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The lungs are the main organs of respiration which are two in number placed one on either side within the thorax, and separated from each other by the heart (Fig: 1). Lungs are highly elastic. The lung surface is smooth, shining, and marked out into numerous polyhedral areas, indicating the lobules of the organ. The right lung usually weighs about 625 gm, the left being 567 gm. Both of them are conical in shape. The right lung, although 2.5 cm shorter than the left, in consequence of the diaphragm rising higher on the right side to accommodate the liver. It is broader due to the inclination of the heart to the left side. Its total capacity is greater and it weighs more than the left lung. The main structures constituting the root of each lung are the two upper pulmonary veins in front, the pulmonary artery in the middle, and the bronchus, along with the bronchial vessels at behind.
Posterior it is in contact with the esophagus. The Right Bronchus is broader, shorter, and more vertical in direction than the left, which is about 2.5 cm long and enters the right lung nearly opposite to fifth thoracic vertebra. About 2 cm. from its starting point it gives off a branch to the upper lobe of the right lung which is termed as the eparterial branch of the bronchus. The bronchus now passes below the artery, and is known as hyparterial branch where it devides into two branches for the middle and lower lobes. The Left Bronchus is smaller in diameter but longer than the right, about 5 cm long. It enters the lung root opposite to sixth thoracic vertebra. The left bronchus contains no eparterial branch.
Cancer of the lung arises from aberrations in body's cell. Normally, the body maintains a system of checking and cell growth balancing in order to let cells divide to produce new cells when there is a need of new cells. Any change in this system results in uncontrolled division and proliferation of cells that eventually leads to a mass known as a tumour.
Tumours may be benign or malignant. In case of cancer we refer to malignant tumours. Benign tumours can be removed in most cases and do not disseminate to other body parts. Malignant tumours, on the other hand, grow aggressively and invade other body tissues, that allows tumour cell entry into the circulatory system and then to other parts of the body. This process is termed as metastasis. Since lung cancer tends to metastasize very early after it forms, it is very life-threatening and also difficult to treat. Adrenal glands, liver, brain and bone are the most common regions of lung cancer metastasis. The lung is also a very common site for metastasis from tumours in other parts of the body. Tumour metastases are made up of the same type of cells as the original (primary) tumour. For example, if prostate cancer metastasizes through bloodstream to lungs, it is metastatic prostate cancer of lungs and is not a lung cancer.
Lung cancer incidence is strongly correlated to cigarette smoking, with about 90% of lung cancers arise due to tobacco smoking.
Passive smoking or intake of tobacco smoke by non-smokers, who live with / stays in the vicinity of smokers, also possess risks for lung cancer development. Research has shown that non-smokers who stays with a smoker have a 24% increase in risk for developing lung cancer when compared with non-smokers who do not stay with a smoker.
Other factors / causes include exposure to asbestos fibers, radon gas, genetic factors such as heredity and other modes of air pollution such as from motor vehicles.
Lung cancers otherwisely called bronchogenic carcinomas are broadly devided into two major categories: small cell lung cancers (SCLC) and non-small cell lung cancers (NSCLC). This classification is based on microscopic view of the tumour cells. These two cancer types grow and metastasize in different ways and may have different treatment strategies.
SCLC comprises about 20% of all lung cancer types possessing rapid growth characteristics. SCLC is strongly related to cigarette smoking, with only 1% of these tumours occurring in non-smokers. SCLC usually originates form the cells lining the trachea or bronchi exhibiting rapid metastasis. NSCLC are the most common lung cancers, accounting for about 80% of all lung cancers. NSCLC can be divided into three main types that are named according to the type of cells found in the tumour:
Adenocarcinomas which comprise up to 50% of all NSCLC. While adenocarcinomas are related to smoking, like other lung cancers, this type is seen as well in nonsmokers who develop lung cancer. Most adenocarcinomas generate in the outer, or peripheral, areas of the lungs. Bronchioloalveolar carcinoma, a subtype of adenocarcinoma frequently occurs in many regions in lungs and metastasizes along the pre-existing alveolar walls.
Squamous cell carcinomas were initially more common than adenocarcinomas. Presently, they account for about 30% of all NSCLC. Squamous cell cancers arise mostly in the central chest area in the bronchi.
Large cell carcinomas othewisely known as undifferentiated carcinomas, are the least common NSCLC type.
Mixtures of different types of NSCLC are also seen .
The most common symptoms include: having cough most times, breathe shortening, blood in sputum while coughing, appetite loss, fatigue, weight Loss.
Some less common symptoms include: hoarse voice, difficulty in swallowing, swelling face or discomfort feelings under the ribs on the right side (from the liver) and breadth shortening .
CANCER OF TRACHEA
Tracheal cancers are uncommon comprising about 0.1% (1 in 1000) of all cancers. The most common types include squamous cell carcinoma and adenoid cystic carcinoma. Squamous cell cancers commence from cells which line different parts of the body such as theÂ airways, the mouth and the gullet. Adenoid cystic cancers are rarer and arise from glandular tissue.
Exact causes of tracheal cancers are still unknown. However, smoking is associated with squamous cell cancer of the trachea. There is no evidence relating adenoid cystic carcinoma of the trachea to smoking. Causes are still unknown like many cancers.
The most common symptoms are: dry cough, breath shortening, hoarse voice, difficulty in swallowing, fevers, repetitive chest infections, ejecting blood while coughing.
Bronchial cancer is a kind of lung cancer type which primarily affects the bronchial tubes. The bronchial tubes, also known as bronchi, are thin passages that connect the trachea/windpipe to the lungs and facilitate the intake of oxygen and exhalation of carbon dioxide. This cancer typically grows as a tumour on the bronchi, but can spread to other parts of the body and make the affected parts unable to function properly.
One of the first notable symptoms of bronchial cancer is persistent cough. As the cancerous tumours grow on the bronchi, they can cause a person to begin ejecting blood and mucus during coughing. Since bronchi constitute a large part of the respiratory system, cancer can result in breath shortening because the tumours may cause a partial obstruction between the windpipe and lungs.
Bronchial cancer doesn't have a completely established cause. Smoking cigarettes or its exposure for long periods may enhance a person's risk of developing cancerous bronchi tumours greatly. People exposed to second hand smoke may still develop this type of cancer rarely, although the exact reason behind this is still unknown.
GENETIC LEVEL UNDERSTANDING OF SMOKING INDUCED LUNG CANCER
Lung cancer & tumour vasculature
In an article of tobacco related disease research program (TRDRP) a plan for vascular targeting strategy has been established where the difference of protein expression in endothelial cells of normal tissue vasculature and cancerous tissue vasculature has been exploited. 2-D gel electrophoresis results of normal tissue vasculature endothelial proteins were compared with the tumour vasculature endothelial proteins revealed several well separated protein spots in tumour vasculature endothelial protein analysis. 5 new spots (TE: 1-5) appeared in tumour vasculature endothelial proteins that appear to be lung tumour induced. Antibodies used against TE-3 revealed it to be lung tumour specific by western blot analysis, tissue immunostaining and in vivo targeting. Monoclonal antibodies (m-Abs) were developed and one of them - TC004 appears to be lung tumour specific. Cyclooxygenase - 2 (COX-2) is an enzyme, found in lung cancer cells responsible for suppressing patient's immune system, thus contributing to the growth of lung cancer. The next objective is to find out how COX-2 regulates the progression of tumour growth in lung cancer, which can form the basis of designing COX-2 inhibitors in prevention and therapy for lung cancer.
COX-2 has a slight role on overall survival in NSCLC (without statistical significance). Stage-I NSCLC showed statistically substantial effect of COX-2 on survival. RT-PCR showed significant results. COX-2 overexpression is related to micro-vascular angiogenesis and resistance to apoptosis in case of lung cancer. COX-2 also decreases host immunity and changes cell adhesion with enhancement of intrusion and metastasis. COX-2 appears early in oncogenesis for squamous cell carcinoma as well as for adeno carcinoma. COX-2 expression enhances in bronchial preneoplastic lesions at the stage of severe dysplasiaand particularly in clones of cells showing atypia, which indicates an active role of COX-2 in bronchial epithelium cells transformation to malignancy.
Mascaux et al. Has COX-2 a Prognostic Role in NSCLC: A Systematic Review of Literature with Meta-analysis of Survival Results. British Journal of Cancer. 2006
Two COX-2 inhibitors (NS398 & Nimesulide) were related to the induction of p21 m-RNA and protein expression. However antigrowth effect of COX-2 inhibitors and their potential to induce p21 were not affected by COX-2 si-RNA indicating their actions were COX-2 independent. Rather activation of MEK-1/ErK pathway was essential since COX-2 inhibitors induced the phosphorylation of ErK's and their effects were stopped by PD98095, an inhibitor of this pathway. Both NS398 and nimesulide induced p21 gene promoter activity and this was prevented by PD98095. COX-2 inhibitors enhanced nuclear protein binding to the sp1 site in the promoter region of p21 gene. P21 antisense oligonucleotides prevented the effects of COX-2 inhibitors on cell growth. Finally determined that COX-2 inhibitors inhibit NSCLC cell growth by inducing the expression of p21 gene via MEK-1/ErK signalling and DNA - protein interactions involving sp1. These observations disclosed a mechanism for p21 gene regulation by COX-2 inhibitors in lung carcinoma cell growth and this pathway acts as a potential target for therapy.
Hana et al. COX-2 Inhibitors Suppress Lung Cancer Cell Growth by inducing p21 via COX-2 Independent Signals. International Journal for Lung Cancer and Other Thoracic Malignancies. 2006.
8473Tâ†’C polymorphism in the 3'-UTR site of the COX-2 gene was linked with the risk of lung cancer, and particularly for the risk of AC. Campa et al. reported linking of 8473 TC and the 8473 CC genotypes with an importantly increased risk of lung cancer in a Norwegian population.
Park et al. Relationship between cyclooxygenase 8473T>C polymorphism and the risk of lung cancer: a case-control study. BMC Cancer. 2006. 6(70):1-7.
TP53 has a central role in mediating cellular responses to genotoxic abuses through its effects on gene transcription, DNA synthesis and repair, genomic plasticity and apoptosis. Smoking is the main risk factor for lung cancer and tobacco carcinogens have been shown to exert a direct mutagenic action on DNA of cancer-related genes and TP53 particularly.
Matakidou et al. TP53 polymorphisms and lung cancer risk: a systematic review and meta-analysis. Mutagenesis. 2003. 18(4):377-385.
The study showed an elevated risk of lung cancer among subjects containing the ERCC2/XPD 751Gln/Gln genotype. Evidences suggest that some polymorphisms in DNA repair genes play a role in carcinogenesis, most notably the ERCC2/XPD Lys751Gln and XPA G23A polymorphisms.
Chikako Kiyohara1 and Kouichi Yoshimasu. Genetic polymorphisms in the nucleotide excision repair pathway and lung cancer risk: A Meta analysis. Int. J. Med. Sci. 2007. 4.
P16/CDKN2A is a crucial tumour suppressor gene which is frequently altered in lung cancer through 5â€² CpG island promoter hypermethylation and homozygous deletion, and rarely through point mutation. A total of 5 (6.7%) mutations was found in the whole series, including 2 asbestos-exposed cases and 3 unexposed cases, and all mutations were detected in former or current smokers (more than 20 P-Y). All mutations were point mutations resulting in amino acid changes. One mutation was identified in exon 1Î±, at codon 42 (124Aâ†’T,N42Y), changing the encoded asparagine into tyrosine. Four mutations were identified in exon 2, at codon 57 (170Câ†’T,A57V), changing the encoded alanine into valine, at codon 80 (238Câ†’T,R80X), changing arginine into a stop codon, at codon 85 (253Gâ†’C,A85P), changing alanine into proline, and at codon 85 (322Gâ†’T,D108Y), changing aspartic acid into tyrosine.
Andujar et al. p16INK4A inactivation mechanisms in non-small-cell lung cancer patients occupationally exposed to asbestos. Lung Cancer. Author Manuscript. 1-10.
Smokers discovered a chromosome 15 site that when mutated significantly increases risk for developing lung cancer in case of smokers by another (30-80) %. Overall (20-30) % lung cancer risk is imparted to smokers depending upon whether the individual has one / two copies of what researches say the 15q24 susceptibility locus. Mutations can be found in exons or regulatory sites. Chromosome 15 encodes many genes including few that encode nicotinic acetylcholine receptors. These receptors bind to nicotine and its derivatives which are found in lung cells. GWAS (Genome Wide Association Studies) technique was applied by researchers to isolate this portion on chromosome 15. Relationship between SNP's and lung cancer were found on chromosome 15 using this technique. Also found a relation between SNP variation and number of cigarettes smoked per day. Genes altered in 15q24 locus most probably play a direct casual role in lung cancer by interfering with nicotine acetylcholine receptors and stimulating tumour growth (as opposed to an indirect casual role related to nicotine addiction). Smokers with missing 15q24 locus have 10 fold greater lung cancer risk compared to non-smokers having the locus.
Molecular Biology of Lung Cancer - Overview.
Estimation studies reveal that 90% of lung cancers are caused by cigarette smoking. Still, a variety of genetic aberrations are observed in lung cancer cells. TP53 gene mutations are the commonest occurring in 80-100% of SCLC and 50-80% of NSCLC. RB1 mutations are also observed in most SCLCs ~ (80-90%), while their frequency is less in case of NSCLC (20-30%). CDKN2A (P16 INK4A) aberrations are inversely correlated with RB1 mutations which are found in about 60% of NSLC, CDKN2A abnormalities are negligible in SCLC ~ (less than 1 in 10). Abnormal splicing of FHIT gene is another frequent mutation accounting 75% of both SCLC and NSCLC. This gene has been proposed as a target of tobacco carcinogens and. RNA component of telomerase up regulation is observed in most lung cancers (98% of SCLC) which may prove to be a target for future therapies. Other genes concerning lung cancer includes MYCN, KRAS, TP73, MADH2, MADH4, PPP2R1B, and PTEN. Del (3p) and Del (9p) account for some common chromosomal aberrations. Some common gene polymorphisms are indicators of increased susceptiblity to lung cancer for some smokers (and passive smokers) compared to others. Particularly, the GSTM1 null allele is related to increased risk of lung cancer, especially in women.
Underwent a molecular analysis of the microsatellite alterations within the FHIT gene and the FRA3B site as well as at an independent locus on chromosome 10, D10S197, in lung cancers form heavy smokers and non-smokers. Loss of heterozygosity affecting at least 1 locus of the FHIT gene was found in 41 of 51 tumours in the smokers group (80%) but in only 9 of 40 tumours in non-smokers (22%). These findings indicate FHIT as a molecular target of carcinogens present in tobacco smoke. In lung cancer, abnormal FHIT transcripts, lacking key exons, and LOH, affecting microsatellite markers within the FHIT gene, have been detected in more than 70% of all lung cancer types.
Sozzi et al. Association between Cigarette Smoking and FHIT Gene Alterations in Lung Cancer. Cancer research. Cancer Research. 1997. 57:2121-2123.
Screened 40 primary lung tumours for the presence of point mutations within the exons of FHIT using PCR-single-strand conformational polymorphism. Reanalyses of exon loss using PCR revealed that 13 of 30 tumours failed to generate a PCR product, and 20 of 30 tumours were missing at least one FHIT exon or had loss (loss of heterozygosity or deletion) of one microsatellite marker, indicating that regions of the gene are homozygously deleted. FHIT gene contains a novel pattern of mutational inactivation not observed previously in other tumour suppressor genes, supposed to be influenced by the closeness of the FRA3B region.
Environmental tobacco smoke (ETS) exposure is considered a significant lung cancer risk factor for non-smokers. Comparison between non-smokers with no ETS exposure who developed lung cancer and never smokers with ETS exposure who developed lung cancer were more probable to be GSTM1 activity deficiency (i.e. GSTM1 null) due to a genetic polymorphism in the GSTM1 gene.
Glutathione S-transferases GSTM1, GSTM3, GSTP1 and GSTT1 are involved in the detoxification of active metabolites of several carcinogens in tobacco smoke. Studied the potential role of GSTM3 and GSTP1 gene polymorphisms either separately, or in combination with GSTM1 and GSTT1 gene polymorphisms using blood DNA from 150 lung cancer patients and 172 control individuals, all regular smokers. The frequencies of GSTM3, AA, AB and BB genotypes were 70.7%, 24.0% and 5.3% in cases and 72.7%, 24.4% and 2.9% in control individuals respectively. The frequencies of GSTP1, AA, AG and GG genotypes were 44.7%, 44.0% and 11.3% in cases and 50.0%, 37.2% and 12.8% in control individuals respectively. When studied separately, neither GSTM3 nor GSTP1 genotypes contributed significantly to the risk of lung cancer. The combined GSTM3 AA and GSTP1 (AG or GG) genotype conferred a nearly threefold risk when the GSTM1 gene was concurrently lacking (odds ratio 2.9, 95% confidence interval 0.7-12.1). Significant interactions were observed between pack-years of smoking and the combined GSTM3 AA and GSTP1 (AG or GG) genotype, or the combined GSTM3 AA, GSTP1 (AG or GG) and GSTM1 null genotype. The combination of these three a priori at risk genotypes conferred an increased risk of lung cancer among smokers with a history of at least 35 pack-years (odds ratio 2.7, 95% confidence interval 1.2-6.0), but not in lighter smokers.
It is hypothesized that susceptibility to nitrosamines, as determined by polymorphisms in CYP2E1, predominantly causes lung ACs. CYP2E1 plays a major role in the metabolic activation of nitrosamines. The CYP2E1 RsaI and DraI polymorphisms were not clearly related to SCC risk, but these homozygous variant genotypes were associated with a 10-fold (95% CI, 0.0-0.5) decrease in the risk of overall lung cancer (RsaI variant) and AC (DraI variant) compared to the homozygous wild-type genotypes. Inverse associations with these two closely linked CYP2E1 polymorphisms were also suggested for small cell carcinoma. In agreement with past experimental and epidemiological data, the associations found in this study between CYP1A1 and lung SCC and between CYP2E1 and lung AC suggest a certain specificity of tobacco smoke PAHs for lung SCC and tobacco-specific nitrosamines for lung ACs.
A gene expression signature characteristic of smoking that includes cell cycle genes, especially those involved in the mitotic spindle formation were identified (e.g., NEK2, TTK, and PRC1). Expression of these genes largely differentiates both smokers from non-smokers in lung tumours and early stage tumour tissue from non-tumour tissue, consistent with a significant role for this pathway in lung carcinogenesis induced by smoking.
Examined K-ras mutations in relation to polymorphisms in the cytochrome P4501A1 (CYP1A1) and glutathione S-transferase micro1 (GSTM1) genes in 246 patients with lung adenocarcinoma and 167 patients with lung squamous cell carcinoma. K-ras mutations were found in 33 of 413 (8.0%) tumours, and all K-ras gene mutations were found in habitual smokers. Among smokers with lung adenocarcinoma, K-ras mutations occurred with greater frequency in patients with the GSTM1 (-) genotype than in those with the GSTM1 (+) genotype. Patients with a combination of the CYP1A1 m1/m2 and GSTM1 (-) genotypes showed an increased probability of having mutated K-ras genes (OR, 6.00; p=0.031; 95% CI, 1.18-30.62) in comparison to those with the CYP1A1 m1/m1 and GSTM1 (+) genotypes. No significant association for squamous cell carcinoma was found. These findings suggest that K-ras mutations in smokers with lung adenocarcinoma may be due in part to accumulation of carcinogens, which is not adequately detoxified in individuals with certain CYP1A1 genotypes and the GSTM1(-) genotype.
P53 mutational patterns are different between smokers and non-smokers having lung cancer characterized by excess Gâ†’T transversions in smoking related cancers. The presence of G to T transversions is 30% in smokers' lung cancer and 12% in lung cancers of non-smokers. Tobacco related mutations are mostly G to T and G to A in case of non-tobacco related cancers, exclusively. Two major classes of tobacco smoke carcinogens are polycyclic aromatic hydrocarbons (PAH) and nicotine-derived nitrosamines. Endogenously methylated CpG dinucleotides are favouriter G to T transversion regions, accounting for more than 50% of such mutations in lung tumours. The same dinucleotide, when present within the CpG-methylated mutational reporter genes, is the target of G to T transversion hotspots in cells exposed to PAH compound benzo[a]pyrene-7, 8-diol-9, 10-epoxide.
There are more than 60 carcinogenic tobacco smoke constituents that include polycyclic aromatic hydrocarbons, aromatic amines, aldehydes and nitrosamines. In lung preferential binding of benzo[a]pyrene and Acrolein to guanine residues leads to an increased incidence of GCâ†’TA transversions. The increase in GCâ†’TA transversions was found to be related with an accompanying GCâ†’AT transition percentage decrease. This was observed in both lung and oral tumours and was reflected in laryngeal tumours of smokers. Base level - Gâ†’T transversions confer mutagenic specificity rather than at the codon level changes. Still positional spectra comparisons GCâ†’TA transversion incidences in case of lung, larynx and oral tumours from smokers define many codons as mutational hot spots in TP53, all of which occur in DNA-binding domains of concerned proteins. In ung and larynx both the codons were 157, 245, 248 and 273 which is in agreement with in vitro studies that indicates benzo[a]pyrene derivatives binding with greatest strength to codons 157, 248 and 273.
P16/CDKN2A promoter hypermethylation in lung cancer occurs as a result of tobacco smoking. P16/CDKN2A is an important tumour suppressor gene, frequently altered in lung cancer as a result of promoter 5'-CpG island hypermethylation and homozygous deletion, and rarely through point mutation.
Massively parallel sequencing strategies were used to sequence a small-cell lung cancer cell line, NCI-H209, to study mutational changes linked with tobacco smoking. 22,910 somatic substitutions were, including 134 in exons. Tumour suppressor gene RB1 is inactivated in 60-90 % SCLC cases due to tobacco smoking. RB1 and TP53 combined loss is a characteristic feature of SCLC.
Smoking does not completely explain the higher lung cancer risks within hereditary retinoblastoma survivors, but probably increases an already-elevated risk.
There is a greater risk of lung cancer in patients with hereditary retinoblastoma. RB1 mutation carriers may be highly susceptible to smoking-induced lung cancers.
A new protein called LMO-4 was identified which is a member of gene regulation protein family. Proteins of this group are linked to leukaemia. Large amounts of LMO-4 are found in proliferating bronchial epithelial cells. LMO-4 associated with Trip230 (another protein), which has been shown to bind to the retinoblastoma protein. Retinoblastoma protein aberrations, which normally inhibit cell proliferation, are concerns of lung cancer progression. Thus LMO-4 participates in the regulation of cell proliferation of normal and cancerous cells of lung.
p53 gene in smoking induced tracheal, bronchial & lung cancer.
25 (29%) patients were found to have somatic p53 mutations in their tumours. A large number (40%) of GC to TA transversions were observed, indicating their increased occurrence with increasing cumulative exposure to cigarette smoke.
p16 + smoking induced tracheal, bronchial & lung cancer. Mutations were detected in 3.9% (2/51) of the tumours; the tumours carrying mutations were from smokers. The incidences of loss of heterozygosity, homozygous deletion, and promoter methylation in 37 smokers vs. 14 non-smokers were; 45.9% vs. 28.6%, 16.2% vs. 7.1%, and 35.1% vs. 7.1%, respectively. Only the association between promoter methylation and tobacco smoking was statistically significant (P < 0.05). Epigenetic aberration is considered to be a major causative event in p16 silencing by tobacco smoking. Loss of p16 protein expression was apparent in 49% (25/51) of the tumours, and was associated with tobacco smoking (P < 0.05) and with histological type (P < 0.05). Thus tobacco smoking leads to inactivation of the p16 gene mainly through the epigenetic mechanism, ultimately increasing the risk of NSCLC, especially the squamous cell histological type.
K-ras in smoking induced tracheal, bronchial & lung cancer.
Codon-12 point mutation leading to K-ras activation was described in lung adenocarcinomas in various models of experimental lung tumours caused by chemical carcinogens. Codon 12 mutations were confirmed by sequencing analysis of ten samples, were G to T transversion, was mostly observrd, TGT and GTT in bronchial carinae and lung tumours.
RASSF1A gene in smoking induced tracheal, bornchal & lung cancer.
Methylation rate was 32% for RASSF1A. Hypermethylation of RASSF1A promoter was found to be greatly linked to age at starting smoking. No relationship was found between the methylation status of RASSF1A promoter and other smoking variables, such as pack-years, smoking status, and smoking duration. Young smokers who started smoking before age 19 were 4.23 times more probable to contain RASSF1A promoter hypemethylation compared to smokers who started smoking after the age of 19. Cigarette smoking at an early age is linked to RASSF1A promoter hypermethylation which can act as an independent prognostic factor in primary non-small cell lung cancer.
NORE1A in smoking induced tracheal, bronchial & lung cancer.
Inactivation by hypermethylation of a RASSF1A homologue, NORE1A, has recently been observed in human cancers. Analyzed NORE1A CpG island hypermethylation of 61 non-small-cell lung carcinomas. Epigenetic alteration of NORE1A is confined to lung tumours with a wild-type K-ras: 88% (15 of 17) of the tumours with NORE1 hypermethylation.
Mechanism (Carcinogenesis) Of Tobacco Smoke Induced Lung Cancer With Respect To Genetic Level Understanding.
Direct interactions of metabolically activated carcinogens with crucial genes, such as p53 tumour suppressor gene and Kirsten-ras (KRAS) oncogene, are fundamental to the theory that specific carcinogens form the connection between nicotine addiction and lung cancer. The p53 gene plays a crucial role in balancing of cell proliferation and apoptosis.TP53 mutation is observed in almost half all cancer types. A sample of 550 p53 mutations in lung tumours accounted 33% Gâ†’T transversions, 26% were Gâ†’A transitions. In most cases it is stated that Gâ†’A transitions at CpG regions in the p53 gene is a consequence of deamination of 5-methyl C. Cytosine methylation increases greatly the guanine alkylation at all CpG regions in p53 gene by a variety of carcinogens. O6-Alkylguanines are also likely to cause Gâ†’A transitions. BPDE selectively forms adducts at CpG regions in codons 157, 248, and 273 similar to three major mutation sites in the p53 gene in lung cancer. Methylated CpG islands are targets of a variety of activated carcinogens like BPDE. Diol-epoxides, pyridyloxobutylating intermediates derived from NNK and N8-nitrosonornicotine, hydroxyl amines derived from aromatic amines, as well as acrolein, crotonaldehyde, and 8-oxodeoxy-guanosine adducts can cause Gâ†’T transversions. Codon 12 mutations of the KRAS gene are found in 24%-50% of human primary adenocarcinomas but are rarely observed in other lung tumour types. These mutations are mostly found in smokers and ex-smokers than in non-smokers. The most commonly observed mutation is GGTâ†’TGT, which typically accounts for about 60% of the codon 12 mutations, followed by GGTâ†’GAT (20%) and GGTâ†’GTT (15%). The p16INK4a tumour suppressor gene inactivation is found in more than 70% of human non-small-cell lung cancers, via homozygous deletion or in association with aberrant hypermethylation of the promoter region. Co-ordinate p16 gene methylation is observesd in 75% of carcinoma in. Methylation of p16 was linked with loss of expression in tumours. p16 abnormal methylation has been indicated as an early marker for lung cancer. The cell cycle protein expression is related to the p16 and retinoblastoma (RB) genes. Loss of heterozygosity and exon deletions within the fragile histidine triad (FHIT) gene are linked with smoking habits in lung cancer patients and have been suggested as a target for tobacco smoke carcinogens. Among carcinogen-metabolizing enzymes encoding genes, cytochrome P450 genes - CYP1A1, CYP2D6, CYP2E1 and glutathione S-transferase (GSTM1) polymorphism received most attention. The CYP1A1 gene product, P4501A1, is activated by cigarette smoke in human lung and plays role in metabolism of PAHs. The GSTM1 gene assists in the detoxification of various carcinogens including PAH diol epoxides. Approximately 40%-50% of the human population is GSTM1 null genotype and this group possess higher risk of lung cancer.
Cigarette Smoke free radicals & Oxidative DNA Damage.
Cigarette smoke comprises free radicals and causes oxidative damage in humans. The fresh cigarette smoke gas phase consists about 600 mg of nitric oxide. The particulate phase comprises free radicals which are stable enough to be identified by electron spin resonance and spin trapping. Important free radical species was postulated to be a quinone-hydroquinone complex "held in a tar matrix". Further investigations indicated that the tar radical system is an equilibrium mixture of semiquinones, hydroquinones and quinones. It is indicated that this free radical complex leads to redox cycling that produces superoxide anion from molecular oxygen and contributes hydrogen peroxide and hydroxyl radical formation. The reactive species yielded in this cascade lead to DNA nicking. It has been revealed that nitric oxide in gas phase interactively acts with cigarette "tar" which lead to DNA single-strand nick in pBR322 plasmid DNA and indicated that peroxynitrite, produced from nitric oxide and superoxide anion, might be associated with this effect. In vitro experiments suggested that gas phase of cigarette smoke leads to lipid peroxidation of human blood plasma which is prevented by ascorbic acid addition. Both whole cigarette smoke and gas-phase cigarette smoke leads to the formation of carbonyls in human plasma. The ascorbic acid levels are lower in smokers compared to non-smokers. Measuring the raised circulating products of lipid peroxidation (F2 - isoprostanes) in smokers provided convincing evidences regarding oxidative damage. Fairly raised levels of 8-oxodeoxyguanosine, a miscoding adduct, in DNA from smokers' lungs, leukocytes, and sperm were found. Increased urinary excretion of 8-hydroxydeoxyguanosine were also been noted.
Stephen S. Hect. Tobacco Smoke Carcinogens and Lung Cancer (Review). Journal of National Cancer Institute. 1999. 91(14):1194-1210
Light smokers containing XPG - Asp/Asp genotypes had a less chance of developing lung cancer, particularly decreased risk of squamous cell carcinoma. XPG codon 1104 - His1104Asp confers genetic susceptibility to lung cancer development.
CSC exposure for 3 hrs reduced [35S]-Met/Cys FANCD2 levels approximately 80% as compared to untreated controls. Thus global suppression is suppressed and FANCD2 is not exempted. In vivo suppression of FANCD2 caused by cigarette smoke create recurring cycles of cytotoxicity and chromosomal instability (CIN). FANCD2 translation suppression renders cells more sensitive to clastogen/DNA damaging agents present in cigarette smoke upon cigarette smoke re-exposure. These responses likely create perfect conditions for the selection of new clones which resist apoptotic cues including those present in cigarette smoke.
Hays et al. Cigarette smoke induces genetic instability in airway epipthelial cells by suppressing FANCD2 expression. British Journal of Cancer. 2008. 98:1653-1661
Prostaglandin-endoperoxide synthase (PTGS), also known as cyclooxygenase is the key enzyme in prostaglandin biosynthesis, and acts both as a dioxygenase and as a peroxidase.
Â Location: 1q25.2-q25.3
8473Tâ†’C in the 3'-UTR region
Intron alteration, so no change in expression.
TP53 [13, 23, 34]
Tumour suppressor, apoptosis inducer, cell cycle regulator.
Mostly Gïƒ T transversion
Codon 157 & 158 - common hotspots for the corresponding change in smoking associated alteration. Formation of mutant residue at codon 157 of the p53 protein.
tumour suppressor gene
5â€²-CpG island hypermethylation and homozygous deletion
Tumour suppressor gene that encodes a triphosphate hydrolase.
LOH at D3S1300 and D3S4103 microsatellite markers residing in the epicenter of the fragile region (FRA3B) comprisign exon 5 and intron 5 of FHIT gene and at D3S1234, in the more distal 3' end of the gene.
Loss of all informative (heterozygous) markers reflecting loss of entire allele.
Encodes mu class glutathione S transferases that play a role in variety of carcinogens' detoxification including PAH diol epoxides.
Null genotype Polymorphism, indicates higher risk of lung cancer.
Null genotype indicates absence of glutathione S transferase.
CYP2E1 [6, 7]
Carcinogen metabolizing enzyme, plays a major role in the metabolic activation of nitrosamines in tobacco
Mostly CYP2E1*5B and CYP2E1*6 polymorphism - in 5' regulatory region there is Câ†’T replacement in 1019 position of CYP2E1*5B and Tâ†’A replacement in intron-6 at 7668 position.
A negative relation to adeno carcinoma i.e. higher the frequency of variant C2 allele of CYP2E1*5B and C allele ofCYP2E1*5B polymorphism lower the incidence. Also CYP2E1*5B polymorphism is associated with increased (~10 fold) enzyme activity.
Carcinogen metabolizing enzyme, mainly metabolism of polycyclic aromatic hydrocarbons (PAHs)
A thymine/cytosine point mutation in the MSPI restriction site of CYP1A1
Increased enzyme activity
RASSF1A [1, 2, 3]
Tumour suppressor gene
aberrant methylation of the CpG islands at promoter region
Loss of RASSF1A expression
XRCC1 [14, 15]
DNA repair pathway gene - mainly involved in base excision repair (BER).
Gâ†’A nt. Replacement at codon-399
Non-conservative amino acid change linked with enhanced levels of aflatoxin B1 adducts and glycophorin A somatic mutations.
DNMT-3B [14, 15]
Nucleotide excision repair (NER) gene
A novel promoter region undergoes Câ†’T transition
Promoter activity is enhanced.
XPD/ERCC2C2 [14, 15]
Both BER and NER of DNA lesions caused due to tobacco and other environmental carcinogens
EPHX1 / EPHX (previous name) [16, 17, 18]
Hydrolysis of arene and aliphatic epoxides to less reactive and more water soluble dihydrodiols by trans addition of water.
Exon 3 & 4 polymorphism
Exon-3 accounts for histidine to tyrosine at residue 113 and exon-4 accounts for arginine to histidine at residue 139.
MGMT [14, 19]
Codes for O6-alkylguanine-DNA-alkyltransferase associated with repair of O6-methylguanine-DNA (O6MG-DNA) adducts induced by NNK in cigarette smoke.
Deletion, mutation and rearrangements are rarely observed, but most commonly promoter hypermethylation (such as CpG site methylations)
Gene inactivation and increased Gïƒ A transition in NSCLC, chiefly in adenocarcinomatous cell lines.
XPG [14, 20, 21]
DNA excision repair, makes a 3' incision in DNA nt. Excision repair.
Codon 1104 polymorphism.
XPA [14, 22]
Acts as DNA binding protein in NER pathways, modulates damage recognition of DNA.
A23G (Aïƒ G, a SNP identified in the 5' non coding region of the gene) and G709A polymorphisms
Polymorphisms gave 3 possible genotypes - AA, AG and GG. AA genotypes exhibited decreased DNA repair capacity compared to GG genotypes.
Putative tumour suppressor gene
Exon 6 variation in NCI-H23 cell line. Gïƒ A transition in nt. 547
Aspartic acidïƒ Asparagine at amino acid 183 (GACïƒ AAC)
Intron 7: D16S3029, D16S3096, D16S504 - loss of heterozygosity.
Nucleotide excision repair
Asn118Asn - silent mutation.
Nucleotide excision repair
T2505C at exon 11
Serine is conserved - no amino acid change.