Enhanced expression of the human gene N-myc consequent to amplification of DNA may contribute to malignant progression of neuroblastoma.

Previous studies had revealed that DNA with partial similarity to the myc oncogene (N-myc) is frequently amplified in human neuroblastoma cell lines and neuroblastoma tumors. We show here for one patient that N-myc amplification is confined to the neuroblastoma tumor and is not present in normal tissue. N-myc mRNA approximately equal to 4.0 kilobases in size is detectable in neuroblastoma cell lines and tumors and in a retinoblastoma cell line. By contrast, appreciable amounts of this RNA were not present in a number of cell lines derived from other human tumors and in fibroblasts from a normal individual and from a neuroblastoma patient. Low levels of N-myc RNA were found in human and murine neuroblastoma cell lines lacking amplification of this gene, up to 80-fold greater levels in all cell lines carrying amplified N-myc. In situ hybridization to sections of neuroblastoma tumors revealed high expression of N-myc predominantly in undifferentiated neuroblasts. We hypothesize that amplification and consequent elevated expression of N-myc may be related to malignant progression.     

Human small-cell lung cancers show amplification and expression of the N-myc gene.

We have found that 6 of 31 independently derived human small-cell lung cancer (SCLC) cell lines have 5- to 170-fold amplified N-myc gene sequences. The amplification is seen with probes from two separate exons of N-myc, which are homologous to either the second or the third exon of the c-myc gene. Amplified N-myc sequences were found in a tumor cell line started prior to chemotherapy, in SCLC tumor samples harvested directly from tumor metastases at autopsy, and from a resected primary lung cancer. Several N-myc-amplified tumor cell lines also exhibited N-myc hybridizing fragments not in the germ-line position. In one patient's tumor, an additional amplified N-myc DNA fragment was observed and this fragment was heterogenously distributed in liver metastases. In contrast to SCLC with neuroendocrine properties, no non-small-cell lung cancer lines examined were found to have N-myc amplification. Fragments encoding two N-myc exons also detect increased amounts of a 3.1-kilobase N-myc mRNA in N-myc-amplified SCLC lines and in one cell line that does not show N-myc gene amplification. Both DNA and RNA hybridization experiments show that in any one SCLC cell line, only one myc-related gene is amplified and expressed. We conclude that N-myc amplification is both common and potentially significant in the tumorigenesis or tumor progression of SCLC.     

Evolutionarily conserved regions of the human c-myc protein can be uncoupled from transforming activity.

The myc family of oncogenes contains coding sequences that have been preserved in different species for over 400 million years. This conservation (which implies functional selection) is broadly represented throughout the C-terminal portion of the human c-myc protein but is largely restricted to three clusters of amino acid sequences in the N-terminal region. We have examined the role that the latter three regions of the c-myc protein might play in the transforming function of the c-myc gene. Several mutations, deletions and frameshifts, were introduced into the c-myc gene, and these mutant genes were tested for their ability to collaborate with the EJ-ras oncogene to transform rat embryo fibroblasts. Complete elimination of the first two N-terminal conserved segments abolished transforming activity. In contrast, genes altered in a portion of the second or the entire third conserved segment retained their transforming activity. Thus, the latter two segments are not required for the transformation process, suggesting that they serve another function related only to the normal expression of the c-myc gene.     

v-Ha-ras oncogene insertion: a model for tumor progression of human small cell lung cancer

Small cell lung cancer (SCLC) manifests a range of phenotypes in culture that may be important in understanding its relationship to non-SCLCs and to tumor progression events in patients. Most SCLC-derived cell lines, termed "classic" SCLC lines, have properties similar to SCLC tumors in patients, including high expression of neuroendocrine markers and low c-myc oncogene expression. A significant number of SCLC lines characterized as "biochemical or morphologic variant" SCLC lines have decreased levels of endocrine differentiation markers associated with increased proliferative indices, amplification of the c-myc oncogene, and growth patterns and biochemical markers more typical of non-SCLCs. To delineate further the relationships between these phenotypes and the molecular events involved, we have inserted the v-Ha-ras gene in SCLC cell lines with (biochemical variant) and without (classic) an amplified c-myc gene. These two SCLC subtypes had markedly different phenotypic responses to similar levels of expression of v-Ha-ras RNA. No biochemical or morphologic changes were observed in classic SCLC cells. In contrast, in biochemical variant SCLC cells, v-Ha-ras expression induced features typical of large cell undifferentiated lung carcinoma, including adherent monolayer growth patterns, increased cloning efficiency, increased levels of non-SCLC cell markers, ultrastructural characteristics and an acquired resistance to polyamine depletion typical of large cell carcinoma, but not SCLC, in vitro. Expression of v-Ha-ras in biochemical variant SCLC cells directly demonstrates that important transitions can occur between phenotypes of human lung cancer cells and that these may play a critical role in tumor progression events in patients. The findings provide a model system to study molecular events involved in tumor progression steps within a series of related tumor types.     

Expression of oncogenes in human liver disease

To elucidate the role of oncogene expression in hepatocarcinogenesis, we examined the expression of 8 cellular oncogenes by dot blot and/or northern blot analysis in neoplastic, cirrhotic and non-cirrhotic human liver tissues obtained at surgery. Significantly higher levels of c-myc gene expression were observed in tissues of hepatocellular carcinoma (HCC) and adjacent cirrhotic tissues than in apparently normal liver tissues or those of chronic hepatitis (normal-chronic hepatitis). There was a tendency to higher c-myc mRNA levels in HCC than in liver cirrhosis. However, when tumorous and adjacent cirrhotic tissues from the same patient were compared, c-myc mRNA levels were not consistently higher in HCC. No significant differences in mRNA levels of c-fos, N-myc, N-ras, Ha-ras, c-erbA, c-erbB and c-abl were observed among the HCC, cirrhosis and normal-chronic hepatitis groups. Although the significance of increased c-myc gene expression in liver cirrhosis and HCC is still not known, it is conceivable that the persistent elevation of c-myc gene expression in cirrhosis contributes to the development of HCC.     

Amplification units containing human N-myc and c-myc genes.

The amplification units in human tumors containing amplified myc genes were examined. The amplification unit in all cases consisted of a large genomic region coamplified with the coding region of the myc genes themselves. In eight independent neuroblastomas containing N-myc amplifications, the amplification unit was estimated to be 290 to 430 kilobases. This amplification unit was highly conserved among the different neuroblastomas, with some neuroblastomas containing almost identical units. In contrast, five tumor cell lines containing c-myc amplifications exhibited amplification units that were more variable in size (90 to 300 kilobases) and sequence content; at least three different patterns of c-myc amplification units could be discerned.     

Frequent amplification of c-myc in ground squirrel liver tumors associated with past or ongoing infection with a hepadnavirus.

Persistent infection with hepatitis B virus (HBV) is a major cause of hepatocellular carcinoma (HCC) in humans. HCC has also been observed in animals chronically infected with two other hepadnaviruses: ground squirrel hepatitis virus (GSHV) and woodchuck hepatitis virus (WHV). A distinctive feature of WHV is the early onset of woodchuck tumors, which may be correlated with a direct role of the virus as an insertional mutagen of myc genes: c-myc, N-myc, and predominantly the woodchuck N-myc2 retroposon. In the present study, we searched for integrated GSHV DNA and genetic alterations of myc genes in ground squirrel HCCs. Viral integration into host DNA was detected in only 3/14 squirrel tumors and did not result in insertional activation of myc genes, despite the presence of a squirrel locus homologous to the woodchuck N-myc2 gene. This suggests that GSHV may differ from WHV in its reduced ability to induce mutagenic integration events. However, the high frequency of c-myc amplification (6/14) observed in ground squirrel HCCs indicates that myc genes might be preferential effectors in the tumorigenic processes associated with rodent hepadnaviruses, a feature not reported so far in HBV-induced carcinogenesis. Together with previous observations, our results suggest that hepadnaviruses, despite close genetic and biological properties, may use different pathways in the genesis of liver cancer.     

Expression of L-myc and N-myc proto-oncogenes in human leukemias and leukemia cell lines

The myc proto-oncogenes encode nuclear phosphoproteins, which are believed to participate in the control of cell proliferation and differentiation. Deregulated expression of c-myc has been implicated in several human hematopoietic malignancies. We have studied the expression and mRNA processing of human L-myc, N-myc, and c-myc genes in a panel of human leukemias, leukemia cell lines, and normal hematopoietic cells. L-myc mRNA was expressed in three acute myeloid leukemias (AML) studied and in several myeloid leukemia cell lines. Only low expression levels were observed in adult bone marrow and in fetal spleen and thymus. The K562 and Dami leukemia cell lines showed a unique pattern of L-myc mRNA processing, with approximately 40% of L-myc mRNA lacking exon III and intron I. N-myc was expressed in five of six AML cases studied, in one of nine acute lymphocytic leukemia (ALL) cases, and in several leukemia cell lines, while c-myc mRNA was detected in all leukemias and leukemia cell lines studied. Coexpression of all three myc genes was observed in Dami and MOLT-4 cell lines and in two AMLs, and either L-myc or N-myc was coexpressed with c-myc in several other cases. These results show that in addition to c-myc, the L-myc and N-myc genes are expressed in some human leukemias and leukemia cell lines, and suggest a lack of mutually exclusive cross-regulation of the myc genes in human leukemia cells.     

High-resolution mapping of the 11q13 amplicon and identification of a gene,TAOS1, that is amplified and overexpressed in oral cancer cells

Amplification of chromosomal band 11q13 is a common event in human cancer. It has been reported in about 45% of head and neck carcinomas and in other cancers including esophageal, breast, liver, lung, and bladder cancer. To understand the mechanism of 11q13 amplification and to identify the potential oncogene(s) driving it, we have fine-mapped the structure of the amplicon in oral squamous cell carcinoma cell lines and localized the proximal and distal breakpoints. A 5-Mb physical map of the region has been prepared from which sequence is available. We quantified copy number of sequence-tagged site markers at 42-550 kb intervals along the length of the amplicon and defined the amplicon core and breakpoints by using TaqMan-based quantitative microsatellite analysis. The core of the amplicon maps to a 1.5-Mb region. The proximal breakpoint localizes to two intervals between sequence-tagged site markers, 550 kb and 160 kb in size, and the distal breakpoint maps to a 250 kb interval. The cyclin D1 gene maps to the amplicon core, as do two new expressed sequence tag clusters. We have analyzed one of these expressed sequence tag clusters and now report that it contains a previously uncharacterized gene, TAOS1 (tumor amplified and overexpressed sequence 1), which is both amplified and overexpressed in oral cancer cells. The data suggest that TAOS1 may be an amplification-dependent candidate oncogene with a role in the development and/or progression of human tumors, including oral squamous cell carcinomas. The approach described here should be useful for characterizing amplified genomic regions in a wide variety of tumors.     

c—myc,c—erbB—2等原癌基因在人甲状腺癌中的高水平表达

用~(32)P-dATP标记的c-myc、c-erbB-2、v-sis DNA探针,与15例人甲状腺癌、3例甲状腺瘤、1例甲状腺癌旁组织及5例正常甲状腺组织的细胞总RNA打点杂交表明:所有标本均有c-myc、c-erbB-2基因的表达,其中9例甲状腺癌c-myc RNA较正常组织增高3～11倍,7例甲状腺癌c-erbB-2表达增高10～50倍,未检测至v-sis RNA。c-myc、c-erbB-2探针与上述组织DNA Southernblot杂交表明:4例甲状腺癌出现c-myc重排,其中1例伴有明显扩增。未发现c-erbB-2扩增与重排。提示:c-myc、c-erbB-2的激活可能与甲状腺癌的发生有关。     

Nucleotide sequence of the human N-myc gene.

Human neuroblastomas frequently display amplification and augmented expression of a gene known as N-myc because of its similarity to the protooncogene c-myc. It has therefore been proposed that N-myc is itself a protooncogene, and subsequent tests have shown that N-myc and c-myc have similar biological activities in cell culture. We have now detailed the kinship between N-myc and c-myc by determining the nucleotide sequence of human N-myc and deducing the amino acid sequence of the protein encoded by the gene. The topography of N-myc is strikingly similar to that of c-myc: both genes contain three exons of similar lengths; the coding elements of both genes are located in the second and third exons; and both genes have unusually long 5' untranslated regions in their mRNAs, with features that raise the possibility that expression of the genes may be subject to similar controls of translation. The resemblance between the proteins encoded by N-myc and c-myc sustains previous suspicions that the genes encode related functions.