Januari 12, 2017
the name digeorge syndrome isn’t the mostdescriptive name, which is why it’s often also referred to as 22q11.2 deletion syndrome,which is actually pretty descriptive, and describes a condition in which a small portionof chromosome 22 is deleted, which causes a bunch of developmental abnormalities andcomplications. alright so our chromosomes are composed ofgenes, right? which are essentially instructions for everything from development to day-to-daysurvival, and these genes are spread out across 23 pairs of chromosomes. 22q11.2 is like anaddress, so 22 stands for chromosome 22, with q designating the long arm of the chromosome,then it’s on region 1, band 1, and sub-band 2. this portion of dna, 22q11.2, spans about30 genes and 1.5 to 3 million base pairs,
which classifies it as a microdeletion sinceit’s less than 5 million base pairs. even though this region is relatively small, itencodes for some really important genes, one of which is the tbx1 gene, which is thoughtto play a big role in the disease. the tbx1 gene is involved in normal developmentof the pharyngeal pouches, specifically pouch 3 and 4, which are fetal structures that developinto parts of the head and neck. the third pharyngeal pouch goes on to develop into thethymus and the inferior parathyroid gland, the fourth pouch goes on to develop into thesuperior parathyroid gland. so with a 22q11.2 deletion and therefore no tbx1 gene, the thymusand parathyroid gland both end up underdeveloped, called hypoplasia.
t lymphocytes or t cells are immune cellsthat’re super important for the adaptive immune response, and are produced in the bonemarrow but mature in the thymus. if someone has thymic hypoplasia and thymic dysfunction,the t cells don’t mature, and so these people often have a deficiency in mature t cells.it turns out, though, that most people with digeorge syndrome have mild to moderate thymicdysfunction, called partial digeorge syndrome, which means that the immunodeficiency isn’tlife-threatening. complete digeorge syndrome, though, where thymic dysfunction is severe,can be fatal within the first year of life, due to a severely compromised immune system. now, let’s talk about the parathyroid glands- the other major organ that’s affected
in digeorge syndrome. the parathyroid glandssecrete parathyroid hormone, which helps increase the level of calcium ions in the blood. parathyroidhypoplasia leads to low levels of parathyroid hormone, which causes low levels of calciumions in the blood, called hypocalcemia. in addition to affecting the thymus and parathyroidglands and causing hypoplasia, the 22q11.2 region encodes genes that can also affectother organs and tissues. individuals with 22q11.2 deletion syndrome commonly have congenitalheart defects, in particular truncus arteriosus and tetralogy of fallot, as well as facialabnormalities such as cleft palate and a characteristic “facies†which means that they might havefeatures that might be normal individually like a long face, small teeth, or broad nose,but taken together these and many other features
become the characteristic facies of digeorgesyndrome. patients might have many other characteristic physical findings as well as higher ratesof behavioral and mental health conditions like schizophrenia. diagnosis can be difficult, but genetic testingcan be done if digeorge syndrome is suspected, and certain blood tests looking at t cellnumbers and function, calcium, and parathyroid hormone can also be helpful. because the syndrome is due to a genetic deletion,there’s no known cure, though many of the symptoms can be treated or managed, for exampleinfections due to the underlying immunodeficiency might be treated with antibiotics and hypocalcemiacan be managed with vitamin d and calcium
supplements. surgery might be required formore severe cases though, like a thymus transplant for complete digeorge syndrome or heart surgeryfor children with congenital heart defects. alright as a quick recap, digeorge syndromeor 22q11.2 deletion syndrome, is a genetic condition where the q11.2 portion of dna onchromosome 22 is deleted, which can cause developmental issues like thymic and parathyroidhypoplasia, congenital heart defects, facial abnormalities, mental health conditions, andothers. thanks for watching, you can help supportus by donating on patreon, or subscribing to our channel, or telling your friends aboutus on social media.
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