Small Cell Cancer Of The Lung
Small cell cancer of the lung accounts for about a fourth of the 110,000 new cases of lung cancer that occur annually in the United States. It is clinically distinctive: usually metastases are already present at the time of discovery so that surgery is not used. In contrast to adeno- and squamous carcinoma, SCCL is sensitive to chemotherapy and radiotherapy. Whang-Peng et al. (1982) found a specific, acquired chromosomal abnormality (deletion 3p) in at least one chromosome 3 in all metaphases in all 12 cell lines cultured from human SCCL tissue in 2-day tumor culture specimens from 3 patients. The shortest region of overlap showed the deletion to involve 3p23-p14. No other type of lung cancer showed this deletion, nor did lymphoblastoid lines cultured from SCCL patients whose tumors had the 3p deletion. SCCL is 'caused' by cigarette smoking as are other types of lung cancer. Thus, like chronic myeloid leukemia, this is an example of an exogenously induced malignancy with a specific chromosomal change. Cytogenetic effects of cigarette smoke are relevant in this connection (Madle et al., 1981).
Several biochemical markers were found to be associated with small cell cancer of the lung (Gazdar et al., 1981; Tapia et al., 1981). Perhaps genes in the 3p14-23 region have something to do with these markers as well as with the genesis of SCCL. (The cell of origin of SCCL is thought to be the Kulchitsky cell, an argentaffine cell situated in the bronchial epithelium, although this is not proved.) Erisman et al. (1982) showed that SCCL contains bombesin, a tetradecapeptide from anuran skin. It had been identified in human fetal and neonatal lung but not in adult lung. Some symptoms of SCCL may be attributable to bombesin. The syndrome of inappropriate secretion of antidiuretic hormone and Cushing syndrome, occurring with SCCL, are due to ectopic production of antidiuretic hormone and ACTH, respectively. The relation between ectopic hormone production and the aberration involving chromosome 3 is unknown. Baylin et al. (1982) found 12 distinguishing surface proteins on SCCL that were not shared by any of the 3 other carcinogen-induced forms of lung cancer (squamous, adeno-, and large cell undifferentiated carcinoma) or by human lymphoblastoid cells and fibroblasts. The neuroendocrine nature of SCCL was supported by the fact that 6 of the 12 were shared by human neuroblastoma cells. On human SCCL cells and tumors, Ruff and Pert (1984) demonstrated 4 surface antigens previously recognized only in macrophages. They suggested that cancerous cells may arise from macrophage precursors in bone marrow, and these precursors migrate to lung to participate in the repair of tissue damage produced by continuous heavy smoking. About 5% of SCCL patients have no apparent pulmonary involvement and the early, rapid and widespread dissemination of tumor to extrathoracic sites requires explanation. Naylor et al. (1984) used an anonymous, polymorphic DNA probe, D3S3, to confirm the presence of deletion of 3p in SCCL. This probe had been assigned to 3p21-cen. Studying 7 SCCL tumors and normal tissue from the same persons, they found that 6 of the 'normal' DNA samples were heterozygous for the D3S3 MspI polymorphism, whereas in all cases the tumor tissues were homozygous. De Leij et al. (1985) isolated 3 new, well-growing cell lines from SCCL. Deletions in 3p, with 3p23-p21 as the smallest region of overlap, were found. Mooibroek et al. (1987) used a recombinant DNA fragment detecting a RFLP presumably at 3p21 to probe DNA isolated from leukocytes of 12 patients with small cell lung cancer. Four of these patients were heterozygous. Analysis of tumor material from the 4 patients showed homozygosity for either one or the other restriction fragment in every case. Gerber and Scoggin (1987) and Naylor et al. (1987) demonstrated loss of constitutional heterozygosity in SCCL. Comparing tumor and constitutional genotypes of 9 patients with small cell lung cancer, Naylor et al. (1987) found a loss of alleles of chromosome 3p markers in tumor DNA of all 9 patients. Brauch et al. (1987) concluded that loss of alleles on 3p is a consistent change in small cell lung cancer but occasionally occurs in non-small cell lung cancer as well. Using a molecular genetic approach, Kok et al. (1987) found evidence for a consistent deletion at the 3p21 region not only in SCCL but in all major types of lung cancer. Yokota et al. (1987) found loss of heterozygosity for RFLPs on chromosome 3p in 7 of 7 patients, on 13q in 10 of 11 patients, and on 17p in 5 of 5 patients. Deletions at these loci in small cell carcinomas were observed even in tumors without any clinical evidence of metastasis. Furthermore, loss of heterozygosity on 3p and 13q occurred before NMYC amplification and before chromosome 11p deletion. (Loss of heterozygosity on 3p was also detected in the adenocarcinomas from 5 of 6 patients. Heterozygosity of chromosomes 13q and 17p was lost in 10 of 31 patients and in 3 of 12 patients, respectively, of lung cancers other than small cell carcinomas.) Johnson et al. (1988) found unequivocal loss of heterozygosity in the DNA from tumor tissue of 23 of 25 patients who were constitutionally heterozygous for at least 1 marker in the region 3p14-p21.
Birrer and Minna (1988) pointed to at least 3 molecular mechanisms involved in the development of lung cancer: deletion of 3p, deregulated expression of the MYC family of genes, and growth factors such as gastrin-releasing hormone (137260). Drabkin et al. (1988) demonstrated that the SCLC 'locus' is proximal to ERBA2 (190160) and also to the constitutive 3p14.2 fragile site. Using 15 chromosome 3 probes that identified 19 different RFLPs, Daly et al. (1991) identified a single 3p deletion extending proximal to the D3S2 locus at 3p21-p14.2 and including at least 3p14-p13. The locus D3F15S2 was excluded from the deleted region, an uncharacteristic feature of SCLC deletions. Moreover, D3S30 and D3S4 were included within this deletion, and thus map within the proximal half of chromosome 3p. Leduc et al. (1989) found that virtually all cases of SCLC had lost heterozygosity at the ERBA2 locus. A smaller but substantial portion of non-small cell carcinomas of the lung had lost heterozygosity at this locus. Among all of the non-small cell tumors, some had lost heterozygosity at a proximal locus but not at ERBA2, whereas none were found where the reverse was true. Thus, the locus that plays a role in non-small cell tumorigenesis probably lies proximal to ERBA2 and is almost certainly not the ERBA2 gene. Sellers et al. (1989) presented epidemiologic data supporting the role of mendelian factors in the susceptibility to human lung cancer. The specific pathology of the tumors studied was not stated in the report.
From studies of allele loss by use of 13 RFLP probes on 3p, Hibi et al. (1992) concluded that 3 distinct regions on 3p are frequently deleted in lung cancer: 3p25, 3p21.3, and 3p14-cen.
Killary et al. (1992) described a rapid genetic assay system that allowed functional analysis of defined areas of 3p in the suppression of tumorigenicity in vivo. Human/mouse microcell hybrids containing fragments of chromosome 3p were constructed and screened for tumorigenicity in athymic nude mice. Hybrid clones were obtained that showed a dramatic tumor suppression and contained a 2-megabase fragment of human chromosomal material encompassing the region 3p21 near the interface with 3p22. This should be the first step toward isolating the tumor suppressor gene.
Chen et al. (1996) analyzed microsatellite repeat markers in plasma, tumor samples, and normal cells from 21 patients with a confirmed diagnosis of small cell lung carcinoma (SCLC). They reported that microsatellite alteration (including loss of heterozygosity or the appearance of new size forms) occurred in 16 out of 21 SCLC tumors and in 15 out of 21 plasma samples. In 52% of cases the marker UT762 on chromosome 21 was altered, and in 38% of cases the marker AR (313700) on the X chromosome was altered. Chen et al. (1996) concluded that analysis of plasma DNA may constitute a new tool for tumor staging, management, or possibly detection. See also 275355.
Watkins et al. (2003) investigated a role for the Sonic hedgehog (SHH; 600725) pathway in regeneration and carcinogenesis of airway epithelium. They demonstrated extensive activation of the hedgehog pathway within the airway epithelium during repair of acute airway injury. This mode of hedgehog signaling is characterized by the elaboration and reception of the SHH signal within the epithelial compartment, and immediately precedes neuroendocrine differentiation. A similar pattern of hedgehog signaling in airway development during normal differentiation of pulmonary neuroendocrine precursor cells, and in a subset of small cell lung cancer, was also observed. Small cell lung cancer tumors maintain their malignant phenotype in vitro and in vivo through ligand-dependent hedgehog pathway activation. Watkins et al. (2003) proposed that some types of small cell lung cancer might recapitulate a critical hedgehog-regulated event in airway epithelial differentiation. This requirement for hedgehog pathway activation identified a common lethal malignancy that may respond to pharmacologic blockade of the hedgehog signaling pathway.
By sequencing 29 SCLC exomes, 2 genomes, and 15 transcriptomes, Peifer et al. (2012) found that compared to other tumor types in global sequencing studies, SCLC exhibits an extremely high mutation rate of 7.4 protein-changing mutations per million basepairs. Integrated analyses of various data sets to identify pathogenetically relevant mutated genes found evidence in all cases for inactivation of TP53 (191170) and RB1 (614041) and identified recurrent mutations in the CREBBP (600140), EP300 (602700), and MLL (159555) genes, which encode histone modifiers. Peifer et al. (2012) also observed mutations in PTEN (601728), SLIT2 (603746), and EPHA7 (602190), as well as focal amplification of the FGFR1 (136350) tyrosine kinase gene. Finally, Peifer et al. (2012) detected many of the alterations found in human SCLC tumors in Tp53 and Rb1 double-knockout mice.
In a study using exome, transcriptome, and copy-number alteration data from 36 primary human SCLC-normal tissue pairs, 17 matched SCLC and lymphoblastoid cell lines, 4 primary tumors, and 23 SCLC cell lines, Rudin et al. (2012) identified 22 significantly mutated genes in SCLC, including genes encoding kinases, G protein-coupled receptors, and chromatin-modifying proteins. Rudin et al. (2012) found that several members of the SOX family of genes were mutated in SCLC. They also found SOX2 (184429) amplification in approximately 27% of the samples. Suppression of SOX2 using shRNAs blocked proliferation of SOX2-amplified SCLC lines. RNA sequencing identified multiple fusion transcripts and a recurrent RLF (180610)-MYCL1 (164850) fusion. Silencing of MYCL1 in SCLC cell lines that had the RLF-MYCL1 fusion decreased cell proliferation.
George et al. (2015) sequenced the genomes of 110 SCLCs and identified biallelic inactivation of TP53 and RB1 (614014), sometimes by complex genomic rearrangements, in nearly all tumors. Two tumors with wildtype RB1 had evidence of chromothripsis leading to overexpression of cyclin D1 (CCND1; 168461), revealing an alternative mechanism of RB1 deregulation. George et al. (2015) concluded that loss of the tumor suppressors TP53 and RB1 is obligatory in SCLC. The authors also discovered somatic genomic rearrangements of TP73 (601990) that create an oncogenic version of this gene that lacks exons 2 and 3 (TP53-delta-ex2/3). In rare cases, SCLC tumors exhibited kinase gene mutations, providing a possible therapeutic opportunity for individual patients. Finally, George et al. (2015) observed inactivating mutations in NOTCH family genes in 25% of human SCLCs. Accordingly, activation of Notch signaling in a preclinical SCLC mouse model strikingly reduced the number of tumors and extended the survival of the mutant mice. Furthermore, neuroendocrine gene expression was abrogated by Notch activity in SCLC cells.