Cancer Genetics Health Article

Definition

A continuous process in which multiple alterations occur in genes that control cell division and differentiation that leads to cancer—the uncontrolled division and proliferation of cells. These genetic alterations are referred to as mutations, which are changes in the normal DNA sequence of a particular gene. Mutations may include deletions, chromosomal translocations, inversions, amplifications, or point mutations.

Cancer genetics is the understanding of the genetic processes underlying the actual disease occurrence. This understanding plays a significant role in early detection, therapy, prevention, and prognosis.

Description

Nearly all cancers originate from a single cell and are the result of genetic alterations, although most of them are not inherited. Individuals who are genetically predisposed to a particular cancer will not necessarily develop the disease in the absence of somatic mutations. Somatic mutations occur in non-sex determining cells, meaning they will not be passed on to offspring. These mutations can be influenced by environment and other causes, such as an individual's habits (i.e. smoking). A single genetic error or mutation in a cell does not typically induce malignancy; instead it develops after a series of mutations over a period of time.

Regulation of cell death and survival

A balance between cell division and death of the old, degenerated cells is essential for proper cellular functioning of any organism. Cells that can no longer replicate or that have sustained injuries (like hypoxia, heat, extreme cold, or ultraviolet radiation) are candidates for cell death. Alternatively, cells can be killed if infected by intracellular organisms (pathogens), or damaged cells may be engulfed by a host's lymphocytes (white blood cells involved in cellular defense mechanisms). Another form of cell death in the disease process is a suicide mechanism initiated by cells known as apoptosis. In this process, extracellular or intracellular signals may trigger the degradation of nuclear material resulting in cell death. Some of the apoptotic genes like bcl2 family members (bcl-X, A1, bax, bad) are shown to be involved in various cancers. Studies to alter the activity of bcl2 family members and related genes will be of potential use in designing cancer therapies.

Oncogenes and tumor suppressor genes

The incessant cell proliferation in cancer may either be due to over-activation of a specific gene that promotes cell division or due to the improper functioning of a gene that will otherwise restrain growth. Genes that promote cell division are proto-oncogenes—positive regulators of cell division. Overexpression of proto-oncogenes results in uncontrolled cell growth. Genes that suppress or restrain growth are tumor suppressor genes and loss of their function results in unregulated cell division. An alteration in the function of genes in each of these classes is due to a change, or mutation, in the DNA within the cell. The different types of mutations include point mutations, amplifications, and chromosomal alterations.

Point mutations

DNA is composed of a string of nucleotides, each containing a phosphate group, deoxyribose, and one of four bases; adenine (A), guanine (G), cytosine (C), and thymine (T). These bases are paired as either A-T or C-G and these pairs compose the "rungs" in the double helix structure of DNA. The order of the bases creates the genetic code for development. A sample genetic code is CAG-TAA-CCA-GCG, etc. These triplets code for synthesis of specific proteins.

A point mutation is a single nucleotide change in a DNA strand. This may alter the genetic code, thus altering the function of the protein. In the above example, a point mutation in the thymine base of the second triplet would look like: CAG-AAA-CCA-GCG. Changing the code from TAA to AAA could alter the function of a protein and thus could cause a predisposition to disease such as cancer. One example of a point mutation that has been identified is the ras family of oncogenes (such as H ras, K-ras, N-ras), present in 15% of all human cancers.

DNA amplification

Another mechanism of oncogene activation—DNA amplification—results in an increase in the amount of DNA in the cell. A large number of genes are amplified in human cancers.

DNA amplification can be detected by cytological staining (a method in which the amplified DNA is stained), or by another fluorescent technique called comparative genomic hybridization (CGH). CGH allows the specific recognition of regions of gene amplifications in tumor DNA and is a more sensitive diagnostic tool.

Chromosomal alteration

Chromosomal alteration may involve translocations and is often seen in lymphoid tumors. Translocation is the transfer of one part of a chromosome to another chromosome during cell division and may involve transcription factors (i.e., nuclear factors), signal transduction proteins, and cellular regulatory molecules.

DNA repair genes

In addition to oncogenes and tumor suppressor genes, DNA repair genes may lead to cancer. DNA repair genes are capable of correcting the errors that occur during cell division. Malfunction of these repair genes, either through inherited mutation or acquired mutation, may affect cell division resulting in malignancies.