Chromosomes are structures found inside the nucleus of cells, carrying our genetic information in the form of DNA. Humans typically have 23 pairs of chromosomes, totaling 46. One of these pairs is chromosome 22, which stands out as the smallest of the 22 pairs of non-sex chromosomes, known as autosomes. Despite its relatively compact size, chromosome 22 contains a significant amount of genetic material that influences various bodily functions.
Characteristics and Key Genes
Chromosome 22, the smallest autosome, is gene-dense, containing an estimated 500 to 600 genes. This concentration means it plays a significant role in human biology. Its genes are involved in a wide array of biological processes fundamental for normal development and function.
These genes contribute to the proper development and operation of the nervous system, influencing brain structure and cognitive functions. They also participate in metabolic pathways, which are the chemical reactions that occur in the body to maintain life. Additionally, genes on chromosome 22 are involved in the development of various organs and tissues throughout the body.
Associated Genetic Conditions
Abnormalities on chromosome 22 are linked to several distinct genetic conditions, each arising from specific changes in its structure or gene sequence. One of the most frequently observed is 22q11.2 Deletion Syndrome, also known as DiGeorge syndrome or Velocardiofacial syndrome. This syndrome results from a microdeletion at position q11.2 on the long arm of chromosome 22, leading to symptoms including heart defects, immune system problems, and characteristic facial features.
Neurofibromatosis Type 2 (NF2) is caused by a mutation in the NF2 gene on chromosome 22. Individuals with NF2 often develop benign tumors, particularly schwannomas, which affect the nerves involved in hearing and balance. These tumors can lead to hearing loss, ringing in the ears, and problems with balance, with symptoms appearing in late adolescence or early adulthood.
Cat Eye Syndrome (CES) involves a partial tetrasomy of 22q11.2, meaning extra copies of a small segment of the long arm of chromosome 22. This duplication leads to features such as coloboma (a gap in the iris of the eye), heart defects, and kidney problems. The severity and specific manifestations of CES can vary widely among affected individuals.
The Philadelphia chromosome involves a translocation between chromosome 9 and chromosome 22. This occurs when a piece of chromosome 9 breaks off and attaches to chromosome 22, and vice versa, creating a new fusion gene called BCR-ABL. Its presence is strongly associated with chronic myeloid leukemia (CML), a type of cancer affecting blood-forming cells, as the fusion gene produces an abnormal protein that promotes uncontrolled cell growth.
Genetic Variations and Diagnostic Approaches
Genetic variations on chromosome 22 can take several forms, including deletions where a segment of DNA is missing, or duplications where a segment is present in extra copies. Translocations involve the rearrangement of genetic material between chromosome 22 and other chromosomes, while point mutations are single base changes within a gene. These variations can arise spontaneously during the formation of egg or sperm cells, or early embryonic development.
Identifying these variations requires specialized diagnostic methods. Karyotyping visualizes and examines the entire set of chromosomes, detecting large-scale structural changes. Fluorescence In Situ Hybridization (FISH) uses fluorescent probes to bind to specific DNA sequences on chromosome 22, enabling detection of smaller deletions or duplications missed by karyotyping.
Chromosomal microarray analysis (CMA) offers a more detailed approach, scanning the entire genome for subtle gains or losses of genetic material with higher resolution. Gene sequencing, particularly next-generation sequencing, provides precise identification of point mutations within specific genes on chromosome 22. These diagnostic tools help confirm diagnoses and guide management strategies for conditions linked to chromosome 22 abnormalities.