutations lead to MTC remains incomplete. We report here that RET(C618F), a mutation identified in MTC patients, displays moderately high basal activity and requires the ligand for its full activation. To assess the biological significance of RET(C618F) in organogenesis, we generated a knock-in mouse line conditionally expressing RET(C618F) cDNA by the Ret promoter. The RET(C618F) allele can be made to be Ret-null and express mCherry by Cre-loxP recombination, which allows the assessment of the biological influence of RET(C618F) in vivo. Mice expressing RET(C618F) display mild C cell hyperplasia and increased numbers of enteric neurons, indicating that RET(C618F) confers gain-of-function phenotypes. This mouse line serves as a novel biological platform for investigating pathogenetic mechanisms involved in MTC and enteric hyperganglionosis.

Mutations in RAD21 disrupt regulation of APOB in patients with chronic intestinal pseudo-obstruction

BACKGROUND & AIMS

Chronic intestinal pseudo-obstruction (CIPO) is characterized by severe intestinal dysmotility that mimics a mechanical subocclusion with no evidence of gut obstruction. We searched for genetic variants associated with CIPO to increase our understanding of its pathogenesis and to identify potential biomarkers.

METHODS

We performed whole-exome sequencing of genomic DNA from patients with familial CIPO syndrome. Blood and lymphoblastoid cells were collected from patients and controls (individuals without CIPO); levels of messenger RNA (mRNA) and proteins were analyzed by quantitative reverse-transcription polymerase chain reaction, immunoblot, and mobility shift assays. Complementary DNAs were transfected into HEK293 cells. Expression of rad21 was suppressed in zebrafish embryos using a splice-blocking morpholino (rad21a). Gut tissues were collected and analyzed.

RESULTS

We identified a homozygous mutation (p.622, encodes Ala>Thr) in RAD21 in patients from a consanguineous family with CIPO. Expression of RUNX1, a target of RAD21, was reduced in cells from patients with CIPO compared with controls. In zebrafish, suppression of rad21a reduced expression of runx1; this phenotype was corrected by injection of human RAD21 mRNA, but not with the mRNA from the mutated p.622 allele. rad21a Morpholino zebrafish had delayed intestinal transit and greatly reduced numbers of enteric neurons, similar to patients with CIPO. This defect was greater in zebrafish with suppressed expression of ret and rad21, indicating their interaction in the regulation of gut neurogenesis. The promoter region of APOB bound RAD21 but not RAD21 p.622 Ala>Thr; expression of wild-type RAD21 in HEK293 cells repressed expression of APOB, compared with control vector. The gut-specific isoform of APOB (APOB48) is overexpressed in sera from patients with CIPO who carry the RAD21 mutation. APOB48 also is overexpressed in sporadic CIPO in sera and gut biopsy specimens.

CONCLUSIONS

Some patients with CIPO carry mutations in RAD21 that disrupt the ability of its product to regulate genes such as RUNX1 and APOB. Reduced expression of rad21 in zebrafish, and dysregulation of these target genes, disrupts intestinal transit and the development of enteric neurons.

Thyroid C-Cell Biology and Oncogenic Transformation

The thyroid parafollicular cell, or commonly named "C-cell," functions in serum calcium homeostasis. Elevations in serum calcium trigger release of calcitonin from the C-cell, which in turn functions to inhibit absorption of calcium by the intestine, resorption of bone by the osteoclast, and reabsorption of calcium by renal tubular cells. Oncogenic transformation of the thyroid C-cell is thought to progress through a hyperplastic process prior to malignancy with increasing levels of serum calcitonin serving as a biomarker for tumor burden. The discovery that multiple endocrine neoplasia type 2 is caused by activating mutations of the RET gene serves to highlight the RET-RAS-MAPK signaling pathway in both initiation and progression of medullary thyroid carcinoma (MTC). Thyroid C-cells are known to express RET at high levels relative to most cell types; therefore, aberrant activation of this receptor is targeted primarily to the C-cell, providing one possible cause of tissue-specific oncogenesis. The role of RET signaling in normal C-cell function is unknown though calcitonin gene transcription appears to be sensitive to RET activation. Beyond RET, the modeling of oncogenesis in animals and screening of human tumors for candidate gene mutations have uncovered mutation of RAS family members and inactivation of Rb1 regulatory pathway as potential mediators of C-cell transformation. A growing understanding of how RET interacts with these pathways, both in normal C-cell function and during oncogenic transformation, will help in the development of novel molecular-targeted therapies.

Apolipoprotein B is a new target of the GDNF/RET and ET-3/EDNRB signalling pathways

BACKGROUND

GDNF/RET and Endothelin-3 (ET-3)/EDNRB regulate survival, differentiation, migration, and proliferation of neural crest-derived cells. Although several RET and EDNRB signalling mediators have been characterized, most of the genes targeted by these two pathways are still largely unknown. We focused our study on apolipoprotein B (APOB) as a novel target gene of the RET and EDNRB pathways, based on previous data obtained using a Caenorhabditis elegans strain mutant for the homologue of mammalian ECE1.

METHODS

Molecular and cellular studies of Apob were performed in the murine Neuro2a cells, an in vitro model for studying neural crest-derived cell development, along with a mouse knock-in for the Hirschsprung-associated mutation Ret(C620R). Silencing for Apob and Ret has been performed via shRNA.

KEY RESULTS

GDNF/RET and ET-3/EDNRB cooperated in inducing neuronal differentiation resulting in Apob activation in Neuro2a cell line. Apob expression was downregulated in mouse embryos homozygous for the Ret(C620R) mutation and presenting a severe Hirschsprung phenotype. Ret silencing prevented Apob expression increase. MAPK P38 kinase activation evoked Apob expression via GDNF/RET signalling in Neuro2a cells. A p53-dependent repressor element in Apob promoter resulted in a reduced Apob expression. Silencing of Apob reduced HuD protein expression.

CONCLUSIONS & INFERENCES

Apob is a novel downstream target of the RET/EDNRB pathways with a role in neuronal survival and maintenance, as indicated by its effect on HuD expression. Our data provide a conceptual framework to investigate and establish the role of APOB gene in severe gut dysmotility.

Sprouty1 is a candidate tumor-suppressor gene in medullary thyroid carcinoma

Medullary thyroid carcinoma (MTC) is a malignancy derived from the calcitonin-producing C-cells of the thyroid gland. Oncogenic mutations of the Ret proto-oncogene are found in all heritable forms of MTC and roughly one half of the sporadic cases. However, several lines of evidence argue for the existence of additional genetic lesions necessary for the development of MTC. Sprouty (Spry) family of genes is composed of four members in mammals (Spry1-4). Some Spry family members have been proposed as candidate tumor-suppressor genes in a variety of cancerous pathologies. In this work, we show that targeted deletion of Spry1 causes C-cell hyperplasia, a precancerous lesion preceding MTC, in young adult mice. Expression of Spry1 restrains proliferation of the MTC-derived cell line, TT. Finally, we found that the Spry1 promoter is frequently methylated in MTC and that Spry1 expression is consequently decreased. These findings identify Spry1 as a candidate tumor-suppressor gene in MTC.

Mouse models for inherited endocrine and metabolic disorders

In vivo models represent important resources for investigating the physiological mechanisms underlying endocrine and metabolic disorders, and for pre-clinical translational studies that may include the assessments of new treatments. In the study of endocrine diseases, which affect multiple organs, in vivo models provide specific advantages over in vitro models, which are limited to investigation of isolated systems. In recent years, the mouse has become the popular choice for developing such in vivo mammalian models, as it has a genome that shares ∼85% identity to that of man, and has many physiological systems that are similar to those in man. Moreover, methods have been developed to alter the expression of genes in the mouse, thereby generating models for human diseases, which may be due to loss- or gain-of-function mutations. The methods used to generate mutations in the mouse genome include: chemical mutagenesis; conventional, conditional and inducible knockout models; knockin models and transgenic models, and these strategies are often complementary. This review describes some of the different strategies that are utilised for generating mouse models. In addition, some mouse models that have been successfully generated by these methods for some human hereditary endocrine and metabolic disorders are reviewed. In particular, the mouse models generated for parathyroid disorders, which include: the multiple endocrine neoplasias; hyperparathyroidism-jaw tumour syndrome; disorders of the calcium-sensing receptor and forms of inherited hypoparathyroidism are discussed. The advances that have been made in our understanding of the mechanisms of these human diseases by investigations of these mouse models are described.

Hirschsprung disease and congenital anomalies of the kidney and urinary tract (CAKUT): a novel syndromic association

Congenital anomalies of the kidney and urinary tract (CAKUT) can be associated with Hirschsprung disease (HSCR). Based on the common genetic background of enteric nervous system and kidney development, the reported association of CAKUT and HSCR seems underestimated. Therefore, we designed a prospective study aimed at determining the prevalence of CAKUT in HSCR patients and at identifying RET, glial cell line-derived neurotrophic factor (GDNF), and GDNF family receptor alpha1 (GFRalpha1) mutations or haplotypes associated with this subset of HSCR patients. Eighty-four HSCR patients consecutively admitted to our department between July 2006 and July 2007 underwent interviews, notes review, ultrasound screening (further investigation according to detected anomaly), urinalysis, and DNA extraction for molecular genetics study. Another 27 patients with isolated CAKUT were included as a control group for the molecular genetics study. Twenty-one patients (25%) with HSCR had associated CAKUT, with hydronephrosis and hypoplasia being the most frequent diagnoses. Nine of 21 CAKUT were symptomatic. Six additional patients had other non-CAKUT anomalies (for example, stones, Barter syndrome) that were excluded from association and molecular genetics analysis to avoid bias of inclusion criteria. RET mutations were found in 5 patients (4 HSCR, 1 HSCR + CAKUT, 0 CAKUT) and GDNF mutations in 3 (2 HSCR, 1 CAKUT, 0 HSCR + CAKUT). No GFRalpha1 mutations were found. Finally, the HSCR-predisposing T haplotype of RET proto-oncogene was found in 64% of HSCR, 50% of HSCR + CAKUT, and in 24% of CAKUT patients. The incidence of CAKUT in HSCR patients is 4- to 6-fold higher than expected. Therefore, a patient with HSCR has a 3- to 18-fold higher risk of developing a CAKUT, particularly hydronephrosis or hypoplasia. If we consider that the proportion of predisposing haplotype in HSCR + CAKUT patients resembles that of other syndromic HSCR, we can conclude that HSCR + CAKUT has to be considered a novel syndromic association. These results need to be confirmed in a larger series. At present, we strongly suggest considering ultrasound screening of the urinary tract in every patient with a diagnosis of HSCR.

Renal aplasia in humans is associated with RET mutations

In animal models, kidney formation is known to be controlled by the proteins RET, GDNF, and GFRA1; however, no human studies to date have shown an association between abnormal kidney development and mutation of these genes. We hypothesized that stillborn fetuses with congenital renal agenesis or severe dysplasia would possess mutations in RET, GDNF, or GFRA1. We assayed for mutations in these genes in 33 stillborn fetuses that had bilateral or unilateral renal agenesis (29 subjects) or severe congenital renal dysplasia (4 subjects). Mutations in RET were found in 7 of 19 fetuses with bilateral renal agenesis (37%) and 2 of 10 fetuses (20%) with unilateral agenesis. In two fetuses, there were two different RET mutations found, and a total of ten different sequence variations were identified. We also investigated whether these mutations affected RET activation; in each case, RET phosphorylation was either absent or constitutively activated. A GNDF mutation was identified in only one fetus with unilateral agenesis; this subject also had two RET mutations. No GFRA1 mutations were seen in any fetuses. These data suggest that in humans, mutations in RET and GDNF may contribute significantly to abnormal kidney development.