الكروموسوم 21، هوواحد من 23 زوج من الكروموسومات البشرية. عادة ما يمتلك البشر زوجان من هذا الكروموسوم. التثلث بهذا الكروموسوم يسبب متلازمة داون. الكروموسوم 21 هوأصغر كروموسوم بشري، ويحتوي على 47 مليون نيوكليوتيد (مادة بناء الدنا)، ويمثل حوالي 1.5% من إجمالي الدنا في الخلايا.

في 2000، أعرب الباحثون العاملون على مشروع الجينوم البشري أنهم قاموا بتحديد تسلسل الأزواج القاعدية المكونة لهذا الكروموسوم. الكرومسوم 21 كان ثاني كروموسوم بشري يتم تحديد تسلسله بالكامل، بعد الكروموسوم 22.

نظراً لاختلاف المناهج التي يستخدمها الباحثون في التنبؤ بعدد الجينات في جميع كروموسوم، هناك تباين في أعداد الجينات المقدرة. من الأرجح حتى الكروموسوم 21 يحتوي على 300-400 جين.

الجينات

القائمة التالية لبعض الجينات الموجودة على الكروموسوم 21:

• APP: amyloid beta (A4) precursor protein (peptidase nexin-II, Alzheimer disease)
• C21orf59: Chromosome 21 open reading frame 59
• CBS: cystathionine-beta-synthase
• CLDN14: claudin 14
• HLCS: holocarboxylase synthetase (biotin-(proprionyl-Coenzyme A-carboxylase (ATP-hydrolysing)) ligase)
• KCNE1: potassium voltage-gated channel, Isk-related family, member 1
• KCNE2: potassium voltage-gated channel, Isk-related family, member 2
• LAD: leukocyte adhesion deficiency (symbols are ITGB2, CD18, LCAMB)
• SOD1: superoxide dismutase 1, soluble (amyotrophic lateral sclerosis 1 (adult))
• TMPRSS3: transmembrane protease, serine 3
• PCNT: centrosomal pericentrin
• DSCR1: Down Syndrome critical region 1
• DYRK1A: dual specificity tyrosine-(Y)-phosphorylation regulated kinase 1A
• RRP1B: ribosomal RNA processing 1 homolog B
• s100B: calcium binding protein

الأمراض والاضطرابات

القائمة التالية لبعض الأمراض المرتبطة بالجينات الموجودة على الكروموسوم 21:

• سقم ألزيامر
  • سقم ألزيامر النوع 1
• Amyotrophic lateral sclerosis
  • Amyotrophic lateral sclerosis type 1
• Autoimmune polyendocrine syndrome
  • Autoimmune polyendocrine syndrome type 1
• متلازمة داون
• متلازمة إرندو-سيمت
• Holocarboxylase synthetase deficiency
• Homocystinuria
• متلازمة جرڤل ولانگ-نايلسن
• Leukocyte adhesion deficiency
• Majewski osteodysplastic primordial dwarfism type II (MOPD II, or MOPD2)
• Nonsyndromic deafness
  • Nonsyndromic deafness, autosomal recessive
• متلازمة رومانو-وارد

حالات كروموسومية

الحالات التالية تحدث بسبب تغيرات في بنية أوعدد نسخ كروموسوم 21:

• السرطان: Rearrangements (translocations) of genetic material between chromosome 21 and other chromosomes have been associated with several types of cancer. For example, acute lymphoblastic leukemia (a type of blood cancer most often diagnosed in childhood) has been associated with a translocation between chromosomes 12 and 21. Another form of leukemia, acute myeloid leukemia, has been associated with a translocation between chromosomesثمانية and 21.
• في حالات قليلة، متلازمة داون قد تحدث by a rearrangement of chromosomal material between chromosome 21 and another chromosome. As a result, a person has the usual two copies of chromosome 21, plus extra material from chromosome 21 attached to another chromosome. These cases are called translocation Down syndrome. Researchers believe that extra copies of genes on chromosome 21 disrupt the course of normal development, causing the characteristic features of Down syndrome and the increased risk of medical problems associated with this disorder.
• Other changes in the number or structure of chromosome 21 can have a variety of effects, including mental retardation, delayed development, and characteristic facial features. In some cases, the signs and symptoms are similar to those of Down syndrome. Changes to chromosome 21 include a missing segment of the chromosome in each cell (partial monosomy 21) and a circular structure called ring chromosome 21. A ring chromosome occurs when both ends of a broken chromosome are reunited.
• Duplication in Amyloid precursor protein (APP) locus (duplicated segment varies in length but includes APP) on Chromosome 21 was found to cause early onset familial Alzheimer's disease in a French family set (Rovelet-Lecrux et al.) and a Dutch family set . Compared to Alzheimer's caused by missense mutations in APP, the frequency of the Alzheimer's caused by APP duplications is significant. All patients that have an extra copy of APP gene due to the locus duplication show Alzheimer's with severe cerebral amyloid angiopathy.

المصادر

• Antonarakis SE, Lyle R, Dermitzakis ET, Reymond A, Deutsch S (2004). "Chromosome 21 and down syndrome: from genomics to pathophysiology". Nat Rev Genet. 5 (10): 725–38. doi:10.1038/nrg1448. PMID 15510164.CS1 maint: multiple names: authors list (link)
• Antonarakis SE, Lyle R, Deutsch S, Reymond A (2002). "Chromosome 21: a small land of fascinating disorders with unknown pathophysiology". Int J Dev Biol. 46 (1): 89–96. PMID 11902692.CS1 maint: multiple names: authors list (link)
• Antonarakis SE (2001). "Chromosome 21: from sequence to applications". Curr Opin Genet Dev. 11 (3): 241–6. doi:10.1016/S0959-437X(00)00185-4. PMID 11377958.
• Gilbert F (1997). "Disease genes and chromosomes: disease maps of the human genome. Chromosome 21". Genet Test. 1 (4): 301–6. doi:10.1089/gte.1997.1.301. PMID 10464663.
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• Sawinska M, Ladon D (2004). "Mechanism, detection and clinical significance of the reciprocal translocation t(12;21)(p12;q22) in the children suffering from acute lymphoblastic leukaemia". Leuk Res. 28 (1): 35–42. doi:10.1016/S0145-2126(03)00160-7. PMID 14630078.
• Rovelet-Lecrux A,Hannequin D,Raux G,Le Meur N,Laquerriere A, Vital A,Dumanchin C,Feuillette S,Brice A,Vercelletto M, Dubas F,Frebourg T,Campion D. (2005). "APP locus duplication causes autosomal dominant early-onset Alzheimer disease with cerebral amyloid angiopathy". Nature Genetics. 38 (1): 24–6. doi:10.1038/ng1718. PMID 16369530.CS1 maint: multiple names: authors list (link)
• Anita Rauch, Christian T. Thiel, Detlev Schindler, Ursula Wick, Yanick J. Crow, Arif B. Ekici, Anthonie J. van Essen, Timm O. Goecke, Lihadh Al-Gazali, Krystyna H. Chrzanowska, Christiane Zweier, Han G. Brunner, Kristin Becker, Cynthia J. Curry, Bruno Dallapiccola, Koenraad Devriendt, Arnd Dörfler, Esther Kinning, André Megarbane, Peter Meinecke, Robert K. Semple, Stephanie Spranger, Annick Toutain, Richard C. Trembath, Egbert Voss, Louise Wilson, Raoul Hennekam, Francis de Zegher, Helmut-Günther Dörr, André Reis (2008). "Mutations in the Pericentrin (PCNT) Gene Cause Primordial Dwarfism". Science Online: 7.CS1 maint: multiple names: authors list (link)