Once you have a tumor cells there is just a massive accumulation of more mutations. Tumors replicate more, and faster -- each division is an opportunity for mutation. Sporadic cancers are more common -- you inherit susceptibility but most important mutations are somatic during your lifetime. There can be strong, almost mendellian predisposition. Mutations in tumor suppressor (especially p53), tumor promoter (proto-oncogenes, which mutate to become oncogenes) and DNA mismatch repair genes. Cancer is any tumor which can spread -- a tumor develops in the following steps:
- Initiating mutation.
- Uncontrolled proliferation, dramatically increasing likelihood of additional mutations.
- Reduced dependence on growth signals and even more mutations.
- Anchorage, independence and metastasis.
Familial vs Sporadic Cancers
|Familial Cancers||Sporadic Cancer|
|Inherited mutation predisposes to cancer.|
|Additional somatic mutations are required.||Accumulation of somatic mutations.|
|Mutation rate impacted by environment.||Mutation rate impacted by environment.|
|Mutation rate impacted by background genetic factors.||Mutation rate impacted by background genetic factors.|
Chromosome abnormalities (like translocations), loss of heterozygosity, positional cloning of familial cancer genes and functional cloning all tip off which cancer genes are involved in a cancer.
|Tumor suppressor genes||Prevent inappropriate division and must be mutated in both alleles for a deleterious effect. The two-hit model is: in sporadic cancers, both tumor-suppressor alleles mutate somatically; in familial cancers, one defective tumor-suppressor allele is inherited and the other mutates somatically.|
|Tumor promoter genes||Aka proto-oncogenes, these promote division during development but are normally silent in adults. They become oncogenes through mutation and force the cell into continuous division.|
|DNA repair genes||Protect against DNA mutation due to replication or the environment. Mutation in a DNA repair genes leads to increased mutation of tumor suppressor and proto-oncogenes.|
|Oncogenes||Tumor Suppressor Genes|
|Normally activate growth.||Normally inhibit growth.|
|Dominant gain of function.||Recessive loss of function.|
|Somatic mutations.||Germline mutations in one allele are heritable, and mutation in second allele is somatic.|
|Active in gene transfection assay.||Not active in gene transfection assay.|
|Caused by translocations, gene amplifications and point mutations.||Caused by deletions, point mutations and other inactivating mutations.|
A notable tumor suppressor gene is p53.
Loss of Heterozygosity
Loss of heterozygosity is characterized by normal tissues showing two bands when probed -- one for each allele -- and tumor tissues only showing one band. This is an important mechanism of cancer, whereby a healthy cell with one faulty allele and one healthy allele suddenly has two faulty alleles. Loss of heterozygosity can occur via:
- Local events, with one allele garnering point mutations or the like.
- Somatic recombination, which is most likely.
- Loss and duplication, where one allele is lost and the other is duplicated in its place.
- Chromosome loss or partial deletion, leaving only the defective allele.
Recombination events are what usually lead to loss of heterozygosity. This happens via unequal crossing-over (improper recombiation) whereby part of one chromosome replaces the homologous region on the other chromosome, which leads to both chromosomes carrying the mutant region.
Normal cell require stimulatory signals to proliferate; tumor cells achieve autonomy where they proliferate without stimultion. Known proto-oncogenes include growth factors and their receptors (PDGF, FGF, EGF, CSF), signal transducers (ras, src, abl) and signal effectors (myc, fos, jun).
Oncogenes arise via:
- Translocation, caused by breakpoints near proto-oncogene leading to inappropriate expression, or even fusion with another protein. Different fusions are associated with different cancers.
- Structural mutation, caused by changes in coding regions of growth factor receptors and signal transducing proteins. A point mutation in ras leads to constitutive activation in the absence of growth factor stimulation.
- Amplification or regulatoroy mutation, leading to structurally normal oncogenes that are just produced at high levels. For example, multiple copies of a single gene or insertion of a viral LTR before a gene.
Oncogenes are isolated via the DNA transformation assay:
- Human tumor DNA transfected into mouse 3T3 cells, which are not immortal and are contact-inhibited (do not grow when crowded).
- Oncogenically transformed cells form proliferating colonies. DNA from these colonies is purified and used to oncogenically transform another plate of 3T3 cells.
- Step 'b' is repeated
- DNA is isolated from proliferating colonies, and used to form a genomic library.
- The genomic library is screened for human Alu sequences, which separates clones of murine DNA from clones of human DNA.
- Remaining clones should contain the oncogene.