Cloned fusion product from a rare t(15;19)(q13.2;p13.1) inhibit S phase in vitro.

BACKGROUND: Somatically acquired chromosomal translocation is a common mechanism of oncogene activation in many haematopoietic tumours and sarcomas. However, very few recurrent chromosomal translocations have been reported in more common epithelial tumours such as lung carcinomas. METHODS: We established a cell line HCC2429 from an aggressive, metastatic lung cancer arising in a young, non-smoking woman, demonstrating a t(15;19)(q13.2;p13.1). The breakpoints on chromosomes 15 and 19 were cloned using long distance inverse PCR and we determined by RT-PCR that the translocation results in a novel fusion transcript in which the 3' end Brd4 on chromosome 19p is fused to the 5' end of NUT on chromosome 15q. RESULTS: In total, 128 lung cancer tissues were screened using fluorescent in situ hybridisation, but none of the tumours screened demonstrated t(15;19), suggesting that this translocation is not commonly found in lung cancer. Consistent with previous literature, ectopic expression of wild type Brd4 was shown to inhibit G(1) to S progression. However, we also found that the Brd4-NUT fusion augments the inhibition of progression to S phase compared with wild type Brd4. CONCLUSION: Alteration in cell cycle kinetics is important in tumorigenesis, although the exact role of Brd4-NUT fusion protein in the pathogenesis of lung cancers remains unclear and need to be further elucidated.

Reduced expression and impaired kinase activity of a Chk2 mutant identified in human lung cancer.

The checkpoint kinase Chk2 is phosphorylated and activated in response to DNA damage such as ionizing radiation. Recently, we found a somatic mutation of CHK2 with clear loss of the wild-type allele in human lung cancer. Here we show that the mutant Chk2 exhibits modestly reduced in vitro kinase activity compared with wild type, whereas it is normally phosphorylated and activated after ionizing radiation. Interestingly, this mutant Chk2 protein was found to be less stable than wild type and could be expressed in various cell types only at a significantly reduced (20%) level of wild type. These findings confirm that the DNA damage checkpoint pathway involving CHK2 is indeed inactivated in this fatal adult cancer and also suggest that reduced expression of Chk2 may also be an important inactivating mechanism, contributing to the development of lung cancer.

Search for mutations and examination of allelic expression imbalance of the p73 gene at 1p36.33 in human lung cancers.

We examined 61 lung cancer cases to determine whether alterations of p73, a novel monoallelically expressed p53-like molecule, may be involved in the pathogenesis of lung cancer. Allelic loss at the p73 locus at 1p36.33 was observed in 42% (11 of 26 informative cases), and squamous cell carcinoma tended to carry this lesion most frequently. Somatic mutations in the p73 gene itself, however, were not detected, despite our extensive search. We found interindividual difference in the allelic expression of p73 in normal lung, as well as intertissue variance, even within the same individual, but preferential loss of the expressed allele appeared to be an unlikely mechanism for p73 inactivation. This study, consequently, suggests the presence of an as yet unidentified tumor suppressor gene or genes within the subtelomeric region of 1p, warranting further studies aimed at its isolation.

Histological type-selective, tumor-predominant expression of a novel CHK1 isoform and infrequent in vivo somatic CHK2 mutation in small cell lung cancer.

Inactivation of p53, which represents the most prevalent genetic alteration in lung cancer, has been shown to play a crucial role in the acquisition of genomic instability. We examined 44 lung cancer specimens to search for mutations in the CHK1 and CHK2 genes, which have been suggested to play roles in regulating p53 after DNA damage. We found that the CHK2 gene was somatically mutated in lung cancer in vivo, although at a low frequency, and that a previously undescribed shorter isoform of CHK1 was expressed preferentially in small cell lung cancer in a tumor-predominant manner. Additional studies are warranted to investigate the functional significance of these changes as well as the potential involvement of other components in this important pathway to maintain genomic stability.

Identification of frequent impairment of the mitotic checkpoint and molecular analysis of the mitotic checkpoint genes, hsMAD2 and p55CDC, in human lung cancers.

The mitotic checkpoint is thought to be essential for ensuring accurate chromosome segregation by implementing mitotic delay in response to a spindle defect. To date, however, very little data has become available on the defects of the mitotic checkpoint in human cancer cells. In the present study, impaired mitotic checkpoint was found in four (44%) of nine human lung cancer cell lines. To our knowledge, this is the first demonstration of frequent impairment of the mitotic checkpoint in this leading cause of cancer deaths. As an initial step towards elucidation of the underlying mechanism, we further undertook a search for mutations in a key component of the mitotic checkpoint, known as hsMAD2, and its immediate downstream molecule, p55CDC. No such mutations were found, however, in either 21 lung cancer cell lines or 25 primary lung cancer cases, although we could identify silent polymorphisms and the transcribed and processed hsMAD2 pseudogene that was subsequently mapped at 14q21-q23. The present observations appear to warrant further investigations, such as search for alterations in other components, to better understand the molecular pathogenesis of this fatal disease, and warn against potential misinterpretation when performing mutational analyses for other cancer types based on cDNA templates.

Search for in vivo somatic mutations in the mitotic checkpoint gene, hMAD1, in human lung cancers.

We previously reported the presence of mitotic check-point impairment in about 40% of lung cancer cell lines. To gain an insight into the molecular basis of this impairment, we examined 49 lung cancer specimens for alterations in the hMAD1 mitotic checkpoint gene and identified a somatic, non-conservative missense mutation, which substitutes alanine (GCG) for threonine (ACG) at codon 299, together with a number of amino acid substituting, single nucleotide polymorphisms. This is the first demonstration of hMAD1 mutation in any type of human cancers. The present finding marks hMAD1 as a potential target, although with low frequency, for genetic alterations in lung cancer. Thus, further studies of hMAD1 dysfunction caused by other mechanisms appear to be warranted, as well as potential involvement of other components of the mitotic checkpoint.

High frequency of clonally related tumors in cases of multiple synchronous lung cancers as revealed by molecular diagnosis.

In patients with multiple synchronous lung tumors, discrimination of multicentric lung cancers from intrapulmonary metastasis is important for treatment decision, but this is sometimes difficult. The aim of this study was to retrospectively distinguish multicentric lung cancers from intrapulmonary metastases in 14 such cases by loss of heterozygosity (LOH) and p53 mutational status. DNA was extracted from microdissected tumor cells in paraffin-embedded archival tissue, and 3p14.2, 3p21, 3p25, 9p21, and 18q21.1 were investigated for LOH. Exons 5-8 of the p53 gene were examined for mutations by the PCR, followed by single-strand conformation polymorphism analysis and DNA sequencing. For cases with the same LOH pattern, we calculated a clonality index, the probability of the given LOH pattern when these tumors were hypothesized to be independent in origin. Eleven of 14 cases (79%) were thus diagnosed as having pulmonary metastasis and only one case as having genuinely multicentric lung cancers. Two cases presented difficulty in diagnosis. In several cases, the LOH patterns conflicted with p53 mutation patterns, suggesting that clonal evolution is directly affected by certain genetic changes. The combination of p53 with LOH helped increase both the sensitivity and specificity of the assay.

Molecular analysis of the mitotic checkpoint genes BUB1, BUBR1 and BUB3 in human lung cancers.

Our previous studies showed that mitotic checkpoint impairment is present in about 40% of human lung cancer cell lines but that mutations in the MAD mitotic checkpoint genes are infrequent. In the present study, we examined 44 lung cancer cases for the potential involvement of the other gene family involved in the mitotic checkpoint, i.e. BUB. We found that the BUB gene family members including BUB1, BUBR1 and BUB3 are not frequent targets for mitotic checkpoint defects in lung cancers, if present at all. Further studies are thus warranted to elucidate the molecular basis for the acquisition of mitotic checkpoint defects in order to better understand the molecular pathogenesis of lung cancers.

Expression of human telomerase subunit genes in primary lung cancer and its clinical significance.

BACKGROUND: Three major components of human telomerase, RNA component (hTERC), telomerase-associated protein (TEP1), and catalytic subunit (hTERT) have been cloned recently. The aim of this study was to examine the expression of these genes and to search for clinical usefulness. METHODS: Expression of these genes was evaluated by reverse transcription-polymerase chain reaction in 92 human lung cancers and in 32 non-neoplastic lung tissues. In 15 patients, both telomerase activity by telomeric repeat amplification protocol assay and expression were evaluated. RESULTS: hTERT expression was best associated with telomerase activity with a concordance of 77%. In 92 lung cancer tissues, hTERC, TEP1, and hTERT were expressed in 100%, 93%, and 89%, respectively. Whereas most adjacent non-neoplastic lung tissues expressed hTERC and TEP1 (94% and 100%, respectively), hTERT was detected in only 1 of 32 normal lungs. However, there was no relationship between hTERT expression and clinicopathologic features. CONCLUSIONS: hTERT expression can be a surrogate for telomerase activity that may serve as a novel biomarker of lung cancer with high specificity and sensitivity.

Frequent allelic imbalance suggests involvement of a tumor suppressor gene at 1p36 in the pathogenesis of human lung cancers.

The short arm of chromosome 1 is among the most frequently affected regions in various types of common adult cancers as well as in neuroblastoma. In a previous study of ours, frequent allelic imbalance at the TP73 locus at 1p36 was noted in lung cancer despite the absence of TP73 mutations. This suggested the possible existence of an as yet unidentified tumor suppressor gene on 1p. Our initial attempt using the candidate gene approach did not yield any somatic mutations in the 14-3-3sigma gene (official gene symbol, SFN), a mediator of G2 arrest by TP53. Detailed deletion mapping of the telomeric region of 1p was thus carried out as an initial step toward positional cloning. We used seven polymorphic markers in addition to TP73 to examine 61 primary lung cancers. Allelic imbalance at one or more loci of 1p36 was observed in 30 of the 61 cases, whereas D1S508 at 1p36.2 exhibited the highest frequency (45%) of allelic imbalance among the 1p36 markers examined. In contrast, two proximal markers at 1p32-34 showed significantly less frequent (11-14%) allelic imbalance. Consequently, the present study identified the shortest region of overlap between D1S507 and TP73, which included the most frequently affected marker, D1S508. In addition, several cases exhibited allelic imbalance confined to a subtelomeric region distal to D1S2845 at 1p36.3. The present findings warrant future studies to identify the putative tumor suppressor gene(s) at 1p36 to gain a better understanding of the molecular pathogenesis of lung cancer. Genes Chromosomes Cancer 28:342-346, 2000.

Characterization of high-grade neuroendocrine tumors of the lung in relation to menin mutations.

It has been suggested that mutations in the menin gene play a role in the development of multiple endocrine neoplasia type 1 (MEN1)-associated and of sporadic forms of low- and intermediate-grade neuroendocrine tumors of the lung. In the present study, eight tumor specimens of large cell neuroendocrine carcinoma (LCNEC) and 13 of small cell lung cancer (SCLC), which represent a high-grade category of neuroendocrine tumors, were examined for the potential involvement of menin alterations as well as for the expression of various neuroendocrine markers and p53 and Rb abnormalities. All specimens expressed multiple neuroendocrine markers as expected and almost invariably carried p53 and Rb alterations. Unexpectedly, however, mutations in the menin gene were not detected in any of the high-grade neuroendocrine tumors examined. We thus conclude that menin mutations do not play a crucial role in the pathogenesis of high-grade subsets, in contrast to their suggested significant role in the development of low- and intermediate-grade subsets. Interestingly, loss of heterozygosity (LOH) in the menin gene appeared to be more prevalent in LCNEC (50%) than in SCLC (22%), suggesting a possible distinction between SCLC and LCNEC.

Persistent increase in chromosome instability in lung cancer: possible indirect involvement of p53 inactivation.

Karyotype and fluorescence in situ hybridization analyses have demonstrated the frequent presence of an altered static state of the number of chromosomes (ie, aneuploidy) in lung cancer, but it has not been directly established whether aneuploidy is in fact associated with a persistent increase in the rate of chromosomal losses and gains (ie, chromosome instability, or CIN). The study presented here used a panel of 10 lung cancer cell lines to provide for the first time direct evidence that CIN is a common feature in lung cancer cell lines in association with the presence of significant aneuploidy. In addition, we found that the CIN phenotype correlates well with the presence of p53 mutations. However, human papilloma virus 16-E6-directed inactivation of p53 in a representative non-CIN lung cancer cell line did not result in the induction of CIN, at least up to the 25th generation, suggesting that inactivation of p53 itself is unlikely to directly induce CIN in lung cancer cells. Interestingly, however, significant CIN could be induced in conjunction with the generation of aneuploid populations when the mitotic spindle formation was transiently abrogated in p53-inactivated cells. These results suggest that inactivation of p53 may allow lung cancer cells to go through an inappropriate second division cycle under certain forms of mitotic stresses, which would result in the induction of the CIN phenotype in conjunction with the generation of aneuploidy.