C to A single nucleotide polymorphism in intron 18 of the human MST1R (RON) gene that maps at 3p21.3

A Novel FGFR3 Splice Variant Preferentially Expressed in African American Prostate Cancer Drives Aggressive Phenotypes and Docetaxel Resistance

Alternative splicing (AS) has been shown to participate in prostate cancer development and progression; however, a link between AS and prostate cancer health disparities has been largely unexplored. Here we report on the cloning of a novel splice variant of FGFR3 that is preferentially expressed in African American (AA) prostate cancer. This novel variant (FGFR3-S) omits exon 14, comprising 123 nucleotides that encode the activation loop in the intracellular split kinase domain. Ectopic overexpression of FGFR3-S in European American (EA) prostate cancer cell lines (PC-3 and LNCaP) led to enhanced receptor autophosphorylation and increased activation of the downstream signaling effectors AKT, STAT3, and ribosomal S6 compared with FGFR3-L (retains exon 14). The increased oncogenic signaling imparted by FGFR3-S was associated with a substantial gain in proliferative and antiapoptotic activities, as well as a modest but significant gain in cell motility. Moreover, the FGFR3-S-conferred proliferative and motility gains were highly resistant to the pan-FGFR small-molecule inhibitor dovitinib and the antiapoptotic gain was insensitive to the cytotoxic drug docetaxel, which stands in marked contrast with dovitinib- and docetaxel-sensitive FGFR3-L. In an in vivo xenograft model, mice injected with PC-3 cells overexpressing FGFR3-S exhibited significantly increased tumor growth and resistance to dovitinib treatment compared with cells overexpressing FGFR3-L. In agreement with our in vitro and in vivo findings, a high FGFR3-S/FGFR3-L expression ratio in prostate cancer specimens was associated with poor patient prognosis. IMPLICATIONS: This work identifies a novel FGFR3 splice variant and supports the hypothesis that differential AS participates in prostate cancer health disparities.

RON Signaling Is a Key Mediator of Tumor Progression in Many Human Cancers

With an increasing body of literature covering RON receptor tyrosine kinase function in different types of human cancers, it is becoming clear that RON has prominent roles in both cancer cells and in the tumor-associated microenvironment. RON not only activates several oncogenic signaling pathways in cancer cells, leading to more aggressive behavior, but also promotes an immunosuppressive, alternatively activated phenotype in macrophages and limits the antitumor immune response. These two unique functions of this oncogene, the strong correlation between RON expression and poor outcomes in cancer, and the high tolerability of a new RON inhibitor make it an exciting therapeutic target, the blocking of which offers an advantage toward improving the survival of cancer patients. Here, we discuss recent findings on the role of RON signaling in cancer progression and its potential in cancer therapy.

Variant RONΔ160 of the RON receptor tyrosine kinase promotes the growth and invasion in vitro and in vivo in gastric cancer cell lines

BACKGROUND

Recepteur d'origine nantais (RON) is a receptor tyrosine kinase whose overexpression has been observed in human gastric cancers. This study aimed to determine whether overexpression of the variant RONΔ160 could induce tumorigenicity of gastric cancer cells in vitro or in vivo, and whether its specific small molecule inhibitor (Compound I) could inhibit the effect of RONΔ160.

METHODS

We constructed human gastric cancer cell line MGC-803 that was stably transfected with a recombinant plasmid expressing RONΔ160, and the effect of RONΔ160 overexpression and macrophage-stimulating protein (MSP) activation on proliferation, migration and invasion abilities of MGC-803 cells were evaluated. Tumor-bearing mice with gastric cancer cells were used to analyze the effects of RONΔ160 overexpression and Compound I on implanted tumor growth.

RESULTS

In vitro, overexpression of RONΔ160 in MGC-803 cells resulted changes to their cell morphology, and promoted cell proliferation, migration and invasion. In addition, overexpression of RONΔ160 increased the proportion of cells in the S phase. The effect of RONΔ160 was significantly enhanced by induction of MSP inducing (p < 0.05). In vivo, RONΔ160 promoted the growth of MGC-803 cells in nude mice, including increased tumor size and weight, and lower tumor incubation period. The Compound I inhibited the tumorigenic abilities of RONΔ160 (p <0.05).

CONCLUSIONS

The results indicate that overexpression of the variant RONΔ160 altered the phenotype and tumorigenicity of MGC-803 cells. Its specific small molecule inhibitor could inhibit the effect of RONΔ160. Therefore, the variant RONΔ160 may become a potential therapeutic target for gastric cancer.

Identification of common genetic variation that modulates alternative splicing

Alternative splicing of genes is an efficient means of generating variation in protein function. Several disease states have been associated with rare genetic variants that affect splicing patterns. Conversely, splicing efficiency of some genes is known to vary between individuals without apparent ill effects. What is not clear is whether commonly observed phenotypic variation in splicing patterns, and hence potential variation in protein function, is to a significant extent determined by naturally occurring DNA sequence variation and in particular by single nucleotide polymorphisms (SNPs). In this study, we surveyed the splicing patterns of 250 exons in 22 individuals who had been previously genotyped by the International HapMap Project. We identified 70 simple cassette exon alternative splicing events in our experimental system; for six of these, we detected consistent differences in splicing pattern between individuals, with a highly significant association between splice phenotype and neighbouring SNPs. Remarkably, for five out of six of these events, the strongest correlation was found with the SNP closest to the intron–exon boundary, although the distance between these SNPs and the intron–exon boundary ranged from 2 bp to greater than 1,000 bp. Two of these SNPs were further investigated using a minigene splicing system, and in each case the SNPs were found to exert cis-acting effects on exon splicing efficiency in vitro. The functional consequences of these SNPs could not be predicted using bioinformatic algorithms. Our findings suggest that phenotypic variation in splicing patterns is determined by the presence of SNPs within flanking introns or exons. Effects on splicing may represent an important mechanism by which SNPs influence gene function.

Genetic variation, through its effects on gene expression, influences many aspects of the human phenotype. Understanding the impact of genetic variation on human disease risk has become a major goal for biomedical research and has the potential of revealing both novel disease mechanisms and novel functional elements controlling gene expression. Recent large-scale studies have suggested that a relatively high proportion of human genes show allele-specific variation in expression. Effects of common DNA polymorphisms on mRNA splicing are less well studied. Variation in splicing patterns is known to be tissue specific, and for a small number of genes has been shown to vary among individuals. What is not known is whether allele-specific splicing events are an important mechanism by which common genetic variation affects gene expression. In this study we show that allele-specific alternative splicing was observed in six out of 70 exon-skipping events. Sequence analysis of the relevant splice sites and of the regions surrounding single nucleotide polymorphisms correlated with the splicing events failed to identify any predictive bioinformatic signals. A genome-wide study of allele-specific splicing, using an experimental rather than a bioinformatic approach, is now required.

Multiple variants of the RON receptor tyrosine kinase: Biochemical properties, tumorigenic activities, and potential drug targets

Aberrant expression of the RON (Recepteur d'Origine Nantais) receptor tyrosine kinase, accompanied by generation of multiple splicing or truncated variants, contributes to pathogenesis of epithelial cancers. Currently, six variants including RONDelta170, Delta165, Delta160, Delta155, Delta110, and Delta55 with various deletions or truncations in the extracellular or intracellular regions have been identified. The extracellular sequences contain functional structures such as sema domain, PSI motif, and IPT units. The deletion or truncation results in constitutive phosphorylation and increased kinase activities. Oncogenic RONDelta160, generated by exclusion of the first IPT unit, is a typical example. In contrast, the deletion adjacent to the conserved MET(1254) in the kinase domain converts RON into a dominant negative agent. Among three mechanisms underlying isoform production, the switch from constitutive to alternative pre-mRNA splicing is the major event in producing RON variants in cancer cells. Most of the RON variants have the ability to activate multiple signaling cascades with a different substrate specificity and phosphorylation profile. They regulate cell migration, invasion, and proliferation, which contribute to the invasive phenotype and promote the malignant progression. Thus, determining the pathogenesis of RON variants is critical in understanding the mechanisms underlying cancer initiation and progression. Targeting oncogenic signals elicited by RON or its variants by special antibody or small interfering RNA could provide a novel strategy for the treatment of malignant epithelial cancers.

Oncogenesis of RON receptor tyrosine kinase: a molecular target for malignant epithelial cancers

Recepteur d'origine nantais (RON) belongs to a subfamily of receptor tyrosine kinases (RTK) with unique expression patterns and biological activities. RON is activated by a serum-derived growth factor macrophage stimulating protein (MSP). The RON gene transcription is essential for embryonic development and critical in regulating certain physiological processes. Recent studies have indicated that altered RON expression contributes significantly to cancer progression and malignancy. In primary tumors, such as colon and breast cancers, overexpression of RON exists in large numbers and is often accompanied by the generation of different splicing variants. These RON variants direct a unique program that controls cell transformation, growth, migration, and invasion, indicating that altered RON expression has the ability to regulate motile/invasive phenotypes. These activities were also seen in transgenic mice, in which targeted expression of RON in lung epithelial cells resulted in numerous tumors with pathological features of human bronchioloalveolar carcinoma. Thus, abnormal RON activation is a pathogenic factor that transduces oncogenic signals leading to uncontrolled cell growth and subsequent malignant transformation. Considering these facts, RON and its variants can be considered as potential targets for therapeutic intervention. Experiments using small interfering RNA and neutralizing monoclonal antibodies demonstrated that suppressing RON expression and activation decreases cancer cell proliferation, increases apoptotic death, prevents tumor formation in nude mice, and reduces malignant phenotypes. Thus, blocking RON expression and activation has clinical significance in reversing malignant phenotypes and controlling tumor growth.

Discovery of frequent homozygous deletions in chromosome 3p21.3 LUCA and AP20 regions in renal, lung and breast carcinomas

We searched for chromosome 3p homo- and hemizygous losses in 23 lung cancer cell lines, 53 renal cell and 22 breast carcinoma biopsies using 31 microsatellite markers located in frequently deleted 3p regions. In addition, two sequence-tagged site markers (NLJ-003 and NL3-001) located in the Alu-PCR clone 20 region (AP20) and lung cancer (LUCA) regions, respectively, were used for quantitative real-time PCR (QPCR). We found frequent (10-18%) homozygous deletions (HDs) in both 3p21.3 regions in the biopsies and lung cancer cell lines. In addition, we discovered that amplification of 3p is a very common (15-42.5%) event in these cancers and probably in other epithelial malignancies. QPCR showed that aberrations of either NLJ-003 or NL3-001 were detected in more than 90% of all studied cases. HDs were frequently detected simultaneously both in NLJ-003 or NL3-001 loci in the same tumour (P<3-10(-7)). This observation suggests that tumour suppressor genes (TSG) in these regions could have a synergistic effect. The exceptionally high frequency of chromosome aberrations in NLJ-003 and NL3-001 loci suggests that multiple TSG(s) involved in different malignancies are located very near to these markers. Precise mapping of 15 independent HDs in the LUCA region allowed us to establish the smallest HD region in 3p21.3C located between D3S1568 (CACNA2D2 gene) and D3S4604 (SEMA3F gene). This region contains 17 genes. Mapping of 19 HDs in the AP20 region resulted in the localization of the minimal region to the interval flanked by D3S1298 and D3S3623 markers. Only four genes were discovered in this interval, namely, APRG1, ITGA9, HYA22 and VILL.