Shirley M. Myers

A Novel RET Kinase–β-Catenin Signaling Pathway Contributes to Tumorigenesis in Thyroid Carcinoma

The RET receptor tyrosine kinase has essential roles in cell survival, differentiation, and proliferation. Oncogenic activation of RET causes the cancer syndrome multiple endocrine neoplasia type 2 (MEN 2) and is a frequent event in sporadic thyroid carcinomas. However, the molecular mechanisms underlying RETs potent transforming and mitogenic signals are still not clear. Here, we show that nuclear localization of beta-catenin is frequent in both thyroid tumors and their metastases from MEN 2 patients, suggesting a novel mechanism of RET-mediated function through the beta-catenin signaling pathway. We show that RET binds to, and tyrosine phosphorylates, beta-catenin and show that the interaction between RET and beta-catenin can be direct and independent of cytoplasmic kinases, such as SRC. As a result of RET-mediated tyrosine phosphorylation, beta-catenin escapes cytosolic down-regulation by the adenomatous polyposis coli/Axin/glycogen synthase kinase-3 complex and accumulates in the nucleus, where it can stimulate beta-catenin-specific transcriptional programs in a RET-dependent fashion. We show that down-regulation of beta-catenin activity decreases RET-mediated cell proliferation, colony formation, and tumor growth in nude mice. Together, our data show that a beta-catenin-RET kinase pathway is a critical contributor to the development and metastasis of human thyroid carcinoma.

Investigation of germline GFRα-1 mutations in Hirschsprung disease

Inactivating mutations of the RET proto-oncogene and of one of its soluble ligand molecules, glial cell line derived neurotrophic factor (GDNF), have been found in a subset of patients with Hirschsprung disease (HSCR). However, the majority of HSCR mutations remain unidentified. As normal RET function requires a multicomponent ligand complex for activation, other members of the RET ligand complex are primary candidates for these mutations. We investigated the presence of mutations in another member of the RET signalling complex, GDNF family receptor alpha-1 (GFRα-1), in a panel of 269 independent cases of HSCR. We identified 10 polymorphisms at the GFRα-1 locus. Surprisingly, however, we did not identify any sequence variants in our HSCR population that were not also present in a normal control population. Our data suggest that mutations of the GFRα-1 gene are not a common aetiological event in HSCR.

Expression of RET 3' splicing variants during human kidney development

The mature mammalian kidney arises through a series of reciprocal inductive interactions between two different cell groups, the ureteric bud epithelium and the metanephric mesenchyme. The RET receptor tyrosine kinase is required for induction and development of the metanephric kidney. Differential splicing at the 3′ end of RET results in transcripts encoding three isoforms that differ with respect to their C-terminal 9 (RET9), 51 (RET51) or 43 (RET43) amino acids. In vitro assays have identified differences in the abilities of the RET9 and RET51 isoforms to induce differentiation suggesting functional differences between these proteins. We examined the relative expression levels of the three RET 3′ splicing variants in developing human kidney using semi-quantitative RT–PCR. We observed consistent expression of the RET9 and RET43 variants in kidney samples spanning 7.5 through 24 weeks gestation. At early gestational ages (7.5–8.5 weeks), RET51 expression was very low (±5%) compared to RET9; however, a rapid seven fold increase in expression was detected by 9 weeks. Our data suggest that RET51 may contribute to differentiation-related events occurring after 8.5 weeks gestation rather than to induction of the human kidney.

The RET Receptor Is Linked to Stress Response Pathways

RET is a transmembrane receptor required for the development of neuroendocrine and urogenital cell types. Activation of RET has roles in cell growth, migration, or differentiation, yet little is known about the gene expression patterns through which these processes are mediated. We have generated cell lines stably expressing either the RET9 or RET51 protein isoforms and have used these to investigate RET-mediated gene expression patterns by cDNA microarray analyses. As seen for many oncogenes, we identified altered expression of genes associated generally with cell–cell or cell-substrate interactions and up-regulation of tumor-specific transcripts. We also saw increased expression of transcripts normally associated with neural crest or other RET-expressing cell types, suggesting these genes may lie downstream of RET activation in development. The most striking pattern of expression was up-regulation of stress response genes. We showed that RET expression significantly up-regulated the genes for heat shock protein (HSP) 70 family members, HSPA1A, HSPA1B, and HSPA1L. Other members of several HSP families and HSP70-interacting molecules that were associated with stress response protein complexes involved in protein maturation were also specifically up-regulated by RET, whereas those associated with the roles of HSP70 in protein degradation were down-regulated or unaffected. The major mechanism of stress response induction is activation of the heat shock transcription factor HSF1. We showed that RET expression leads to increased HSF1 activation, which correlates with increased expression of stress response genes. Together, our data suggest that RET may be directly responsible for expression of stress response proteins and the initiation of stress response.

RET-mediated gene expression pattern is affected by isoform but not oncogenic mutation

The inherited cancer syndrome multiple endocrine neoplasia type 2 (MEN 2) is caused by mutations of the RET receptor tyrosine kinase and is characterized by medullary thyroid carcinoma. MEN 2 subtypes have distinct mutational spectrums and vary in severity. The most severe disease subtype, MEN 2B, is associated with a specific RET mutation (M918T) that has been predicted to alter downstream signaling and target gene expression patterns. We used gene expression microarray analysis to identify target genes modulated by RET. We compared two oncogenic RET mutants, associated with MEN 2A (2ARET) or MEN 2B (2BRET) disease subtypes, that are predicted to have distinct downstream target genes. We showed that overall, 2ARET and 2BRET modulated genes with similar functional ontologies. Further, when we validated our microarray data by quantitative real time PCR, we did not detect major differences in gene expression associated with these mutants when differences in receptor activity levels were considered. We did, however, detect differences in gene expression induced by two RET COOH‐terminal isoforms, RET9 and RET51, irrespective of the RET form present (wildtype, 2ARET, or 2BRET). Our data suggest that similar transcriptional programs contribute to all forms of MEN 2 but that differences in target gene expression may contribute to developmental pattern differences observed between RET isoforms.

Characterisation of the human GFRalpha-3 locus and investigation of the gene in Hirschsprung disease.

BACKGROUND The GDNF family receptor alpha (GFRα) proteins are extracellular cell surface bound molecules that act as adapters in binding of the GDNF family of soluble neurotrophic factors to the RET receptor. These molecules are essential for development of many neural crest derived cell types and the kidney. Mutations in RET and in two members of the GDNF ligand family are associated with Hirschsprung disease (HSCR), a congenital absence of the enteric ganglia. Members of the GFRα family are also candidates for HSCR mutations. One such gene isGFRα-3, which is expressed in the peripheral nervous system and developing nerves. OBJECTIVE We have characterised the structure of the human GFRα-3 locus and investigated the gene for sequence variants in a panel of HSCR patients. METHODS Long range PCR or subcloning of PAC clones was used to investigateGFRα-3 intron-exon boundaries. A combination of single strand conformation polymorphism (SSCP) analysis and direct sequencing was used to investigateGFRα-3 sequence variants. RESULTS GFRα-3spans eight coding exons and has a gene structure and organisation similar to that of GFRα-1. We identified three polymorphic variants in GFRα-3 in a normal control population, a subset of which also occurred in HSCR patients. We did not detect any sequence variants within the coding sequence of GFRα-3. We found a base substitution in the 5′ UTR of GFRα-3, 15 base pairs upstream of the translation start site. A second substitution was identified in intron 4 (IVS4-30G>A) between the splice branch site and the splice acceptor site. The final variant was a 2 base pair insertion within the splice donor consensus sequence of exon 7 (IVS7+4ins GG). CONCLUSIONS We did not detect any correlation between variants of GFRα-3 and the HSCR phenotype. Our data suggest that mutations of this gene are not a cause of HSCR.

The β subunit locus of the human fibronectin receptor: DNA restriction fragment length polymorphism and linkage mapping studies

SummaryThe beta subunit of the human fibronectin receptor (FNRB) is a transmembrane protein belonging to the VLA (very late antigens of activation) family. Using pGEM-32, a 2.5-kb partial cDNA clone corresponding to the 3′ portion of the human FNRB locus, multiple restriction fragment length polymorphisms (RFLPs) were revealed on DNAs from unrelated Caucasians. RFLPs detected by five enzymes, BanII, HinfI, KpnI, BglII, and SacI, are of the simple two-allele form, and pairwise linkage analyses of these RFLPs with numerous known DNA markers from the chromosome-10 pericentromeric region not only confirmed the chromosome-10 assignment of the functional FNRB gene but also supported its localization at p11.2 suggested by in situ hybridization. An infrequent MspI RFLP was detected by pB/R2, a 4.6-kb genomic clone from the FNRB locus. Another type of DNA polymorphism was also revealed by the cDNA clone and it was visualized on the Southern blot analyses as the presence or absence of an extra band (or a set of extra bands). It seems to stem from a stretch of DNA sequence present in some individuals at one single locus but absent in others, and is of non-chromosome-10 origin based on linkage analyses with known chromosome 10 markers. This “presence/absence” type of polymorphism could be revealed by all of the 25 restriction enzymes tested and is similar in nature to that previously reported with one of the human dihydrofolate reductase pseudogenes, DHFRP1. Dissection of the pGEM-32 clone demonstrated that the region revealing the non-chromosome-10 sequences is within a fragment about 1.7 kb in length extending from about 600 nucleotides preceding the stop codon down to the end of the cloned FNRB 3′ untranslated region. Due to its high polymorphism information content (PIC) value (0.71 for haplotypes of BanII, HinfI, and KpnI RFLPs) and proximity to the centromere, FNRB will prove to be a highly useful marker for genetic linkage studies of multiple endocrine neoplasia type 2A (MEN2A) as well as for chromosome-10 linkage studies in general.

A linkage group of five DNA markers on human chromosome 10.

Five chromosome 10 DNA markers (D10S1, D10S3, D10S4, D10S5, and RBP3) were typed in five large pedigrees with multiple endocrine neoplasia type 2A (MEN-2A) and in five non-MEN-2A pedigrees. Linkage analyses showed that these loci and the locus for MEN-2A (MEN2A) are in one linkage group spanning at least 70 cM. The order of the marker loci is RBP3-D10S5-D10S3-D10S1-D10S4, with interlocus recombination frequencies of 7, 13-19, 19, and 19%, respectively, all on the same side of MEN2A. Analyses of sex-specific recombination frequencies indicated no significant differences between males and females for any of the map intervals studied. Previous localization of D10S5 and RBP3 to the proximal region of the long arm and the pericentric region, respectively, comparison of results with other studies, and our preliminary results with other chromosome 10 markers suggest that the D10S4 end of the map extends into the long arm. Our linkage map has been constructed using only two- and three-locus analyses. It will be possible to combine our results with those of other groups to construct a more detailed and accurate genetic map of chromosome 10.

Characterization of a Distinctive Motif of the Low Molecular Weight Neurotrophin Receptor that Modulates NGF-mediated Neurite Growth

The cytoplasmic region of the common neurotrophin receptor (p75NGFR) (rat, human, chick) contains a putative membrane‐associating domain implicated in intracellular signalling. A peptide (R3) identical to this domain (p75NGFR 367–379) and various analogues of this peptide displayed circular dichroism spectra in aqueous and non‐polar environments identical to the amphiphilic tetradecapeptide mastoparan (MP) and were internalized by PC12 rat pheochromocytoma cells. The R3 peptide enhanced neurite growth in PC12 cells, embryo chick primary sensory neurons and fetal rat primary sensory neurons in vitro in the presence of sub‐saturating concentrations of NGF. Peptide analogues of R3 not faithful to the distance and angular relationships of ionic groups and the putative amphiphilic structure of p75NGFR 367–379 displayed reduced potency to enhance NGF‐mediated neurite growth. Exposure of NGF and the R3 peptide to a cell line displaying predominantly p75NGFR (PC12nnr5), had no influence on neurite growth. The R3 peptide had no effects on cell survival, cell binding or uptake of [125I]NGF, affinity cross‐linking of [125I]NGF to p75NGFR or trkA monomers and homodimers, or NGF‐mediated trkA monomer tyrosine phosphorylation. The studies implicate a role for a highly conserved motif of p75NGFR in the downstream modulation of NGF‐mediated neurite growth.

A Refined Linkage Map for DNA Markers around the Pericentromeric Region of Chromosome 10

A refined genetic linkage map for the pericentromeric region of human chromosome 10 has been constructed from data on 12 distinct polymorphic DNA loci as well as the locus for multiple endocrine neoplasia type 2A (MEN 2A), a dominantly inherited cancer syndrome. The map extends from D10S24 (at 10p13-p12.2) to D10S3 (at 10q21-q23) and is about 70 cM long. Overall, higher female than male recombination frequencies were observed for this region, with the most remarkable female excess in the immediate vicinity of the centromere, as previously reported. Most of the DNA markers in this map are highly informative for linkage and the majority of the interlocus intervals are no more than 6 cM apart. Thus this map should provide a fine framework for future efforts in more detailed mapping studies around the centromeric area. A set of ordered cross-overs identified in this work is a valuable resource for rapidly and accurately localizing new DNA clones isolated from the pericentromeric region.