Chromosome Rearrangements
Chromosome rearrangements
Definition
A chromosome rearrangement is a structural change in a chromosome such as a deletion, translocation, inversion, or gene amplification. Chromosome rearrangements can contribute to the transformation of a normal cell into a cancerous cell and are therefore found in many cancer cells.
Description
Chromosomes and genes
A chromosome is a microscopic structure which is composed of proteins and DNA and is found in every cell of the body. Each cell of the body, except for the egg and the sperm cells, contains 23 pairs of chromosomes and 46 chromosomes in total. All cells of the body except for the egg and sperm cells are called the somatic cells. The egg and sperm cells each contain 23 chromosomes. Both males and females have 22 pairs of chromosomes, called the autosomes, that are numbered one to twenty-two in order of decreasing size. The final pair of chromosomes, called the sex chromosomes, determine the sex of the individual. Women possess two identical chromosomes called the X chromosomes while men possess one X chromosome and one Y chromosome.
Each type of chromosome contains different genes that are found at specific locations along the chromosome. Men and women possess two of each type of autosomal gene since they inherit one of each type from each parent. Each gene contains the instructions for the production of a particular protein. The proteins produced by genes have many functions and work together to create the traits of the human body, such as hair and eye color, and are involved in controlling the body's basic functions. Some genes produce proteins that are involved in controlling the growth cycle of the cell and are therefore involved in preventing the development of cancer.
Types of chromosome rearrangements
Sometimes a spontaneous break or breaks occur in a chromosome or chromosomes in a particular cell and can result in a deletion, inversion, or translocation. If the break or breaks result in the loss of a piece of chromosome, it is called a deletion. An inversion results when a segment of chromosome breaks off, is reversed (inverted), and is reinserted into its original location. When a piece of one chromosome is exchanged with a piece from another chromosome it is called a translocation.
Sometimes a small segment of chromosome is amplified, which results in the presence of multiple copies of that section of the chromosome. In most cases the segment of the chromosome that is duplicated contains only one gene, although it is possible for more than one gene to be amplified. Sometimes amplified genes form a separate and unique chromosome and sometimes they are located within an otherwise normal chromosome.
Chromosome rearrangements and cancer
A chromosome rearrangement can delete or disrupt the functioning of genes that are located on the chromosomal pieces involved. Chromosome rearrangements that delete or disrupt genes that regulate the cell cycle can contribute to the transformation of a normal cell into a cancerous cell. That is why chromosomal rearrangements are found in many cancers.
THE TRANSFORMATION OF A NORMAL CELL INTO A CANCEROUS CELL.
The process by which a normal cell is transformed into a cancerous cell is a complex, multi-step process involving a breakdown in the normal cell cycle. Normally a somatic cell goes through a growth cycle during which it produces new cells. The process of cell division is necessary for the growth of tissues and organs of the body and for the replacement of damaged cells.
Cell division is tightly regulated by genes. Normal cells have a limited lifespan and only go through the cell cycle a certain number of times. Genes regulate the cell cycle by producing regulatory proteins. Different types of regulatory proteins regulate cell growth and division in different types of cells. For example, a skin cell may be regulated by a different combination of proteins than a breast cell or a liver cell.
A cell that loses control of its cell cycle and replicates out of control is called a cancer cell. Cancer cells undergo many cell divisions, often at a quicker rate than normal cells, and do not have a limited lifespan. They also have loss of apoptosis, or cell death, which is characteristic of a normal cell. This allows them to eventually overwhelm the body with a large number of abnormal cells and hurt the functioning of the normal cells.
A cell becomes cancerous only after changes or deletions occur in a number of genes that are involved in the regulation of its cell cycle. However, a change or deletion of one regulatory gene can result in the change or deletion of other regulatory genes.
Proto-oncogenes and tumor-suppressor genes are the two most common types of genes involved in regulating the cell cycle. We inherit two of each type of proto-oncogene and two of each type of tumor-suppressor gene. Tumor-suppressor genes produce proteins that are involved in helping to prevent uncontrolled cell growth and division. Only one normal copy of a tumor-suppressor gene needs to be present to maintain its normal role in the regulation of the cell cycle. If both copies of a tumor-suppressor gene are changed, however, then not enough normal tumor-suppressor protein will be produced and the cell is more likely to become cancerous.
Proto-oncogenes produce proteins that are largely involved in stimulating the growth and division of cells in a controlled manner. A change in a proto-oncogene can convert it into an oncogene. An oncogene produces an abnormal protein, which is involved in stimulating uncontrolled cell growth. Only one proto-oncogene of a pair needs to be changed into an oncogene for it to promote the transformation of a normal cell into a cancerous cell.
A chromosome rearrangement involving a tumor-suppressor gene or proto-oncogene can contribute to the transformation of a normal cell into a cancerous cell. Certain types of chromosome rearrangements are found more commonly in cancers of certain types of cells. This is because these chromosome rearrangements involve genes that regulate the cell cycle in those specific cells. More than one chromosome rearrangement is usually present in a particular cancer cell since it is necessary for more than one regulatory gene to be altered during the transformation of a normal cell into a cancerous cell. Different types of chromosome rearrangements contribute to the formation of cancer cells in different ways. Researchers don't always know how a chromosome rearrangement contributes to the development of cancer.
How specific types of rearrangements contribute to the development of cancer
Deletions
A deletion of a piece of chromosome that contains a tumor suppressor gene can contribute to the transformation of a normal cell into a cancerous cell. If both copies of a tumor suppressor gene are deleted or changed then little or no tumor suppressor protein is produced. This in turn can impact the regulation of the cell cycle and contribute to the transformation of the normal cell.
A deletion of a segment of chromosome 13, for example, can result in the loss of a tumor-suppressor gene that helps to prevent an eye cancer calledretinoblastoma . If both retinoblastoma tumor-suppressor genes are deleted or changed in one of the cells of the eye then that cell can become cancerous.
Translocations
A translocation involving a proto-oncogene can result in its conversion into an oncogene which can contribute to the development of cancer. A translocation involving a proto-oncogene results in the transfer of the proto-onco-gene from its normal location on a chromosome to a different location on another chromosome. Sometimes this results in the transfer of a proto-oncogene next to an activating gene. This activating gene abnormally activates the proto-oncogene and converts it into an oncogene. When this oncogene is present in a cell, it contributes to uncontrolled cell growth and the development of cancer.
For example, the translocation of the c-myc protooncogene from its normal location on chromosome eight to a location on chromosome 14 results in the abnormal activation of c-myc. This type of translocation is involved in the development of a type of cancer called Burkitt's lymphoma . The translocated c-myc protooncogene is found in the cancer cells of approximately 85% of people with Burkitt's lymphoma.
A translocation involving a proto-oncogene can also result in the fusion of the proto-oncogene with another gene. The resulting fused gene is an oncogene that produces an unregulated protein which stimulates uncontrolled cell growth. One example is the Philadelphia chromosome translocation, found in the leukemia cells of greater than 95% of patients with a chronic form of leukemia. The Philadelphia chromosome translocation results in the fusion of the c-abl proto-oncogene, normally found on chromosome nine, to the bcr gene that is found on chromosome 22. The fused gene produces an abnormal protein that is involved in the formation of cancer cells.
Inversions
An inversion, like a translocation, can result in the creation of an oncogene through either the activation of a proto-oncogene or the creation of a fusion gene. An inversion involving a proto-oncogene results in the movement of the gene to another location on the same chromosome. For example, an inversion of chromosome ten can move a proto-oncogene called RET and cause it to fuse with a gene called ELEI or a gene called H4. The fusion of RET with either of these genes creates an oncogene. When the RET oncogene is present in a thyroid cell it promotes the transformation of that cell into a cancerous cell.
Gene amplification
Gene amplification can also contribute to the development of cancer. Amplification of a segment of chromosome that contains a proto-oncogene can result in the formation of many copies of a proto-oncogene. Each copy of the proto-oncogene produces protein that is involved in stimulating cell growth. This can result in a significant increase in the amount of protein produced, which can promote uncontrolled cell growth. Multiple copies of proto-oncogenes are found in many tumors.
See Also Cancer genetics
Resources
BOOKS
Vogelstein, Bert and Kenneth Kinzler, eds. The Genetic Basis of Human Cancer New York, NY: McGraw-Hill, 1998.
OTHER
"Chromosome Rearrangements" University of Wisconsin- Madison Department of Genetics 3 July 2001 <http://www1.genetics.wisc.edu/466/Fall98/lect08/index.html>.
"The Genetics of Cancer—an Overview" Robert H. Lurie Comprehensive Cancer Center of Northwestern University. 17 Feb. 1999. 29 June 2001 <http://www.cancergenetics.org/gncavrvu.htm>.
Lisa Andres, M.S., CGC
KEY TERMS
Chromosome
—A microscopic structure, made of a complex of proteins and DNA, that is found within each cell of the body.
Deletion
—A piece missing from a chromosome.
Gene
—A building block of inheritance, made up of a compound called DNA (deoxyribonucleic acid) and containing the instructions for the production of a particular protein. Each gene is found on a specific location on a chromosome.
Gene amplification
—When multiple copies of a small segment of chromosome containing one or more genes are present as a separate chromosome or as part of an otherwise normal chromosome.
Inversion
—A piece of a chromosome that was removed from the chromosome, inverted, and reinserted into the same location on the chromosome.
Leukemia
—Cancer of the blood-forming organs which results in an overproduction of white blood cells.
Lymphoma
—Cancer involving cells of the immune system.
Oncogene
—A changed proto-oncogene that promotes uncontrolled cell division and growth.
Protein
—A substance produced by a gene that is involved in creating the traits of the human body such as hair and eye color or is involved in controlling the basic functions of the human body.
Proto-oncogene
—A gene involved in stimulating the normal growth and division of cells in a controlled manner.
Somatic cells
—All the cells of the body except for the egg and sperm cells.
Translocation
—An exchange of a piece of one chromosome with a piece from another chromosome.
Tumor-suppressor gene
—Gene involved in controlling normal cell growth and preventing cancer.
Chromosome Aberrations
Chromosome Aberrations
Chromosome aberrations are departures from the normal set of chromosomes either for an individual or from a species. They can refer to changes in the number of sets of chromosomes (ploidy), changes in the number of individual chromosomes (somy), or changes in appearance of individual chromosomes through mutation-induced rearrangements. They can be associated with genetic diseases or with species differences.
Trisomy
Chromosome number, size, and shape (X-shaped or V-shaped) are fixed for each species. In most animals, chromosomes are present in pairs, called homologous pairs, carrying similar genes. Each chromosome pair carries a distinctive set of genes. Genes code for proteins , and the amount of protein produced in a cell from a particular gene is proportional to the number of functional gene copies present.
Trisomy refers to having three copies of one chromosome. It arises through the chromosomal accident of nondisjunction during meiosis , which sends two copies of a particular chromosome into a sperm or egg, rather than one. An individual with a trisomy who survives to be born produces more of the gene products encoded on the trisomic chromosome. The resulting genic imbalance almost always severely impairs growth. Trisomy of the twenty-first chromosome, the smallest in humans, is the cause of Down syndrome, which is associated with mental retardation, congenital heart disease, accelerated aging, and characteristic facial features. Trisomy that occurs after fertilization , during fetal development, results in a "mosaic" individual with only some trisomic cells in the body. Such individuals may display some but not all of the features of the syndrome.
Trisomics for different chromosomes result in different abnormal characteristics. In humans, trisomies 13 and 18 are associated with different birth defects. While these individuals do not live long after birth, trisomies for any of the other non-sex chromosomes die before birth. Monosomies (having only one copy) for any chromosome also do not survive fetal existence, except for the sex chromosomes X and Y. Sex chromosome trisomies and monosomy for the X chromosome are associated with less severe effects on the phenotype .
Translocations
Translocations are the result of a chromosomal-level mutation, with two different (nonhomologous) chromosomes breaking and rejoining, placing the genes from one part of the one chromosome with part of the second chromosome, and vice versa. The number of genes is unchanged. Occasionally, the breakpoint mutation interrupts and inactivates the gene located at that chromosomal site. In other cases, the juxtaposition of new deoxyribonucleic acid (DNA) sequences from the other chromosome next to a gene at the breakpoint results in inappropriate expression. This action may activate an oncogene , for example. The "Philadelphia chromosome" is a translocation that fuses parts of chromosomes 9 and 22, which produces a new gene product that functions as an oncogene called Abl, which is implicated in chronic myelogenous leukemia.
Inherited translocations are passed through generations in a codominant fashion. Since one copy of each chromosome remains normal, both parent and progeny with such a translocation are heterozygous , or "balanced" carriers. Half their gametes will include one copy of each gene, either on the translocated chromosomes or their normal homologs. The other half, however, are unbalanced with some combination of translocated and normal homologs. The result is that the gamete has two copies of some genes, but no copies of other genes, from the translocated chromosomes. Such an "unbalanced" gamete, if it takes part in fertilization, often disrupts development so greatly that the individual does not survive to be born. If the number of unbalanced genes is low, however, children may be born, but often they have growth defects and mental retardation. Couples with recurrent spontaneous abortions may have one partner carrying a balanced translocation. Thus, gene copy number determines the specific phenotypes associated with a translocation, or with any chromosome aberration. Extreme examples of the importance of gene number are triploidy (3n = 69 for humans) and tetraploidy (4n = 92). These individuals nearly always die early in fetal life and are detected only in the remains of early spontaneous abortions. However, intolerance of polypoidy may be a mammalian phenomenon. It is common among plants, and frogs that are triploid are both viable and fertile.
Robertsonian translocations are a special class that result from the fusion of two V-shaped chromosomes at their centromere ends to form a single X-shaped chromosome. Individuals who are balanced for this translocation
have forty-five chromosomes, but are otherwise normal. However, during gamete formation, some gametes will become unbalanced, and their progeny are at risk for being aneuploid (without the correct set of genes). If the two fused chromosomes are homologs, then the risk is 100 percent that the zygote will be aneuploid since it will either have too few or too many genes. If nonhomologs are fused, the risk is usually 50 percent. About 5 percent of Down syndrome cases are caused by Robertsonian translocations.
Chromosome fusions or exchanges at the centromere position can often be correlated with differences between closely related species. The great apes (chimpanzees and gorillas) have forty-eight chromosomes, for example. Their chromosome constitution differs from humans only in having one fewer small X-shaped chromosome pair and two additional small V-shaped pairs. At some point in our past, the V-shaped pairs fused at the centromeres to form the X-shaped pairs. Using DNA sequences that are conserved among vertebrate species as position markers, it is possible to create comparative maps of conserved blocks of genes. All mammalian X chromosomes are alike in the genes present, for instance. Thus, changes visible at the chromosome level are useful markers to follow the evolutionary relatedness of different species. Further comparison of conserved sequences suggests the vertebrate genome is a tetraploid (four-copy) version of the invertebrate genome.
Polyploidy
Changes in ploidy are of evolutionary importance in the flowering plants (angiosperms). Tetraploid plants often grow faster and larger than the diploid plants they derive from, and tend to be selected for agriculture. Alfalfa, coffee, wheat, peanuts, and potatoes are some examples. Commercially grown strawberries are octaploids (eight chromosome sets). Triploid plants, formed by crossing tetraploid with related diploid species, are almost always sterile because their aneuploid seeds abort. Seedless watermelons and bananas are examples of this technique used to improve fruit for human consumption.
An example of plant polyploidy that affected human civilization and history is the origin of wheat. Modern bread wheat, cultivated for about eight thousand years, is a hexaploid with 2n = 42, formed by sequential hybrids formed among three related grass species, each with 2n = 14. To complete the circle, modern hybridizers have created a new species, Triticale, by crossing the ancestral Emmer wheat (2n = 28) with rye (2n = 14) and then doubling the chromosome number to 42 to take advantage of strong wheat growth with the high lysine content of rye. DNA analysis is scrutinizing the hybrid origins of many cultivated plants to identify their ancestors.
see also Angiosperms; Chromosome, Eukaryotic; Gene; Genetic Diseases; Mutation; Oncogenes and Cancer Cells; Patterns of Inheritance
John Merriam
Bibliography
Gardner, R. J. McKinlay, and G. R. Sutherland. Chromosome Abnormalities and Genetic Counseling. Oxford: Oxford University Press, 1996.
John, Bernard. Meiosis. New York: Cambridge University Press, 1990.