Chromosomes are thread-like structures found inside the nucleus of animal and plant cells. They are composed of DNA tightly coiled many times around proteins, serving as organized packages of genetic instructions for the body. Humans typically have 23 pairs of chromosomes, totaling 46, with one set inherited from each parent. Chromosome 13 is one of these pairs, carrying hundreds of genes that direct a wide array of bodily functions and development.
The Structure and Gene Landscape of Chromosome 13
Chromosome 13 is an acrocentric chromosome, meaning its centromere is positioned very close to one end. This results in a very short “p” arm and a much longer “q” arm. The short arm of acrocentric chromosomes generally contains no protein-coding genes.
Chromosome 13 spans approximately 113 to 115 million DNA base pairs, making up about 3.5% to 4% of total human DNA. It contains an estimated 300 to 400 genes that provide instructions for making various proteins. These proteins perform diverse roles throughout the body.
Essential Functions Governed by Key Genes
The RB1 gene provides instructions for producing the pRB protein, which functions as a tumor suppressor. This protein helps regulate cell growth, acting like a cellular “brake” to prevent cells from dividing too rapidly. It stops other proteins from initiating DNA replication, controlling cell proliferation and helping to prevent tumor formation.
The GJB2 gene creates connexin 26, a protein that forms gap junctions. These channels facilitate the transport of nutrients, ions, and signaling molecules between adjacent cells. In the inner ear, gap junctions maintain potassium ion levels, necessary for converting sound waves into nerve impulses.
The ATP7B gene codes for the copper-transporting ATPase 2 protein, primarily found in the liver. This protein manages copper levels by transporting copper to the Golgi apparatus for incorporation into ceruloplasmin, which carries copper throughout the body. If copper levels become too high in liver cells, it facilitates the removal of excess copper for elimination through bile.
Impact of Chromosomal Number and Structural Changes
Changes in the number or structure of chromosome 13 can lead to health conditions. Trisomy 13, or Patau syndrome, occurs when an individual has three copies of chromosome 13 instead of the usual two. This extra genetic material disrupts normal development, causing severe intellectual disability and physical abnormalities affecting many body systems.
Individuals with Trisomy 13 often exhibit heart defects, brain or spinal cord abnormalities, and very small or poorly developed eyes. Other common features include extra fingers or toes, a cleft lip with or without a cleft palate, low muscle tone, and a small head or lower jaw. Most infants with Trisomy 13 do not survive past their first days or weeks of life. This condition occurs in about 1 in 10,000 to 20,000 live births.
Deletion syndromes involving chromosome 13, specifically 13q deletion syndrome, arise when a segment of the long (q) arm is missing. The specific effects and severity depend on the size and exact location of the deleted segment. Common characteristics include varying degrees of intellectual disability and developmental delay.
Affected individuals may also present with distinctive facial features, such as small or wide-set eyes, a thin forehead, or a broad nasal bridge. Vision and hearing impairments are frequently observed. Additionally, individuals can experience central nervous system malformations, heart defects, and genital abnormalities in affected males.
Link to Hereditary Cancers
Inherited changes in genes on chromosome 13 can increase an individual’s lifetime risk for certain cancers. Retinoblastoma, a rare eye cancer that typically affects young children, is linked to mutations in the RB1 gene. While the RB1 gene normally functions as a tumor suppressor, inherited mutations in both copies mean the cellular “brake” on cell division is broken, allowing retinal cells to grow and divide uncontrollably, forming a tumor.
Approximately 40% of retinoblastoma cases are hereditary, meaning the RB1 gene mutation is present in all cells of the body, including reproductive cells. Individuals with this hereditary form often develop tumors in both eyes and face an increased risk of developing other types of cancer later in life. Understanding the specific RB1 mutation is important for managing the affected individual and counseling family members.
The BRCA2 gene, also located on chromosome 13, plays a significant role in DNA repair processes. Its protein helps mend breaks in DNA strands and maintain the stability of a cell’s genetic information. When a person inherits a harmful mutation in the BRCA2 gene, this DNA repair function is impaired, leading to an accumulation of DNA damage.
This impaired DNA repair capacity substantially increases the lifetime risk of developing several cancers. Women with inherited BRCA2 mutations have a significantly elevated risk of breast and ovarian cancers. Men with these mutations also face an increased risk of breast cancer, as well as prostate and pancreatic cancers.