Jacobsen Syndrome

What is Jacobsen syndrome?

• Jacobsen syndrome is a chromosomal condition because it happens when a section of DNA is missing from one copy of chromosome 11. That missing section contains many genes. The symptoms come from not having those genes. Because the missing part is at the tip of the long arm (called 11q), doctors also call it 11q terminal deletion disorder.

 Connection to DNA replication:

• In Jacobsen syndrome, a mistake during replication causes a break in chromosome 11. The cell tries to repair the break but uses a method called microhomology-mediated break-induced replication. This method looks for just a few matching DNA letters and glues the broken ends there, skipping over a large chunk of DNA between. That skipped chunk is the deletion. Scientists called the whole messy process chromoanasynthesis.

Connection to meiosis:

• most cases of Jacobsen syndrome, the deletion happens randomly during meiosis in one of the parents. That parent’s own body is normal, but the egg or sperm carries the missing piece. When that egg or sperm joins with other parent’s, the baby ends up with the deletion in every cell.

Inherited or passed down to children?

• In 85-90% of cases, the deletion is not inherited. In 10-15% of cases, a parent carries something called balanced translocation, and this can be passed down causing Jacobsen syndrome.

The mechanism though which the mutation occurs:

• Chromosome 11 has a naturally weak spot near its tip, called FRA11B. This spot breaks easily when the cell is copying its DBNA. Once it breaks, the cell’s repair system makes a mistake and loses the tip of the chromosome forever. That missing tip is the mutation that causes Jacobsen syndrome.

The results:

• Bleeding disorder: platelets don’t work, causing easy bruising and nosebleeds.
• Heart problem: more than half are born with heart problems that may need surgery.
• Learning disability and delayed speech, motor skills, overall growth, and eating ability.
• Higher chance of ADHD, seizure, ear and sinus infections, and autism
• Distinct appearance: Wide-set eyes, small low-set ears, broad nose bridge, small chin, skin folds on eyelids, curved pinky fingers.

References: 

Cleveland Clinic. (2025, April 8). Jacobsen syndrome: Symptoms, causes & treatment. https://my.clevelandclinic.org/health/diseases/jacobsen-syndrome

National Library of Medicine (U.S.). (2023, August 2). Jacobsen syndrome. MedlinePlus Genetics. https://medlineplus.gov/genetics/condition/jacobsen-syndrome/

O’Brien, M. P., & Morales, A. (2025, April 12). Jacobsen syndrome (11q terminal deletion syndrome). In StatPearls. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK614164/

Mattina, T., Perrotta, C. S., & Grossfeld, P. (2009). Jacobsen syndrome. Orphanet Journal of Rare Diseases, 4, 9. https://pmc.ncbi.nlm.nih.gov/articles/PMC2670819/