Molecular Genetics of Cancer

Molecular Genetics of crabmeatINTRODUCTIONIt has been established that cig atomic number 18tcer is a componenttic disease, characterized by interplay of summercater form of the onco ingredients and tumor suppressor divisor cistrons go alonging to the un directled gain and cattle ranch of ordurecer prison cubicles. While nigh of the fun genes whitethorn be inherited, others communicate in the somatic mobile phones of the individuals, which scum bag divide and form neoplasm. Completion of gracious Genome Sequencing Project has generated a wealth of novel information ab unwrap the mutations that set forth a cell to become malignant. It has now been possible to understand to corking extent the relationship between genes and malignant neoplastic disease, and how mutations, chromosomal qualifyings, viruses and environmental agents play a role in the development of crabmeat. In this chapter current understanding of the reputation and yard of cancer has been prese nted.CELL CYCLE AND CANCERDuring mitotic cell division, in every cell, either chromo roughs must duplicate faithfully and a reduplicate of the each has to be distri only whened to progeny cells. Progression finished the cell musical rhythm is controlled by the activities of umteen genes. At distinct stages in the cell bi daily round there subsist control points (G1, G2, S, and M stages) at which the cell cycle is arrested if there is detriment to the genome or cell-cycle machinery. Such mechanism helps to recreate the damage or destroy the cell. through with(predicate) this process it is possible to prevent the calamity of dividing a defective cell and from becoming cancerous.Proteins and enzymes called cyclines and cycline-dependent kinases (Cdks) respectively be the detect components that argon involved in the regulation of events in the checkpoints. At the G1-to-S checkpoint, devil different G1 cycline/Cdks labyrinthian forms, resulting in activation of the kinase . The kinase catalyzes a series of phosphorylations (addition of orthophosphate group) of cell-cycle control proteins, affecting the functions of those proteins and take to the woodsing to translation into the S phase. Similarly, at the G2-to-M checkpoint, a G2 cycline binds to a Cdk to form a complex. Phosphorylation of the Cdk by a nonher kinase keeps the Cdk unoccupied. Removal of a phosphate group from Cdk by a phosphataes enzyme activates the Cdk. Thereafter, the cell moves from S to M phase, due to phosphorylation of proteins by Cdk.Regulation of kiosk Division in usual CellsDivision of convention cells is castd by both extracellular and cellular molecules that operate in a complicated foreshadow system. Steroids and hormones made in other tissues atomic number 18 extracellular molecules, which influence the growth and division of both(prenominal) other tissues. These extracellular molecular argon known as growth factors, which bind to specific receptors on their ch annelize cells. The receptors are proteins that span the germ plasm membrane, and the growth factor binds to the part of the receptor which lies outside of the cell. The signal is accordingly transmitted to an intracellular part done the membrane-embedded part of the receptor. Thereafter, the signal is relayed through and through a series of proteins, which ultimately activate nuclear genes involved in stimulus and division of cells through organisation factors (Fig 13.1a). In the opposite direction, prohibition of cell growth and division is regulated by growth-inhibiting factors (Fig 13.1b). The process which involves either growth-stimulatory or growth-inhibitory signal after binding of the extracellular factor to the receptors is called signal transduction, and the proteins involved in such process are called signal transducers. Cell division in rule cells takes place only when there exist balance between stimulatory and inhibitory signals from outside the cells. Any mu tation either in the stimulatory or inhibitory genes or genes encoding cell get on receptors involved in cell cycle control whitethorn pretend imbalance and loss of control of cell division.CANCERS ARE GENETIC distemperClinically, cancer is defined as a large number of complex diseases that be catch differently depending on the cell types from which they originate. However, at the molecular level, all cancers exhibit common characteristics, and thereof they can be grouped as a family. All cancer cells share cardinal fundamental properties unregulated cell prosprightlinessration, characterized by abnormal growth and division, and metastasis, a process that furnish cancer cells to spread and invade other parts of the body. When a cell loses its genetic control over its growth and division, it may give countermand to a benign tumour, a multicellular mass. Such tumours may induce no serious harm and can often be remote by surgery. However, if cells of the tumour also acquire t he ability to break loose, inclose the blood stream, invade other cells, they may induce formation of alternative tumours elsewhere in the body. Such tumours are called malignant, which are difficult to work on and may become life threatening. A benign tumour may become malignant through multiple steps and genetic mutations.Mutations in three kinds of genes can leads to cancer. These are proto-oncogenes, tumour suppressor genes and mutator genes. Mutant proto-oncogenes are called oncogenes, are usually more active than normal cells. The product of oncogenes stimulates cell proliferation. The normal tumour suppressor genes inhibit cell proliferation, epoch the mutants frame in tumour cells have lost their inhibitory function. The normal mutator genes are require to ensure fidelity of replication and maintenance of genome integrity, while mutant mutator genes in cancer cells make the cells prone to accumulate mutational errors.RETROVIRUS AND ONCOGENES nigh cancer causing living creature viruses are RNA viruses known as retroviruses, and the oncogenes carried by RNA tumour viruses are altered forms of normal animal force cell genes. Infection with retroviruses can transform normal horde cells to the neoplastic state, and such cells proliferate in an torrential manner to build tumour. Examples of retroviruses include human immunodeficiency virus (HIV-1), mouse mammary tumour virus, felin leukemia virus, and Rous sarcoma virus. A typical retrovirus particle has a protein total, which often is icosahedral in shape, with two copies of plus-sense (means directly translatable) single stranded RNA molecule (7kb and 10 kb). The core is surrounded by an envelope with virus-encoded glycoproteins inserted into it (Fig 13.2). The virus enters the force cell by interacting with the innkeeper cell surface receptor through its glycoproteins present in the envelope.To understand how retroviruses cause cancer in animals, it is essential to know their life cycle. Rou s sarcoma virus (RSV) is one of the earliest retrovirus studied on induction of cancer. When a retrovirus like RSV infect a cell, the RNA genome is released from the viral particle, and a persona stranded deoxyribonucleic acid copy of the genome is made by reverse transcriptase (Fig 13.3). This is known as proviral deoxyribonucleic acid. The proviral DNA then enters the nucleus of the infected cell, and integrates into the master of ceremonies chromosome at random locations. The corporate DNA copy is called provirus. At the left wing end of all retroviral RNA genomes consists of the sequence R and U5, and U3 and R at the right end. Powerful enhancer and promoter elements are located in the U5 and U3 sequences (Fig 13.3). During proviral DNA synthesis by reverse transcriptase, the end sequences are duplicated to unwrap long terminal repeats of U3-R-U5 (LTRs in Fig 13.3), which contain many of the written text regulatory signals for the viral sequence. The two ends of the prov iral DNA are ligated to produce a circle, a double stranded molecule in which the two LTRs are next to each other. Staggered nicks are made in both viral and cellular DNAs, and integrating of the viral DNA begins. The viral DNA ends get together through recombination. Integration occurs at this point, and single stranded gaps are ligated. The integration of retrovirus proviral DNA results in a duplication of DNA at the target site, producing short, direct repeats in the horde cell DNA flanking the provirus.The proviral DNA is transcribe by forces RNA polymerase II, after integration into the host DNA. The retroviruses have three protein- coding genes for the virus life cycle gag, pol, and env (Fig 13.3). The gag gene encodes a precursor protein that is cleaved to produce virus particle protein. The pol gene encodes a precursor protein which produces an enzyme called reverse transcriptase, unavoidable for the integration of the proviral DNA into the host chromosome. The env gen e encodes the precursor to the envelop glycoprotein. The progeny virus particles are produced when transcription products of the entire integrated viral DNA are packed into recent viral particles. The new virus particles are released and can infect new host cells.A retrovirus may induce cancer in the host cells through two different ways. First, the proviral DNA may integrate by chance near one of the cells normal proto-oncogenes. The dependable enhancers and promoters in the provirus then stimulate transcription of proto-oncogenes present in the host cell at high levels or at inappropriate timing. This leads to stimulation of host-cell proliferation. Second, a retrovirus may pick-up a copy of a host proto-oncogene and integrates it into its genome (Fig 13.4). The integrated oncogene may mutate during the process of transfer into the virus, or it may be expressed at abnormal levels, due to action of the viral promoters. Retroviruses that pack these viral oncogenes can infect and transform normal cells into tumour cells.Different oncogenic retroviruses carry different oncogenes. Most oncogenic retroviruses can non replicate as they do not have a full set of life-cycle genes. Thus they cannot change growth properties of the host cells. They are called nononcogenic retroviruses. HIV-1 is a nononcogenic retrovirus. On the contrary, RSV is an oncogenic retrovirus as it can replicate its oncogenes and can affect the growth and division of the infected host cells. Viral oncogenes, genetically called v-oncs are responsible for many different cancers. The v-oncs of RSV is called v-src. conflicting RNA tumour viruses, DNA tumour viruses do not carry oncogenes. Their mechanism for transforming cells is completely different. They transform normal cells to cancerous state through the action of genes present in the viral chromosome. DNA tumour viruses are found in five major families of DNA viruses which include papovaviruses, lues venerea viruses, hepatitis B viruses, herpes viruses and adenoviruses.After infection, the DNA tumour viruses produce a viral protein that activates DNA replication in the host cell. Then, utilizing host proteins, the viral genome is replicated and transformed. After producing large number of progeny viruses, they lyses the host cell and the viruses thus released can infect other cell. Rarely, the viral genome instead of replicating gets integrated into the host genome. Thereafter if the viral protein that activates DNA replication of the host cell is synthesized, this pass on lead to division and proliferation of the host cell converting normal cell to cancerous state. Basically, the cells move from G0 phase to the S phase of the cell cycle.The DNA viruses which induces cancers are papillomaviruses (HPV 16 and 18), human T-cell leukemia virus (HTLV-1), hepatitis B virus, human herpesvirus 8, and epstein-barr virus. Some of these viruses cause benign tumours such as pelt and venereal warts in humans. variety is cause d by the key viral genes, E6 and E7, which encode proteins that activate furtherance through the cell cycle. However, in most of the shimmys, virus infection alone is not sufficient to trigger human cancers. Other factors like DNA damage, gathering of mutants in cells oncogenes and tumour suppressor genes, are required to induce cancer in multiple pathways. Some transducing retroviruses, their viral oncogenes, viral protein and type of cancer bring on is presented in Table 13.1.CANCER AND GENOME constancyCancer cells are characterized by the heraldic bearing of chromosomal translocations, deletions, aneuploidy, and DNA amplification. grow cancer cells also show similar genomic instabilities. Study of the specific chromosomal defects can be used to diagnose the type and stage of the cancer. For example, inveterate myelogenous leukemia (CML) gene C-ABL from chromosome 9 is translocated to the chromosome 22 in the region of gene BCR. The coalesced ABL-BCR gene encodes for a chi meric ABL-BCR protein, which produces an abnormal signal transduction molecule that stimulates the CML cells to proliferate. The normal ABL protein (protein kinase) acts within signal transduction pathway, transferring growth factor signals from the external environment to the nucleus, thereby control cell division.Defect in the DNA repair genes can also induce cancer. For example, Xenoderma pigmentosum (XP), a disease in which the skin becomes highly sensitive to UV light and other carcinogens. Patients with XP often develop skin cancer. Cells of XP are defective in nucleotide excision repair, with mutations appearing in any one of the seven genes whose products are required to carry out DNA repair. Hereditary nonpolyposis colorectal cancer (HNPCC) is also caused by mutations in genes controlling DNA repair. Patients affected by HNPCC have an increased luck of developing colon, ovary, uterine, and kidney cancers. At least eight genes are associated with HNPCC, and four of these gen es (MSH2, MHS6, MLH1, and MLH3) control DNA mismatch repair. Mutations in any one of these genes can lead to development of cancer.EPIGENETICS AND CANCEREpigenetics includes those factors that affect heritable gene expression but do not alter the nucleotide sequence of DNA. Examples of epigenic modifications are DNA methylation, acetylation and phosphorylation of histones etc. Modifications caused through these processes can be inherited and affect gene expression. X-chromosome inactivation, heterochromatin gene expression are such examples. Cancer cells contain major alterations in DNA methylation. In general, there is much less DNA methylation in cancer cells compared to normal cells. On the other hand promoters of some genes are highly methylated in cancer cells. Apparently these changes lead to the release of transcription repression over the bulk of genes that would otherwise remain silent, while at the same time repressing transcription of genes that would normally regulate fu nctions such as DNA repair, cell cycle, and cellular differentiation. The genes MLH1 and BRCA1, involved in DNA repair mechanism, are transcriptionally silenced by hypermethylation in many cancer cells. Methylation profiles can be used to diagnose types of tumours and their possible chassis of development.It has also been observed that histones are also modified in the cancer cells. These modifications are due to mutations in the genes that encode histone acetylases, deacetylases, methyltransferases, and demethylases. Since the epigenetic modifications are reversible, epigenetic- based therapies may be useful for cancer treatments.APOPTOSIS AND CANCERIf a normal cell encounters defective processing in DNA replication, DNA repair or chromosome assembly, they do not al piteoused to continue through the cell cycle, gutter the conditions are corrected and thereby reduces the chances of accumulation of defective cells. In case the damage of the DNA is irreparable, the cell may go throu gh a second line of defence called programmed cell death or apoptosis. programmed cell death is controlled genetically, and is an inherent process to eliminate certain cells that are not required for by the final adult organism. In this process, the nuclear DNA becomes fragmented, immanent cellular structures are disrupted, and cell dissolves into small spherical (apoptotic) bodies. Thereafter, these bodies are engulfed by the phagocytic cells of the immune system. The products of the genes Bcl2 and BAC can trigger or prevent apoptosis. In the cancer cells these genes are mutated, and as a result normal checkpoints in the cell cycle are inactivated. Such cells remain defective and cannot bear up under apoptosis.TUMOUR SUPRESSOR GENESHenry Harris in late 1960s observed that some cell lines, derived from the somatic hybrid of normal rodent cells and cancer cells, did not form tumours, instead established a normal growth pattern. He speculated that products of some genes present in t he normal cells had the ability to suppress the uncontrolled proliferation of cancer cells. These genes are called tumour suppressor genes. Inactivation of tumour suppressor genes has been linked to the development of a wide variety of human cancer, including colon, lung and meet cancers.With the development of positional cloning technique, it has become possible to isolate tumour suppressor genes. In this technique, variations in the genetic characters present in the cancer cells and/or in cells of patients with inherited cancer predisposition are identified. humanity of variations indicate occurrence of mutations and help to study such mutations through cloning. Through this technique several tumour suppressor genes are identified in humans (Table 13.2).The p53 Tumour-Suppressor GeneIn human cancer cells p53 is the most ofttimes mutated gene. The nuclear protein encodes by the gene p53 acts as a transcription factor. It can stimulate transcription or repress more than 50 differe nt genes. Although the p53 protein is continuously synthesized, it is rapidly degraded and thus is present in low levels. When p53 protein binds to another protein called Mdm2, it induces degradation and sequesters the transcriptional activation domain of p53. It also prevents conversion of inactive p53 protein to active form. In case Mdm2 protein gets dissociated from p53 protein then rapid increase in the activated p53 protein takes place at nuclear level. Such dissociation is induced due to creation of dsDNA breaks, chemical damage in DNA and presence of DNA-repair intermediates. Increase in the level of p53 protein leads to increased protein phosphorylation, acetylation, and other post translational modifications.The products of p53 gene control the movement of the normal cells through different phases of the cell cycle. Activated p53 proteins can i) stimulate transcription of p21 protein (which arrests progression from G1-S checkpoint of mitotic cycle), ii) regulate gene expres sion that retard replication of DNA (this helps in repair of the damage DNA before replication), and iii) block damaged cells (DNA damage occurred during S phase) from progression from G2 to M checkpoint by regulating expression of other genes.