15q26 overgrowth syndrome
How is 15q26 overgrowth syndrome passed down in families?
15q26 overgrowth syndrome is a chromosome disorder. Chromosomes contain our body's genetic information. Chromosomes are found in almost every cell of a person's body. Human cells usually contain 46 chromosomes, or 23 pairs of chromosomes. One chromosome of each pair is inherited from our mother, and the other is inherited from our father. The first 22 pairs of chromosomes are numbered 1-22, and they are the same in males and females. The last pair are called the sex chromosomes and they determine our gender. Females have two X chromosomes (XX) and males have one X chromosome and one Y chromosome (XY).
Each chromosome is made up of 2 sections, called "arms" - a short arm and a long arm. The short arm of a chromosome is called the "p arm" while the long arm is called the "q arm". The arms are broken down into even smaller sections which are designated by numbers. 15q26 overgrowth syndrome is a disorder caused when a person has too many copies of section 26 on the q arm of chromosome 15 - this is where "15q26" comes from. The condition is sometimes called "Tetrasomy 15q26 overgrowth syndrome". "Tetra" means 4, and "somy" refers to the chromosomes. People with 15q26 overgrowth syndrome have four copies of the q26 section of chromosome 15 rather than the usual two.
The extra sections of chromosome 15q26 in patients with 15q26 overgrowth syndrome are packaged into structures called "supernumerary marker chromosomes". Most of the time (approximately 70% of the time), small supernumerary marker chromosomes occur as an error during the formation of an egg or a sperm cell. The other 30% of the time, these marker chromosomes are inherited from a parent. When the chromosome changes cause severe symptoms, like those seen in 15q26 overgrowth syndrome, the changes may be more likely to be the result of an error during egg or sperm formation. For those patients with 15q26 overgrowth syndrome that have been reported in the medical literature, information is not always provided on whether the parents have a 15q26 chromosome abnormality.
By taking a detailed family history, learning more about the symptoms of 15q26 overgrowth syndrome in a family and reviewing results of chromosome testing for 15q26 overgrowth syndrome in the family, a genetic counselor can help people understand if 15q26 overgrowth syndrome occurred as an error during the formation of an egg or a sperm cell or if it happened because of a chromosome abnormality in a patient's parent. To help figure this out, the parents of a patient with 15q26 overgrowth syndrome will likely need to be tested for chromosome 15 abnormalities. A genetic counselor can be found by asking your regular doctor for recommendations, or by using the National Society of Genetic Counselor's "Find a Genetic Counselor" tool.
Liehr T and Weise A. Frequency of small supernumerary marker chromosomes in prenatal, newborn, developmentally retardedand infertility diagnostics. International Journal of Molecular Medicine. 19: 719-731, 2007.
Levy B, Tegay D, Papenhausen P, Tepperberg J, Nahum O, Tsuchida T, Pletcher BA, Ala-Kokko L, Baker S, Ferederick B, Hirschhorn K, Warburton P, Shanske A. "Tetrasomy 15q26: a distinct syndrome or Shprintzen-Goldberg syndrome phenocopy?" Genetics in Medicine 2012. 14(9):811-818.