Screening for microsatellite instability target genes in colorectal cancers

BACKGROUND

Defects in the DNA repair system lead to genetic instability because replication errors are not corrected. This type of genetic instability is a key event in the malignant progression of HNPCC and a subset of sporadic colon cancers and mutation rates are particularly high at short repetitive sequences. Somatic deletions of coding mononucleotide repeats have been detected, for example, in the TGFbetaRII and BAX genes, and recently many novel target genes for microsatellite instability (MSI) have been proposed. Novel target genes are likely to be discovered in the future. More data should be created on background mutation rates in MSI tumours to evaluate mutation rates observed in the candidate target genes.

METHODS

Mutation rates in 14 neutral intronic repeats were evaluated in MSI tumours. Bioinformatic searches combined with keywords related to cancer and tumour suppressor or CRC related gene homology were used to find new candidate MSI target genes. By comparison of mutation frequencies observed in intronic mononucleotide repeats versus exonic coding repeats of potential MSI target genes, the significance of the exonic mutations was estimated.

RESULTS

As expected, the length of an intronic mononucleotide repeat correlated positively with the number of slippages for both G/C and A/T repeats (p=0.0020 and p=0.0012, respectively). BRCA1, CtBP1, and Rb1 associated CtIP and other candidates were found in a bioinformatic search combined with keywords related to cancer. Sequencing showed a significantly increased mutation rate in the exonic A9 repeat of CtIP (25/109=22.9%) as compared with similar intronic repeats (p< or =0.001).

CONCLUSIONS

We propose a new candidate MSI target gene CtIP to be evaluated in further studies.

Sema3A-induced growth-cone collapse is mediated by Rac1 amino acids 17-32

BACKGROUND

Neurons project their axons along specific pathways in order to establish appropriate connections with their target cells. The rate and direction of axonal growth is determined by interactions between the highly motile growth cone and environmental cues that can act in either an attractive or a repulsive manner. Locomotion is ultimately dependent upon the reorganisation of the actin cytoskeleton and an established role for the Rho family of small GTPases in regulating this process in non-neuronal cells identifies them as candidate signalling molecules in growth cones. An inactive form of Rac1 has recently been shown to inhibit the 'growth-cone collapse' response induced by chick Sema3A, a protein that has recently been established as an important guidance cue. The molecular basis for this inhibition remains unclear.

RESULTS

We have made a series of overlapping peptides from the amino-terminal region of Rac1 and rendered them cell permeable by synthesis in tandem with an established internalisation vector. We report here that a peptide encompassing Rac1 amino acids 17-32 binds directly to the established Rac1-interacting molecules PAK, WASP, 3BP-1 and p85beta(P13K), but not to p67(Phox). Furthermore, the peptide can compete with activated Rac1 for target binding, and inhibits Sema3A-induced growth-cone collapse. We also synthesised cell-permeable peptides that correspond to the Cdc42/Rac1-binding (CRIB) motifs present in PAK and N-WASP. Our results show that a CRIB-containing peptide from PAK, but not that from N-WASP, inhibits growth-cone collapse and that the inhibitory activity correlates with binding to Rac1 and not to Cdc42.

CONCLUSIONS

Our results suggest that Sema3A-induced growth-cone collapse is mediated by Rac1 amino acids 17-32, and demonstrate the feasibility of designing new cell-permeable inhibitors of small GTPases.

Bmx tyrosine kinase is specifically expressed in the endocardium and the endothelium of large arteries

BACKGROUND

The growth and differentiation of endothelial cells are regulated by signal transduction through tyrosine protein kinases. Recently, a novel cytoplasmic tyrosine kinase gene, Bmx (Bone Marrow tyrosine kinase gene in chromosome X), was identified in human bone marrow RNA and found to be expressed predominantly in myeloid hematopoietic cell lineages. Our preliminary analyses indicated that the Bmx gene was also highly expressed in human heart.

METHODS AND RESULTS

Mouse Bmx cDNA was isolated, sequenced, and found to encode a polypeptide approximately 91% identical to the human Bmx tyrosine kinase. Northern blotting and in situ hybridization of tissue sections indicated that Bmx mRNA is specifically expressed in the endocardium of the developing heart as well as in the endocardium of the left ventricle and in the endothelium of large arteries in adult mice. A weak signal was seen also in coronary arterial endothelium.

CONCLUSIONS

Bmx shows a unique specificity of expression among tyrosine kinase genes and may be involved in signal transduction in endocardial and arterial endothelial cells. The results suggest that specific signal transduction mechanisms are present in such endothelia.

Expression of Mad, an antagonist of Myc oncoprotein function, in differentiating keratinocytes during tumorigenesis of the skin

The Myc oncoprotein is associated with cell proliferation and is often down-regulated during cell differentiation. The related Mad transcription factor, which antagonises Myc activity, is highly expressed in epidermal keratinocytes. Mad also inhibits cell proliferation in vitro. To study Mad expression in keratinocyte proliferation and differentiation, we have analysed Mad RNA expression in regenerating and hyperproliferative epidermal lesions and epidermal tumours of varying degrees of differentiation using the RNA in situ hybridisation and RNAase protection techniques. Mad was strongly expressed in differentiating suprabasal keratinocytes in healing dermal wounds and in benign hyperproliferative conditions, but also in squamous cell carcinomas, in which the keratinocytes retain their differentiation potential. However, Mad expression was lost in palisading basal carcinoma cells and poorly differentiated squamous cell carcinomas, which lacked the epithelial differentiation marker syndecan-1. We therefore suggest that Mad expression is closely associated with epithelial cell differentiation, and that this association is retained in epithelial tumours of the skin.

Determination of sequences responsible for the differential regulation of Myc function by delta Max and Max

The DNA-binding, transcriptional activation and transforming activities of the Myc protein require dimerization with Max. Max can form also homodimers which are able to bind the same DNA sequence as Myc/Max heterodimers and suppress Myc-induced transcription and transformation. We have recently identified a naturally occurring truncated form of Max, delta Max, which in a rat embryo fibroblast enhances transformation by Myc and Ras. Like Max, this delta Max protein contains a b-HLH-Zip domain, except that the end of the leucine zipper is replaced by five delta Max-specific amino acid residues. Delta Max also lacks the C-terminal sequences of Max including a nuclear localisation signal. Here we have dissected the regions responsible for the specific effects of Max and delta Max in Ras-Myc cotransformation of rat embryo fibroblasts. Our results indicate that the suppressive activity of Max requires C-terminal acidic and basic regions and an intact leucine zipper. Replacement of the end of the leucine zipper with the delta Max-specific sequence is responsible for the enhancement of transformation by delta Max. Surprisingly, delta Max does not require the DNA-binding basic region for enhancement of transformation and has no effect on Myc-induced transcription activation from Myc/Max-binding site-containing promoter construct.

Expression of the mad gene during cell differentiation in vivo and its inhibition of cell growth in vitro

Mad is a basic region helix-loop-helix leucine zipper transcription factor which can dimerize with the Max protein and antagonize transcriptional activation by the Myc-Max transcription factor heterodimer. While the expression of Myc is necessary for cell proliferation, the expression of Mad is induced upon differentiation of at least some leukemia cell lines. Here, the expression of the mad gene has been explored in developing mouse tissues. During organogenesis in mouse embryos mad mRNA was predominantly expressed in the liver and in the mantle layer of the developing brain. At later stages mad expression was detected in neuroretina, epidermis, and whisker follicles, and in adult mice mad was expressed at variable levels in most organs analyzed. Interestingly, in the skin mad was highly expressed in the differentiating epidermal keratinocytes, but not in the underlying proliferating basal keratinocyte layer. Also, in the gut mad mRNA was abundant in the intestinal villi, where cells cease proliferation and differentiate, but not in the crypts, where the intestinal epithelial cells proliferate. In the testis, mad expression was associated with the completion of meiosis and early development of haploid cells. In cell culture, Mad inhibited colony formation of a mouse keratinocyte cell line and rat embryo fibroblast transformation by Myc and Ras. The pattern of mad expression in tissues and its ability to inhibit cell growth in vitro suggests that Mad can cause the cessation of cell proliferation associated with cell differentiation in vivo.

c-Myc induces cellular susceptibility to the cytotoxic action of TNF-alpha

Tumor necrosis factor-alpha (TNF) is a multifunctional cytokine which is cytotoxic for some tumor cells and transformed cells. The molecular mechanisms which render transformed and tumor cells sensitive to the cytotoxic action of TNF are unclear. We show here that an increased expression of the c-Myc oncoprotein strongly increases cellular sensitivity to TNF cytotoxicity. In Rat1A fibroblasts, which are resistant to TNF, the addition of TNF with a concomitant activation of a hormone-inducible c-Myc-estrogen receptor chimera (MycER) resulted in apoptotic cell death. Similarly, c-Myc overexpression enhanced the sensitivity of NIH3T3 fibroblasts to TNF-induced death. The c-Myc and TNF-induced apoptosis was inhibited by ectopic expression of the Bcl2 oncoprotein and by the free oxygen radical scavenging enzyme Mn superoxide dismutase. Furthermore, in highly TNF-sensitive fibrosarcoma cells, antisense c-myc oligodeoxynucleotides caused a specific inhibition of TNF cytotoxicity. Our results suggest that the deregulation of c-Myc, which is common in human tumors and tumor cell lines is one reason why these cells are TNF sensitive.

Max activity is affected by phosphorylation at two NH2-terminal sites

Max is a nuclear phosphoprotein that has a dose-dependent role in regulation of Myc function. The DNA-binding activity of Max homodimers, but not of Myc/Max heterodimers, has been reported to be inhibited by NH2-terminal phosphorylation. (S. J. Berberich and M. D. Cole, Genes & Dev., 6: 166-176, 1992). Here, we have mapped the NH2-terminal in vivo phosphorylation sites of Max to Ser2 and Ser11 and show that the NH2 termini of the two major alternatively spliced forms of Max (p21max and p22max) are equally phosphorylated despite differences in their amino acid sequences following Ser11. A Max mutant deficient in the NH2-terminal phosphorylation was found to inhibit both basal and Myc-induced transcription of a reporter gene more efficiently than the wild-type protein. Similarly, the ability of Myc and Ras to induce transformation was more severely impaired by the mutant. These results indicate that the NH2-terminal phosphorylation diminishes the ability of Max to negatively interfere with Myc function. However, we found no evidence that Max phosphorylation would be regulated during cell growth or differentiation. Similarly, we observed no major cell cycle-dependent changes in the extent of phosphorylation between cell populations fractionated by centrifugal elutriation or by cell cycle inhibitors.

Myc protein: partners and antagonists

One of the first oncogenes identified from human tumors was c-myc, which is frequently activated in Burkitt's lymphomas due to chromosomal translocations. Subsequently, members of the myc oncogene family were found to be amplified in neuroblastoma and small-cell lung cancer. In normal cells, Myc activity has been shown to be both necessary and sufficient for resting cells to enter the cell cycle. Interestingly, it appears that Myc not only drives the cell cycle, but also induces cell death by apoptosis in certain situations. Myc contains a transcriptional activation domain and a basic helix-loop-helix-leucine zipper DNA-binding and dimerization domain. As a heterodimer with a structurally related protein, Max, Myc can bind DNA in a sequence-specific manner. These results suggest that the Myc/Max heterodimer functions as a transcriptional activator of genes that are critical for the regulation of cell growth.

Differential expression of myc, max and RB1 genes in human gliomas and glioma cell lines

Deregulated expression of myc proto-oncogenes is implicated in several human neoplasias. We analysed the expression of c-myc, N-myc, L-myc, max and RB1 mRNAs in a panel of human gliomas and glioma cell lines and compared the findings with normal neural cells. The max and RB1 genes were included in the study because their protein products can interact with the Myc proteins, being thus putative modulators of Myc activity. Several gliomas contained c/L-myc mRNAs at levels higher than those in fetal brain, L-myc predominantly in grade II/III and c-myc in grade III gliomas. High-level N-myc expression was detected. In one small-cell glioblastoma and lower levels in five other gliomas. In contrast, glioma cell lines totally lacked N/L-myc expression. The in situ hybridisations revealed mutually exclusive topographic distribution of myc and glial fibrillary acidic protein (GFAP) mRNAs, and a lack of correlation between myc expression and proliferative activity, max and RB1 mRNAs were detected in most tumours and cell lines. The glioma cells displayed interesting alternative splicing patterns of max mRNAs encoding Max proteins which either suppress (Max) or augment (delta Max) the transforming activity of Myc. We conclude that (1) glioma cells in vivo may coexpress several myc genes, thus resembling fetal neural cells; but (2) cultured glioma cells expression only c-myc; (3) myc, max and RB1 are regulated independently in glioma cells; and (4) alternative processing of max mRNA in some glioma cells results in delta Max encoding mRNAs not seen in normal fetal brain.

Myc amplification: regulation of myc function

The myc oncogenes have been implicated in the control of cell proliferation in both normal and neoplastic cells. There is increasing evidence that Myc proteins function as transcriptional regulators of other genes apparently involved in the control of cell proliferation. The effects of Myc on both gene expression and cell growth are differentially regulated by the recently described Max and delta Max proteins that can either cooperate or compete with Myc for sequence-specific DNA binding.

Alternative mRNA forms and open reading frames of the max gene

The max gene encodes a heterodimeric partner of Myc. We have recently identified an alternative max mRNA (delta max) that contains an additional internal exon introducing an in-frame translational termination. Here we have studied the expression of human max mRNAs by Northern blotting analysis. In addition to the major 2.3-kb mRNA form, four bands were identified. Our results indicate that these bands represent differentially spliced mRNA forms, which contain altogether three open reading frames. In addition to the previously identified Max and delta Max proteins, sequence analysis of a 3.5-kb mRNA form predicted a protein that resembles delta Max in structure. Like delta Max, this protein enhanced the number of transformed foci in the ras-myc co-transformation assay. Although the 3.5-kb mRNA represents a minor form in actively proliferating cells, a shift from the major 2.3-kb mRNA to the 3.5-kb form was observed in response to high cell density or acidification of the growth medium. Our results indicate the presence of several differentially spliced mRNA forms of the max gene, and suggest a possible mechanism for the production of functionally distinct Max proteins.

Alternative forms of Max as enhancers or suppressors of Myc-ras cotransformation

Max is a basic-helix-loop-helix-leucine zipper protein capable of forming sequence-specific DNA binding complexes with Myc proteins. An alternatively spliced messenger RNA has been identified that encodes a form of Max truncated at the COOH-terminus. This delta Max protein retained the ability to bind to the CACGTG motif in a complex with c-Myc but lacks the nuclear localization signal and the putative regulatory domain of Max. When tested in a myc-ras cotransformation assay in rat embryo fibroblasts, Max suppressed, whereas delta Max enhanced, transformation. Thus, the max gene may encode both a negative and a positive regulator of c-Myc function.