A chromosome is a thread-like structure inside the nucleus of animal and plant cells, composed of DNA tightly coiled around proteins, carrying genetic information. Humans typically possess 23 pairs of these chromosomes, totaling 46, with one set inherited from each parent. Chromosome 5 is one of the larger human chromosomes, containing a substantial number of genes. It plays a fundamental role in various biological functions, impacting human development and health.
General Characteristics of Chromosome 5
Chromosome 5 is a significant component of the human genome, spanning approximately 181 to 182 million base pairs. This length accounts for nearly 6% of the total DNA found within human cells.
Despite its considerable size, Chromosome 5 exhibits one of the lower gene densities compared to other human chromosomes. This characteristic is partly due to the presence of numerous gene-poor regions, which contain non-coding DNA.
Estimates suggest that Chromosome 5 contains around 900 genes, although these numbers can vary slightly depending on the methods researchers use for gene identification. These genes contribute to various biological processes, including regulating cell growth and division, nervous system development and function, and metabolic pathways involving lipids and glucose. Genes on Chromosome 5 are also involved in immune response and include tumor suppressor genes that prevent uncontrolled cell proliferation. Their diverse roles impact normal physiological functions and overall human health.
Important Genes and Their Roles
Several genes on Chromosome 5 have well-defined functions. One such gene is TERT (Telomerase Reverse Transcriptase), situated on the short arm at 5p15.33. The TERT gene provides instructions for creating a component of telomerase, an enzyme that maintains telomeres, which are protective caps found at the ends of chromosomes.
Telomeres naturally shorten with each cell division; telomerase counteracts this by adding specific DNA segments, allowing cells to divide numerous times without losing genetic information. This process is important for rapidly dividing cells like those in bone marrow and the gastrointestinal tract. TERT helps ensure chromosomal stability and cellular longevity.
Another gene on Chromosome 5 is APC (Adenomatous Polyposis Coli), found at 5q21-q22. The APC gene is a tumor suppressor, controlling cell growth and preventing tumor formation. The APC gene’s protein regulates cell division, adhesion, and migration.
The APC protein is involved in the Wnt/β-catenin signaling pathway, important for cell proliferation and differentiation. It prevents excessive β-catenin accumulation, regulating cell growth and ensuring proper tissue development, especially in the intestinal lining. This helps maintain tissue integrity and normal cell behavior.
The EGR1 (Early Growth Response 1) gene, located on chromosome 5q31, contributes to cellular regulation. It acts as a transcription factor, controlling the activity of many other genes. EGR1 influences cellular processes including growth, differentiation, and stress responses.
EGR1 functions as a tumor suppressor, preventing uncontrolled cell growth. It regulates genes involved in cell cycle control and programmed cell death. EGR1 helps ensure cells grow and divide in a controlled manner.
Conditions Associated with Chromosome 5
Alterations to Chromosome 5 can lead to several genetic conditions, illustrating its important role in human health. One notable disorder is Cri du chat syndrome, also known as 5p- syndrome, resulting from a deletion on the short arm of chromosome 5. The name, French for “cat’s cry,” refers to the distinctive high-pitched cry of affected infants, resembling a kitten’s meow.
Individuals with Cri du chat syndrome often experience intellectual disability, delayed development, and microcephaly. Other features include low birth weight, weak muscle tone, and characteristic facial features. The condition’s severity often correlates with the deleted segment’s size; specific regions are linked to symptoms like the cat-like cry (5p15.3) and severe intellectual disability (5p15.2, involving the CTNND2 gene). Most cases arise spontaneously rather than being inherited.
Another condition linked to Chromosome 5 is Familial Adenomatous Polyposis (FAP), caused by mutations in the APC gene at 5q21-q22. This inherited disorder is characterized by hundreds to thousands of adenomatous polyps primarily in the colon and rectum. These polyps, if left untreated, carry a nearly 100% risk of progressing to colorectal cancer, often by early adulthood.
FAP is passed down in an autosomal dominant pattern; a person with the mutation has a 50% chance of transmitting it to each child. The APC gene’s normal function is to suppress tumor growth, and its malfunction in FAP leads to uncontrolled cell proliferation in the intestinal lining.
Deletions on the long arm of Chromosome 5 can also lead to 5q- syndrome, a type of myelodysplastic syndrome. This disorder affects the bone marrow, causing blood cells to mature improperly. Individuals with 5q- syndrome often suffer from anemia due to reduced red blood cell production and abnormalities in platelet development, which can impair blood clotting.
There is also an increased risk of developing acute myeloid leukemia (AML) in individuals with 5q- syndrome. The deleted region, typically about 1.5 million base pairs, contains approximately 40 genes. The loss of specific genes within this region, such as RPS14 and MIR145, contributes to the red blood cell and platelet abnormalities observed in the syndrome, respectively.
Deletions in the long arm of chromosome 5 are observed in both myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). The EGR1 gene, located within a commonly deleted segment on 5q31, functions as a tumor suppressor. Its loss or impaired function can contribute to myeloid neoplasm development by allowing uncontrolled cell growth and hindering proper differentiation.