The human body’s genetic material is organized into 23 pairs of chromosomes, which are tightly packaged structures containing DNA. Chromosome 13 is one of the 22 pairs of non-sex chromosomes, or autosomes, that individuals inherit, one copy from each parent. This chromosome spans approximately 114 million base pairs, representing between 3.5 and 4% of the total DNA within a cell. Chromosome 13 is characterized as an acrocentric chromosome, meaning its centromere is positioned very close to one end, resulting in one very short arm and one long arm. Despite its size, it has one of the lowest gene densities in the human genome, containing an estimated 308 to over 600 protein-coding genes.
Key Genes and Normal Biological Roles
The genes located on Chromosome 13 govern a wide range of normal cellular functions, particularly those related to growth control and maintaining DNA integrity. Among the most studied is the BRCA2 gene, situated on the long arm of the chromosome. BRCA2 is classified as a tumor suppressor gene, and the protein it produces plays a central role in the homologous recombination repair pathway. This repair system accurately fixes double-stranded breaks in the DNA helix, which are highly damaging forms of genetic injury.
When the BRCA2 gene functions correctly, it ensures the cell’s genetic blueprint remains stable, preventing the accumulation of mutations that can lead to cancer. An inherited mutation in this gene significantly increases the risk of developing breast, ovarian, prostate, and other forms of cancer. Another gene impacting cellular control is the RB1 gene, located nearby on the long arm of Chromosome 13.
The RB1 gene is also a tumor suppressor, named for its association with retinoblastoma, a rare eye cancer. The protein product of RB1 acts as a regulator of the cell cycle, managing the cell’s progression from a resting state to an active state of division. By controlling this transition, the RB1 protein prevents cells from dividing uncontrollably. The co-location of these two tumor suppressors means that the loss of a region of Chromosome 13 can disable multiple cellular safeguards simultaneously.
Consequences of Extra or Missing Chromosome Copies
The conditions associated with Chromosome 13 arise when there is an abnormal number of full chromosome copies present in the body’s cells. The most well-known numerical abnormality is Trisomy 13, also called Patau Syndrome, which occurs when a person has three full copies of Chromosome 13 instead of the usual pair. This surplus of genetic material is the result of an error during the formation of reproductive cells, known as nondisjunction. Trisomy 13 is a severe condition, occurring in roughly 1 out of every 16,000 newborns, with the risk increasing with maternal age.
The presence of the extra chromosome severely disrupts normal fetal development, leading to life-threatening health issues. Infants present with severe intellectual disability and physical abnormalities affecting multiple organ systems. Common features include congenital heart defects, affecting up to 80% of cases, and profound brain abnormalities like holoprosencephaly, where the forebrain fails to divide properly.
External physical markers include a cleft lip or cleft palate, abnormally small eyes (microphthalmia), and extra fingers or toes (polydactyly). The prognosis for individuals with Trisomy 13 is poor, as the disorder is often incompatible with long-term survival. Only about 5 to 10% of babies born with the syndrome survive beyond their first year of life. The opposite condition, Monosomy 13, where an entire copy of the chromosome is missing, is almost always incompatible with life.
Syndromes Linked to Partial Structural Changes
Structural changes involving only parts of Chromosome 13 also lead to specific syndromes. One condition is Partial Monosomy 13q, which results from the deletion of a segment of the chromosome’s long arm (q). The specific physical and developmental effects depend on the size and exact location of the deleted segment and the genes contained within that region.
Deletions encompassing the region near band 13q32 are associated with slow growth, intellectual disability, and various congenital malformations. In some cases, the ends of the chromosome break and fuse together to form a Ring Chromosome 13. This event leads to a phenotype characterized by developmental delays, intellectual disability, and growth deficiencies, as genetic material is typically lost during the ring-forming process.
Chromosome 13 is frequently involved in Robertsonian translocations, a type of rearrangement common to acrocentric chromosomes. This occurs when the long arms of two acrocentric chromosomes, such as Chromosome 13 and Chromosome 14, fuse together, and the short arms are lost. A person with a balanced Robertsonian translocation is generally healthy, having 45 chromosomes instead of 46, as no substantial genetic material is lost. The significance for carriers lies in reproduction, as the rearranged chromosome can increase the risk of having a child with Trisomy 13.