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Reviewed October 2008
What is chromosome 22?
Humans normally have 46 chromosomes (23 pairs) in each cell. Two copies of chromosome 22, one copy inherited from each parent, form one of the pairs. Chromosome 22 is the second smallest human chromosome, spanning about 50 million DNA building blocks (base pairs) and representing between 1.5 percent and 2 percent of the total DNA in cells.
In 1999, researchers working on the Human Genome Project announced they had determined the sequence of base pairs that make up this chromosome. Chromosome 22 was the first human chromosome to be fully sequenced.
Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies. Chromosome 22 likely contains between 500 and 800 genes.
Genes on chromosome 22 are among the estimated 20,000 to 25,000 total genes in the human genome.
There are many genetic conditions related to genes on chromosome 22.
What chromosomal conditions are related to chromosome 22?
The following conditions are caused by changes in the structure or number of copies of chromosome 22.
- cancers
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A rearrangement (translocation) of genetic material between chromosomes 9 and 22 is associated with several types of blood cancer known as leukemias. This chromosomal abnormality, which is commonly called the Philadelphia chromosome, is found only in cancer cells. It fuses part of a specific gene from chromosome 22 (the BCR gene) with part of another gene from chromosome 9 (the ABL gene). The protein produced from this fused gene abnormally signals tumor cells to continue dividing and prevents them from adequately repairing DNA damage.
The Philadelphia chromosome has been identified in most cases of a slowly progressing form of blood cancer called chronic myeloid leukemia (CML). It also has been found in some cases of more rapidly progressing blood cancers known as acute leukemias. The presence of the Philadelphia chromosome can help predict how a cancer will progress and provides a target for molecular therapies.
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22q11.2 deletion syndrome
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Most people with 22q11.2 deletion syndrome are missing about 3 million base pairs on one copy of chromosome 22 in each cell. The deletion occurs near the middle of the chromosome at a location designated as q11.2. This region contains 30 to 40 genes, but many of these genes have not been well characterized. A small percentage of affected individuals have shorter deletions in the same region.
The loss of a particular gene, TBX1, is thought to be responsible for many of the characteristic features of 22q11.2 deletion syndrome such as heart defects, an opening in the roof of the mouth (a cleft palate), distinctive facial features, and low calcium levels. Some studies suggest that a deletion of this gene may contribute to behavioral problems as well. The loss of another gene, COMT, in the same region of chromosome 22 may also help explain the increased risk of behavioral problems and mental illness. Additional genes in the deleted region likely contribute to the signs and symptoms of 22q11.2 deletion syndrome.
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Emanuel syndrome
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Emanuel syndrome is caused by the presence of extra genetic material from chromosome 11 and chromosome 22 in each cell. In addition to the usual 46 chromosomes, people with Emanuel syndrome have an extra (supernumerary) chromosome consisting of a piece of chromosome 22 attached to a piece of chromosome 11. The extra chromosome is known as a derivative 22 or der(22) chromosome.
People with Emanuel syndrome typically inherit the der(22) chromosome from an unaffected parent. The parent carries a chromosomal rearrangement between chromosomes 11 and 22 called a balanced translocation. No genetic material is gained or lost in a balanced translocation, so these chromosomal changes usually do not cause any health problems. As this translocation is passed to the next generation, it can become unbalanced. Individuals with Emanuel syndrome inherit an unbalanced translocation between chromosomes 11 and 22 that introduces extra genetic material in the form of the der(22) chromosome.
As a result of the extra chromosome, people with Emanuel syndrome have three copies of some genes in each cell instead of the usual two copies. The excess genetic material disrupts the normal course of development, leading to mental retardation and birth defects. Researchers are working to determine which genes are included on the der(22) chromosome and what role these genes play in development.
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Opitz G/BBB syndrome
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The autosomal dominant form of Opitz G/BBB syndrome is caused by a deletion in one copy of chromosome 22 in each cell. This condition is considered part of 22q11.2 deletion syndrome because affected people usually have a deletion in the same region of chromosome 22. These cases occur in people with no history of the disorder in their family. It is not yet known which deleted gene(s) cause the signs and symptoms of Opitz G/BBB syndrome.
- other chromosomal conditions
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Other changes in the number or structure of chromosome 22 can have a variety of effects. Mental retardation, delayed development, delayed or absent speech, distinctive facial features, and behavioral problems are common features. Frequent changes to chromosome 22 include an extra piece of the chromosome in each cell (partial trisomy), a missing segment of the chromosome in each cell (partial monosomy), and an abnormal structure called a ring chromosome 22. Ring chromosomes occur when a chromosome breaks in two places and the ends of the chromosome arms fuse together to form a circular structure. Rearrangements (translocations) of genetic material between chromosomes can also lead to extra or missing material from chromosome 22. The most common of these translocations involves chromosomes 11 and 22.
Cat-eye syndrome is a rare disorder most often caused by a chromosomal change called an inverted duplicated 22. In people with this condition, each cell has a small extra chromosome made up of genetic material from chromosome 22 that has been abnormally copied (duplicated). The extra genetic material causes the characteristic signs and symptoms of cat-eye syndrome, including an eye abnormality called an iris coloboma (a gap or split in the colored part of the eye), small skin tags or pits in front of the ear, unusually formed ears, heart defects, kidney problems, malformations of the anus, and, in some cases, delayed development.
Is there a standard way to diagram chromosome 22?
Geneticists use diagrams called ideograms as a standard representation for chromosomes. Ideograms show a chromosome's relative size and its banding pattern. A banding pattern is the characteristic pattern of dark and light bands that appears when a chromosome is stained with a chemical solution and then viewed under a microscope. These bands are used to describe the location of genes on each chromosome.
Where can I find additional information about chromosome 22?
You may find the following resources about chromosome 22 helpful. These materials are written for the general public.
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- Educational resources - Information pages
- MedlinePlus - Health information
You may also be interested in these resources, which are designed for genetics professionals and researchers.
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Gene Tests - DNA tests ordered by healthcare professionals (3 links)
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- PubMed
 - Recent literature
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OMIM - Genetic disorder catalog (3 links)
- Map Viewer - Genetic maps
Where can I find general information about chromosomes?
The Handbook provides basic information about genetics in clear language.
These links provide additional genetics resources that may be useful.
What glossary definitions help with understanding chromosome 22?
The resources on this site should not be used as a substitute for
professional medical care or advice. Users seeking information about
a personal genetic disease, syndrome, or condition should consult with a qualified
healthcare professional.
See How can I find a genetics professional in my area? in the Handbook.
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