Down syndrome pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Syed Hassan A. Kazmi BSc, MD [2]

## Overview

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Down Syndrome (DS) is the consequence of trisomy of human chromosome 21 (Hsa21) and is the most common genetic form of intellectual disability. Additional copy of chromosome 21 results in elevated expression of many of the genes encoded on this chromosome, leading to variying expression of genes associated with this chromosome. Mechanisms leading to trisomy 21 include meiotic non-disjunction during meiosis I (majority) and meiosis II, Robertsonian translocation and mosaicism (rare). In addition, increased maternal age leads to rapid degradation of cellular proteins involved in spindle formation, sister chromatid cohesion and anaphase separation of sister chromatids in oocytes during cell cycle. Absence of chiasmata and suboptimally placed chiasmata are the major mechanisms involved in non-disjunction of chromosome 21.Immaturity of the feto-placental unit has been proposed as an explanation for the reduced maternal serum alpha fetoprotein (AFP) and unconjugated oestriol (UE3) levels and increased hCG levels in Down’s syndrome pregnancies. Reduced synthesis of AFP by the fetal liver is also thought to contribute to low AFP in Down’s syndrome pregnancies. Robertsonian translocation occurrs when the long arms of 2 acrocentric chromosomes (chromosomes with centromeres near their ends) fuse at the centromeres and the 2 short arms are lost. Mosaicism does not have any maternal association and it is a post-fertilization mitotic error. Disbilities found in Down syndrome patients are thought to arise secondary to varied genetic expression associated with the presence an extra 21st chromosome.

## Pathophysiology

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* Down Syndrome (DS) is the consequence of trisomy of human chromosome 21 (Hsa21) and is the most common genetic form of intellectual disability.
* Additional copy of chromosome 21 results in elevated expression of many of the genes encoded on this chromosome, leading to variying expression of genes associated with this chromosome.[1][2][3]
* Recent data points towards a number of ‘susceptibility regions’ located on Hsa21, which are modified by other loci on Hsa21 and other genomic regions, increase the risk of developing specific DS associated phenotypes.[4][5]

### Mechanisms of trisomy 21

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#### Meiotic non-disjunction

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* In trisomy 21 the extra chromosome 21 is maternal in origin in about 95 percent of the cases, and paternal in only about 5 percent.
* In approximately 95% cases, the extra chromosome occurs as a result of meiotic nondisjunction (NDJ) or abnormal segregation of chromosomes. Of these, in the majority of cases the error occurs during maternal oogenesis, specially during meiosis I.[6][7]
* The process of oogenesis is lengthy and involves meiotic arrest, which makes predisposes the process to inappropriate segregation of chromosomes than spermatogenesis.[8]
* In addition, increased maternal age leads to rapid degradation of cellular proteins involved in spindle formation, sister chromatid cohesion and anaphase separation of sister chromatids in oocytes during cell cycle.[9][10][11]
* Nondisjoined chromosomes often show recombination in various patterns and for trisomy 21, achiasmate meioses contribute about 45% of maternal meiotic error cases.[12][13]
* Absence of chiasmata and suboptimally placed chiasmata are the major mechanisms involved in non-disjunction of chromosome 21.[14]
* Exchange of telomeric segments increase the risk for MI error, whereas exchanges in the pericentromeric regions predispose to MII error. A distally placed chiasma probably links the homologue less efficiently to the spindle and leads to suboptimal orientation of the kinetochore towards opposite pole.[15]
* Achiasmate meioses and mono-telomeric exchanges lead to an increased risk of NDJ regardless of maternal age.[16]
* On the other hand, pericentromeric exchange pedispose older mothers to NDJ during meiosis II.[17][18]
* Immaturity of the feto-placental unit has been proposed as an explanation for the reduced maternal serum alpha fetoprotein (AFP) and unconjugated oestriol (UE3) levels and increased hCG levels in Down’s syndrome pregnancies. Reduced synthesis of AFP by the fetal liver is also thought to contribute to low AFP in Down’s syndrome pregnancies.
* In Down’s syndrome pregnancies the normal proportion of syncytiotrophoblasts to cytotrophoblasts is disturbed leading to increased hCG levels.

#### Robertsonian translocation

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* Approximately 4 percent cases of DS arise from Robertsonian translocation.
* This involves non-reciprocal chromosomal translocation that commonly involves chromosome pairs 13, 14, 15, 21 and 22.
* The event occurrs when the long arms of 2 acrocentric chromosomes (chromosomes with centromeres near their ends) fuse at the centromeres and the 2 short arms are lost
* Unbalanced translocations result in miscarriage, stillbirth and chromosomal imbalance (Down's syndrome, Patau syndrome)

#### Mosaicism

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* Down syndrome resulting from mosaicism is very rare and accounts for around 1 percent of cases of DS.
* Mosaicism does not have any maternal association and it is a post-fertilization mitotic error.

### Effects on increased gene dosage

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* The phenotypical characrteristics produced in DS may be attributed to the disturbed gene expression due to trisomy 21.

#### Learning and memory

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* Patients with DS have learning and memory problems and exhibit differences in brain structure compared to the euploid population.[19][20][21]
* Impaired synaptic plasticity in the interstriatal cholinergic system has been though to play a key role in DS-associated motor and cognitive defects.[22]
* Murine models have allowed to identify genes thought to affect memory and learning in DS patients. The main genes involved are:[23][24][25][26][27][28][29]
* DYRK1A
* Synaptojanin 1(SYNJ1)
* SIM2
* DOPEY2
* DSCAM

#### Neurodevelopment

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* DS patients have increased rates of neuronal apoptosis related to oxidative stress.[30]
* Brain size of fetuses carrying trisomy 21 is smaller than euploid fetuses.
* Murine models have suggested that disruption in expression of the following genes may play key roles in affecting neurodevelopment in DS patients:
* PREP1[31]
* Tetratricopeptide repeat domain 3 (TTC3)[32]
* DYRK1A[33][34]
* MECP2[35]

#### Alzheimer's disease

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* Trisomy 21 leads to increased expression of amyloid precursor protein (APP), which is suspected to predispose individuals with DS to early-onset Alzheimer's disease (dementia by the age of 60).[36][37]
* Patients with Alzheimer's disease exhibit brain atrophy, extracellular β-amyloid (Aβ) deposits and the accumulation of neurofibrillary tangles (NFTs) that are composed of hyperphosphorylated Tau. The amyloid precursor protein, (APP), from which Aβ is produced, is encoded on Hsa21.
* Trisomy 21 leads to loss of basal forebrain cholinergic neurons and enlargement of early endosomes.[38][39]
* DYRK1A gene (found on chromosome 21) encodes a kinase which may contribute to phosphorylation of Tau found in patients suffering from Alzheimer's disease.[40]

#### Cancer and leukemias

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* DS predisposes individuals to developing myeloproliferative dosorders, as well as acute megakaryocytic leukemia (AMKL) and acute lymphoblastic leukemia (ALL).[41][42][43]
* Trisomy 21 lies at the heart of these abnormal cell proliferations.
* Varied expression of the following genes may lead to the development of disorders of cellular proliferation:
* GATA binding protein 1 (GATA1) gene\- Transient myeloproliferative disorder[41][44][45]
* Janus kinase 3 (JAK3)- Acute megakaryocytic leukemia (AMKL)[43][46][47]
* Janus kinase 2 (JAK2) point mutation- Acute lymphoblastic leukemia (ALL)[48][49]

## Genetics

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Expression of the following genes may be disturbed in trisomy 21:

Table 1: Some genes located on the long arm of chromosome 21[50] Gene  | OMIM Reference  | Location  | Purported Function

|
|
|

APP | 104760 | 21q21 | Amyloid beta A4 precursor protein. Suspected to have a major role in cognitive difficulties. One of the first genes studied with transgenic mice with Down syndrome.[51]
SOD1 | 147450 | 21q22.1 | Superoxide dismutase. Possible role in Alzheimer's disease. Anti-oxidant as well as possible affects on the immuno-system.
DYRK  | 600855 | 21q22.1 | Dual-specificity Tyrosine Phosphorylation-Regulated Kinase 1A. May have an effect on mental development through abnormal neurogenesis. [52]
IFNAR  | 107450 | 21q22.1 | Interferon, Alpha, Beta, and Omega, Receptor. Responsible for the expression of interferon, which affects the immuno-system.
DSCR1  | 602917 | 21q22.1–21q22.2 | Down Syndrome Critical Region Gene 1. Possibly part of a signal transduction pathway involving both heart and brain.[53]
COL6A1  | 120220 | 21q22.3 | Collagen, type I, alpha 1 gene. May have an effect on heart disease.
ETS2  | 164740 | 21q22.3 | Avian Erythroblastosis Virus E26 Oncogene Homolog 2. Researchers have "demonstrated that overexpression of ETS2 results in apoptosis. Transgenic mice overexpressing ETS2 developed a smaller thymus and lymphocyte abnormalities, similar to features observed in Down syndrome."[54]
CRYA1  | 123580 | 21q22.3 | Crystallin, Alpha-A. Involved in the synthesis of Crystallin, a major component of the lens in eyes. May be cause of cataracts.

## Associated Conditions

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The following conditions may be associated with Down's syndrome:

* Early onset Alzheimer's disease
* Hypothyroidism
* Hyperthyroidism
* Mental retardation
* Leukemias
* Infertility
* Delayed puberty
* Autoimmune diseases:
* Diabetes
* Celiac disease
* Short stature
* Congenital heart defects (atrioventricular canal defect, ventricular septal defect, atrial septal defect, patent ductus arteriosus, tetralogy of Fallot)
* Hearing loss (related to otitis media with effusion or sensorineural)
* Ophthalmic disorders (congenital cataracts, glaucoma, strabismus)
* Gastrointestinal malformations (duodenal atresia, Hirschsprung disease)

## Gross Pathology

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There are no gross pathological findings associated with Down syndrome.

## Microscopic Pathology

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There are no microscopic findings associated with Down syndrome.

## References

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1. ↑ Prandini P, Deutsch S, Lyle R, Gagnebin M, Delucinge Vivier C, Delorenzi M, Gehrig C, Descombes P, Sherman S, Dagna Bricarelli F, Baldo C, Novelli A, Dallapiccola B, Antonarakis SE (August 2007). "Natural gene-expression variation in Down syndrome modulates the outcome of gene-dosage imbalance". Am. J. Hum. Genet. 81 (2): 252–63. doi:10.1086/519248. PMC 1950802. PMID 17668376.
2. ↑ Sultan M, Piccini I, Balzereit D, Herwig R, Saran NG, Lehrach H, Reeves RH, Yaspo ML (2007). "Gene expression variation in Down's syndrome mice allows prioritization of candidate genes". Genome Biol. 8 (5): R91. doi:10.1186/gb-2007-8-5-r91. PMC 1929163. PMID 17531092.
3. ↑ Aït Yahya-Graison E, Aubert J, Dauphinot L, Rivals I, Prieur M, Golfier G, Rossier J, Personnaz L, Creau N, Bléhaut H, Robin S, Delabar JM, Potier MC (September 2007). "Classification of human chromosome 21 gene-expression variations in Down syndrome: impact on disease phenotypes". Am. J. Hum. Genet. 81 (3): 475–91. doi:10.1086/520000. PMC 1950826. PMID 17701894.
4. ↑ Korbel JO, Tirosh-Wagner T, Urban AE, Chen XN, Kasowski M, Dai L, Grubert F, Erdman C, Gao MC, Lange K, Sobel EM, Barlow GM, Aylsworth AS, Carpenter NJ, Clark RD, Cohen MY, Doran E, Falik-Zaccai T, Lewin SO, Lott IT, McGillivray BC, Moeschler JB, Pettenati MJ, Pueschel SM, Rao KW, Shaffer LG, Shohat M, Van Riper AJ, Warburton D, Weissman S, Gerstein MB, Snyder M, Korenberg JR (July 2009). "The genetic architecture of Down syndrome phenotypes revealed by high-resolution analysis of human segmental trisomies". Proc. Natl. Acad. Sci. U.S.A. 106 (29): 12031–6. doi:10.1073/pnas.0813248106. PMC 2709665. PMID 19597142.
5. ↑ Lyle R, Béna F, Gagos S, Gehrig C, Lopez G, Schinzel A, Lespinasse J, Bottani A, Dahoun S, Taine L, Doco-Fenzy M, Cornillet-Lefèbvre P, Pelet A, Lyonnet S, Toutain A, Colleaux L, Horst J, Kennerknecht I, Wakamatsu N, Descartes M, Franklin JC, Florentin-Arar L, Kitsiou S, Aït Yahya-Graison E, Costantine M, Sinet PM, Delabar JM, Antonarakis SE (April 2009). "Genotype-phenotype correlations in Down syndrome identified by array CGH in 30 cases of partial trisomy and partial monosomy chromosome 21". Eur. J. Hum. Genet. 17 (4): 454–66. doi:10.1038/ejhg.2008.214. PMC 2986205. PMID 19002211.
6. ↑ Antonarakis SE (March 1991). "Parental origin of the extra chromosome in trisomy 21 as indicated by analysis of DNA polymorphisms. Down Syndrome Collaborative Group". N. Engl. J. Med. 324 (13): 872–6. doi:10.1056/NEJM199103283241302. PMID 1825697.
7. ↑ Antonarakis SE, Petersen MB, McInnis MG, Adelsberger PA, Schinzel AA, Binkert F, Pangalos C, Raoul O, Slaugenhaupt SA, Hafez M (March 1992). "The meiotic stage of nondisjunction in trisomy 21: determination by using DNA polymorphisms". Am. J. Hum. Genet. 50 (3): 544–50. PMC 1684265. PMID 1347192.
8. ↑ Oliver TR, Feingold E, Yu K, Cheung V, Tinker S, Yadav-Shah M, Masse N, Sherman SL (March 2008). "New insights into human nondisjunction of chromosome 21 in oocytes". PLoS Genet. 4 (3): e1000033. doi:10.1371/journal.pgen.1000033. PMC 2265487. PMID 18369452.
9. ↑ Hawley RS, Frazier JA, Rasooly R (September 1994). "Separation anxiety: the etiology of nondisjunction in flies and people". Hum. Mol. Genet. 3 (9): 1521–8. PMID 7833906.
10. ↑ Wolstenholme J, Angell RR (November 2000). "Maternal age and trisomy--a unifying mechanism of formation". Chromosoma. 109 (7): 435–8. PMID 11151672.
11. ↑ Yoon PW, Freeman SB, Sherman SL, Taft LF, Gu Y, Pettay D, Flanders WD, Khoury MJ, Hassold TJ (March 1996). "Advanced maternal age and the risk of Down syndrome characterized by the meiotic stage of chromosomal error: a population-based study". Am. J. Hum. Genet. 58 (3): 628–33. PMC 1914585. PMID 8644722.
12. ↑ Sherman SL, Allen EG, Bean LH, Freeman SB (2007). "Epidemiology of Down syndrome". Ment Retard Dev Disabil Res Rev. 13 (3): 221–7. doi:10.1002/mrdd.20157. PMID 17910090.
13. ↑ Koehler KE, Hawley RS, Sherman S, Hassold T (1996). "Recombination and nondisjunction in humans and flies". Hum. Mol. Genet. 5 Spec No: 1495–504. PMID 8875256.
14. ↑ Lamb NE, Freeman SB, Savage-Austin A, Pettay D, Taft L, Hersey J, Gu Y, Shen J, Saker D, May KM, Avramopoulos D, Petersen MB, Hallberg A, Mikkelsen M, Hassold TJ, Sherman SL (December 1996). "Susceptible chiasmate configurations of chromosome 21 predispose to non-disjunction in both maternal meiosis I and meiosis II". Nat. Genet. 14 (4): 400–5. doi:10.1038/ng1296-400. PMID 8944019.
15. ↑ Hawley RS, Frazier JA, Rasooly R (September 1994). "Separation anxiety: the etiology of nondisjunction in flies and people". Hum. Mol. Genet. 3 (9): 1521–8. PMID 7833906.
16. ↑ Lamb NE, Yu K, Shaffer J, Feingold E, Sherman SL (January 2005). "Association between maternal age and meiotic recombination for trisomy 21". Am. J. Hum. Genet. 76 (1): 91–9. doi:10.1086/427266. PMC 1196437. PMID 15551222.
17. ↑ Lamb NE, Yu K, Shaffer J, Feingold E, Sherman SL (January 2005). "Association between maternal age and meiotic recombination for trisomy 21". Am. J. Hum. Genet. 76 (1): 91–9. doi:10.1086/427266. PMC 1196437. PMID 15551222.
18. ↑ Oliver TR, Feingold E, Yu K, Cheung V, Tinker S, Yadav-Shah M, Masse N, Sherman SL (March 2008). "New insights into human nondisjunction of chromosome 21 in oocytes". PLoS Genet. 4 (3): e1000033. doi:10.1371/journal.pgen.1000033. PMC 2265487. PMID 18369452.
19. ↑ Vicari S, Carlesimo GA (June 2006). "Short-term memory deficits are not uniform in Down and Williams syndromes". Neuropsychol Rev. 16 (2): 87–94. doi:10.1007/s11065-006-9008-4. PMID 16967345.
20. ↑ Carlesimo GA, Marotta L, Vicari S (January 1997). "Long-term memory in mental retardation: evidence for a specific impairment in subjects with Down's syndrome". Neuropsychologia. 35 (1): 71–9. PMID 8981379.
21. ↑ Aylward EH, Li Q, Honeycutt NA, Warren AC, Pulsifer MB, Barta PE, Chan MD, Smith PD, Jerram M, Pearlson GD (April 1999). "MRI volumes of the hippocampus and amygdala in adults with Down's syndrome with and without dementia". Am J Psychiatry. 156 (4): 564–8. doi:10.1176/ajp.156.4.564. PMID 10200735.
22. ↑ Di Filippo M, Tozzi A, Ghiglieri V, Picconi B, Costa C, Cipriani S, Tantucci M, Belcastro V, Calabresi P (April 2010). "Impaired plasticity at specific subset of striatal synapses in the Ts65Dn mouse model of Down syndrome". Biol. Psychiatry. 67 (7): 666–71. doi:10.1016/j.biopsych.2009.08.018. PMID 19818432.
23. ↑ Ahn KJ, Jeong HK, Choi HS, Ryoo SR, Kim YJ, Goo JS, Choi SY, Han JS, Ha I, Song WJ (June 2006). "DYRK1A BAC transgenic mice show altered synaptic plasticity with learning and memory defects". Neurobiol. Dis. 22 (3): 463–72. doi:10.1016/j.nbd.2005.12.006. PMID 16455265.
24. ↑ Yu HH, Yang JS, Wang J, Huang Y, Lee T (February 2009). "Endodomain diversity in the Drosophila Dscam and its roles in neuronal morphogenesis". J. Neurosci. 29 (6): 1904–14. doi:10.1523/JNEUROSCI.5743-08.2009. PMC 2671081. PMID 19211897.
25. ↑ Best TK, Siarey RJ, Galdzicki Z (January 2007). "Ts65Dn, a mouse model of Down syndrome, exhibits increased GABAB-induced potassium current". J. Neurophysiol. 97 (1): 892–900. doi:10.1152/jn.00626.2006. PMID 17093127.
26. ↑ Ema M, Ikegami S, Hosoya T, Mimura J, Ohtani H, Nakao K, Inokuchi K, Katsuki M, Fujii-Kuriyama Y (August 1999). "Mild impairment of learning and memory in mice overexpressing the mSim2 gene located on chromosome 16: an animal model of Down's syndrome". Hum. Mol. Genet. 8 (8): 1409–15. PMID 10400987.
27. ↑ Best TK, Cho-Clark M, Siarey RJ, Galdzicki Z (June 2008). "Speeding of miniature excitatory post-synaptic currents in Ts65Dn cultured hippocampal neurons". Neurosci. Lett. 438 (3): 356–61. doi:10.1016/j.neulet.2008.04.039. PMID 18490108.
28. ↑ Meng X, Shi J, Peng B, Zou X, Zhang C (April 2006). "Effect of mouse Sim2 gene on the cell cycle of PC12 cells". Cell Biol. Int. 30 (4): 349–53. doi:10.1016/j.cellbi.2005.11.012. PMID 16530433.
29. ↑ Rachidi M, Delezoide AL, Delabar JM, Lopes C (June 2009). "A quantitative assessment of gene expression (QAGE) reveals differential overexpression of DOPEY2, a candidate gene for mental retardation, in Down syndrome brain regions". Int. J. Dev. Neurosci. 27 (4): 393–8. doi:10.1016/j.ijdevneu.2009.02.001. PMID 19460634.
30. ↑ Busciglio J, Yankner BA (1995). "Apoptosis and increased generation of reactive oxygen species in Down's syndrome neurons in vitro". Nature. 378 (6559): 776–9. doi:10.1038/378776a0. PMID 8524410.
31. ↑ Micali N, Longobardi E, Iotti G, Ferrai C, Castagnaro L, Ricciardi M, Blasi F, Crippa MP (June 2010). "Down syndrome fibroblasts and mouse Prep1-overexpressing cells display increased sensitivity to genotoxic stress". Nucleic Acids Res. 38 (11): 3595–604. doi:10.1093/nar/gkq019. PMC 2887940. PMID 20110257.
32. ↑ Suizu F, Hiramuki Y, Okumura F, Matsuda M, Okumura AJ, Hirata N, Narita M, Kohno T, Yokota J, Bohgaki M, Obuse C, Hatakeyama S, Obata T, Noguchi M (December 2009). "The E3 ligase TTC3 facilitates ubiquitination and degradation of phosphorylated Akt". Dev. Cell. 17 (6): 800–10. doi:10.1016/j.devcel.2009.09.007. PMID 20059950.
33. ↑ Lepagnol-Bestel AM, Zvara A, Maussion G, Quignon F, Ngimbous B, Ramoz N, Imbeaud S, Loe-Mie Y, Benihoud K, Agier N, Salin PA, Cardona A, Khung-Savatovsky S, Kallunki P, Delabar JM, Puskas LG, Delacroix H, Aggerbeck L, Delezoide AL, Delattre O, Gorwood P, Moalic JM, Simonneau M (April 2009). "DYRK1A interacts with the REST/NRSF-SWI/SNF chromatin remodelling complex to deregulate gene clusters involved in the neuronal phenotypic traits of Down syndrome". Hum. Mol. Genet. 18 (8): 1405–14. doi:10.1093/hmg/ddp047. PMID 19218269.
34. ↑ Canzonetta C, Mulligan C, Deutsch S, Ruf S, O'Doherty A, Lyle R, Borel C, Lin-Marq N, Delom F, Groet J, Schnappauf F, De Vita S, Averill S, Priestley JV, Martin JE, Shipley J, Denyer G, Epstein CJ, Fillat C, Estivill X, Tybulewicz VL, Fisher EM, Antonarakis SE, Nizetic D (September 2008). "DYRK1A-dosage imbalance perturbs NRSF/REST levels, deregulating pluripotency and embryonic stem cell fate in Down syndrome". Am. J. Hum. Genet. 83 (3): 388–400. doi:10.1016/j.ajhg.2008.08.012. PMC 2556438. PMID 18771760.
35. ↑ Kuhn DE, Nuovo GJ, Terry AV, Martin MM, Malana GE, Sansom SE, Pleister AP, Beck WD, Head E, Feldman DS, Elton TS (January 2010). "Chromosome 21-derived microRNAs provide an etiological basis for aberrant protein expression in human Down syndrome brains". J. Biol. Chem. 285 (2): 1529–43. doi:10.1074/jbc.M109.033407. PMC 2801278. PMID 19897480.
36. ↑ Rovelet-Lecrux A, Hannequin D, Raux G, Le Meur N, Laquerrière A, Vital A, Dumanchin C, Feuillette S, Brice A, Vercelletto M, Dubas F, Frebourg T, Campion D (January 2006). "APP locus duplication causes autosomal dominant early-onset Alzheimer disease with cerebral amyloid angiopathy". Nat. Genet. 38 (1): 24–6. doi:10.1038/ng1718. PMID 16369530.
37. ↑ Cabrejo L, Guyant-Maréchal L, Laquerrière A, Vercelletto M, De la Fournière F, Thomas-Antérion C, Verny C, Letournel F, Pasquier F, Vital A, Checler F, Frebourg T, Campion D, Hannequin D (November 2006). "Phenotype associated with APP duplication in five families". Brain. 129 (Pt 11): 2966–76. doi:10.1093/brain/awl237. PMID 16959815.
38. ↑ Salehi A, Delcroix JD, Belichenko PV, Zhan K, Wu C, Valletta JS, Takimoto-Kimura R, Kleschevnikov AM, Sambamurti K, Chung PP, Xia W, Villar A, Campbell WA, Kulnane LS, Nixon RA, Lamb BT, Epstein CJ, Stokin GB, Goldstein LS, Mobley WC (July 2006). "Increased App expression in a mouse model of Down's syndrome disrupts NGF transport and causes cholinergic neuron degeneration". Neuron. 51 (1): 29–42. doi:10.1016/j.neuron.2006.05.022. PMID 16815330.
39. ↑ Seo H, Isacson O (June 2005). "Abnormal APP, cholinergic and cognitive function in Ts65Dn Down's model mice". Exp. Neurol. 193 (2): 469–80. doi:10.1016/j.expneurol.2004.11.017. PMID 15869949.
40. ↑ Ryoo SR, Jeong HK, Radnaabazar C, Yoo JJ, Cho HJ, Lee HW, Kim IS, Cheon YH, Ahn YS, Chung SH, Song WJ (November 2007). "DYRK1A-mediated hyperphosphorylation of Tau. A functional link between Down syndrome and Alzheimer disease". J. Biol. Chem. 282 (48): 34850–7. doi:10.1074/jbc.M707358200. PMID 17906291.
41. ↑ 41.0 41.1 Wechsler J, Greene M, McDevitt MA, Anastasi J, Karp JE, Le Beau MM, Crispino JD (September 2002). "Acquired mutations in GATA1 in the megakaryoblastic leukemia of Down syndrome". Nat. Genet. 32 (1): 148–52. doi:10.1038/ng955. PMID 12172547.
42. ↑ Izraeli S, Rainis L, Hertzberg L, Smooha G, Birger Y (2007). "Trisomy of chromosome 21 in leukemogenesis". Blood Cells Mol. Dis. 39 (2): 156–9. doi:10.1016/j.bcmd.2007.04.004. PMID 17532652.
43. ↑ 43.0 43.1 Malinge S, Ben-Abdelali R, Settegrana C, Radford-Weiss I, Debre M, Beldjord K, Macintyre EA, Villeval JL, Vainchenker W, Berger R, Bernard OA, Delabesse E, Penard-Lacronique V (March 2007). "Novel activating JAK2 mutation in a patient with Down syndrome and B-cell precursor acute lymphoblastic leukemia". Blood. 109 (5): 2202–4. doi:10.1182/blood-2006-09-045963. PMID 17068151.
44. ↑ Groet J, McElwaine S, Spinelli M, Rinaldi A, Burtscher I, Mulligan C, Mensah A, Cavani S, Dagna-Bricarelli F, Basso G, Cotter FE, Nizetic D (May 2003). "Acquired mutations in GATA1 in neonates with Down's syndrome with transient myeloid disorder". Lancet. 361 (9369): 1617–20. doi:10.1016/S0140-6736(03)13266-7. PMID 12747884.
45. ↑ Stepensky P, Brooks R, Waldman E, Revel-Vilk S, Izraeli S, Resnick I, Weintraub M (July 2010). "A rare case of GATA1 negative chemoresistant acute megakaryocytic leukemia in an 8-month-old infant with trisomy 21". Pediatr Blood Cancer. 54 (7): 1048–9. doi:10.1002/pbc.22331. PMID 20108342.
46. ↑ Sato T, Toki T, Kanezaki R, Xu G, Terui K, Kanegane H, Miura M, Adachi S, Migita M, Morinaga S, Nakano T, Endo M, Kojima S, Kiyoi H, Mano H, Ito E (May 2008). "Functional analysis of JAK3 mutations in transient myeloproliferative disorder and acute megakaryoblastic leukaemia accompanying Down syndrome". Br. J. Haematol. 141 (5): 681–8. doi:10.1111/j.1365-2141.2008.07081.x. PMID 18397343.
47. ↑ De Vita S, Mulligan C, McElwaine S, Dagna-Bricarelli F, Spinelli M, Basso G, Nizetic D, Groet J (May 2007). "Loss-of-function JAK3 mutations in TMD and AMKL of Down syndrome". Br. J. Haematol. 137 (4): 337–41. doi:10.1111/j.1365-2141.2007.06574.x. PMID 17456055.
48. ↑ Gaikwad A, Rye CL, Devidas M, Heerema NA, Carroll AJ, Izraeli S, Plon SE, Basso G, Pession A, Rabin KR (March 2009). "Prevalence and clinical correlates of JAK2 mutations in Down syndrome acute lymphoblastic leukaemia". Br. J. Haematol. 144 (6): 930–2. doi:10.1111/j.1365-2141.2008.07552.x. PMC 2724897. PMID 19120350.
49. ↑ Hertzberg L, Vendramini E, Ganmore I, Cazzaniga G, Schmitz M, Chalker J, Shiloh R, Iacobucci I, Shochat C, Zeligson S, Cario G, Stanulla M, Strehl S, Russell LJ, Harrison CJ, Bornhauser B, Yoda A, Rechavi G, Bercovich D, Borkhardt A, Kempski H, te Kronnie G, Bourquin JP, Domany E, Izraeli S (February 2010). "Down syndrome acute lymphoblastic leukemia, a highly heterogeneous disease in which aberrant expression of CRLF2 is associated with mutated JAK2: a report from the International BFM Study Group". Blood. 115 (5): 1006–17. doi:10.1182/blood-2009-08-235408. PMID 19965641.
50. ↑ See Leshin, L. (2003). "Trisomy 21: The Story of Down Syndrome". Retrieved 2006-05-21.
51. ↑ Chandra Shekhar (6 July 2006). "Down syndrome traced to one gene". The Scientist. Retrieved 2006-07-11. Check date values in: `|date=` (help)
52. ↑ Song, W.-J., Sternberg, L. R., Kasten-Sportes, C., Van Keuren, M. L., Chung, S.-H., Slack, A. C., Miller, D. E., Glover, T. W., Chiang, P.-W., Lou, L.; Kurnit, D. M. (1996). "Isolation of human and murine homologues of the Drosophila minibrain gene: human homologue maps to 21q22.2 in the Down syndrome 'critical region". Genomics. 38: 331–339.CS1 maint: Multiple names: authors list (link)
53. ↑ Fuentes JJ, Pritchard MA, Planas AM, Bosch A, Ferrer I, Estivill X (1995). "A new human gene from the Down syndrome critical region encodes a proline-rich protein highly expressed in fetal brain and heart". Hum Mol Genet. 4 (10): 1935–1944.CS1 maint: Multiple names: authors list (link)
54. ↑ OMIM, NIH. "V-ETS Avian Erythroblastosis virus E26 Oncogene Homolog 2". Retrieved 2006-06-29.

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