Trisomy 21 is the most common genetic cause of mental retardation and one of the few aneuploidies compatible with post-natal survival. It occurs in 1 out of 700 live births in all ethnic groups. The vast majority of meiotic errors leading to the trisomic condition occur in the egg, as nearly 90% of cases involve an additional maternal chromosome (1). Besides mental retardation, present in every individual with Down syndrome, trisomy 21 is associated with more than 80 clinical traits including congenital heart disease, duodenal stenosis or atresia, imperforate anus, Hirschprung disease, muscle hypotonia, immune system deficiencies, increased risk of childhood leukaemia and early onset Alzheimer's disease (1). The Human Down Syndrome Critical Region 1 gene is located on chromosome 21 in the region of 21q22.12. This gene is involved in cellular adaptation to oxidative stress, transiently protecting cells that have been primed with a low dose of an oxidant stress against subsequent, potentially lethal, challenges. It has been shown to be involved in Alzheimer's disease as well as the prevention of muscle hypertrophy and is likely to be involved in several disease pathways (2). The DSCR1 gene, also referred to as Adapt 78 and MCIP1, is known to bind and regulate calcineurin. It has been found to be expressed in diverse cell types and tissues, including heart/cardiac muscle, striated muscle, brain/neuronal cells, and T-cells, and plays an important role in calcineurin regulation in these cell types (3). This gene, encodes a protein that binds to and inhibits the catalytic subunit of calcineurin.
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Figure 1. Chromosome 21: DSCR1 (Adapt78) is located on chromosome 21 in the region q22.12. This lies outside of the Down Syndrome Candidate Region (DSCR). DSCR1 Genomic DNA: DSCR1 consists of seven exons separated by six introns that are alternatively spliced and vary in their 5' exon, but all contain exons 5, 6 and 7. There is a cluster of 15 NFAT binding
sites on the DSCR1 gene(2).
Calcineurin/NFAT Signalling Pathway
The nuclear factor of activated T-cells (NFAT) family of transcription factors was originally identified in lymphocytes for its role in cytokine gene expression. Beyond the immune system, NFAT proteins are expressed in many cell types including cardiac, skeletal, and vascular smooth muscle (4). NFAT proteins are downstream effectors in the calcineurin signalling pathway-a critical pathway in the transduction of many extracellular, adaptive stimuli. Calcineurin is a calcium-dependent, serine/threonine protein phosphatase that dephosphorylates NFAT to enable nuclear translocation and target gene transcription (4). Recently, it has been reported that the protein products of DSCR1, calcipressin 1s, are able to bind to and inhibit the catalytic subunit of calcineurin (protein phosphatase 2B). Calcineurin is under the control of calcium/calmodulin, it is a heterodimer, which consists of a catalytic subunit, calcineurin A, and a regulatory subunit, calcineurin B (2). Since DSCR1 proteins have been discovered to be endogenous inhibitors of calcineurin, they have been dubbed calcipressins. which has several known substrates - of these, the best characterized is the transcription factor NFAT.
Fig. 2. Calcineurin subunits and NFAT isoforms in mouse, Drosophila, and C. elegans. Conserved protein domains of calcineurin A, calcineurin B, and NFAT are presented. The regulatory part of calcineurin A includes calcineurin B and calmodulin interacting regions and an auto inhibitory domain. The four conserved EF-hand calcium-binding domains (C) of calcineurin B are indicated. Within NFAT, the transactivation domain (TAD) is located at the N-terminus, adjacent to a regulatory domain containing sequences required for calcineurin binding, serine-rich motifs, and nuclear localization signals. The conserved DNA binding domain is of the rel homology class(5).
Calcineurin signalling transduced by NFATs activation was first characterized in the immune system. Substrates of calcineurin phosphatase activity include most members of the NFAT family of transcription factors (5). Loss of specific NFAT isoforms results in cardiovascular, skeletal muscle, cartilage, neuronal, or immune system defects. Calcineurin and NFATs have also been implicated in the differentiation of bone, cartilage, muscle, skin, and fat in tissue culture experiments. Postnatally, this signalling pathway contributes to normal homeostasis as well as pathological conditions in the skin, cardiovascular system, skeletal muscle, immune cells, and central nervous system (5). These are many of the same organ systems that require calcineurin/NFAT signalling during embryonic development. Thus, there is accumulating evidence for the widespread utilization of calcineurin/NFAT signalling mechanisms in a broad spectrum of developmental and disease processes (5). Calcineurin dephosphorylates NFATs in response to increased intracellular calcium and regulates gene expression in a variety of calcium-sensitive tissues such as brain, muscle, and lymphocytes . Transcriptional activity of NFATs is dependent on dephosphorylation by calcineurin, which leads to nuclear translocation (5). An additional level of regulation of calcineurin activity is through calcineurin interacting proteins including Down syndrome Critical Region 1. NFATs bind DNA with low affinity and usually act in conjunction with other transcription factors such as AP-1, MEF2, or GATA4 . Thus, the control of NFAT transcriptional regulatory activity in association with other proteins likely represents a nodal point in intersecting signal transduction pathways including calcineurin, MAPK, p38, JNK, and Wnt (5). These include requirements in neural and muscular tissues that are among the organ systems affected in vertebrate embryos with altered calcineurin and NFAT signalling (5).
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Down Syndrome Critical Region Control on Organ and Vessel Development
Since DSCR1 has been confirmed to bind to calcineurin catalytic A subunit and act as a regulator of the calcineurin-mediated signalling pathway, it was hypothesized that the up-regulation of DSCR1 gene expression in endothelial cells might act as an endogenous regulator of angiogenesis by regulating the calcineurin-NFAT signalling pathway. Activation of calcineurin in endothelial cells by vascular epidermal growth factor (3) and calcium stimulators dephosphorylates NFATs, resulting in NFAT translocation to the nucleus where it activates transcription of target genes, including Cox-2, tissue factor, and isoform 4 of DSCR1. Newly synthesized DSCR1 protein then binds to and inhibits calcineurin. Therefore, DSCR1 synthesis serves as a negative feedback mechanism which dampens calcineurin activity and affects the expression of the downstream response genes (3). Studies have demonstrated that the promoter regions of DSCR1 isoform 4 genes contain multiple NFAT binding sites. These genes could therefore be potential downstream targets of DSCR1. Over expression of DSCR1 significantly down-regulates DSCR1 isoform 4 promoter activity (3). This inhibitory effect correlated with the number of NFAT binding sites in the promoter region: DSCR1 contains about 15 NFAT sites. Of particular importance, it should be noted that DSCR1 strongly regulates its own promoter activity via the calcineurin-NFAT signalling pathway(3). Since NFAT activity is important for the activation of many pro-angiogenic genes, including Cox-2 and tissue factor, feedback inhibition of NFAT by DSCR1 may represent a important molecular mechanism underlying the regulation of angiogenic genes activated by the calcineurin-NFAT signalling pathway in endothelial cells (3). With over expression of DSCR1 due to trisomy of chromosome 21, inhibiting angiogenesis would cause underdevelopment of vessels and organs and so is maybe a factor that causes Down Syndrome features.
Fig. 3. Hypothesized schematic profile of DSCR1 gene expression and its regulation in response to VEGF treatment in endothelial cells (3).
Down Syndrome Critical Region Control On Heart Development
Approximately 40-50% of the surviving Down Syndrome patients have characteristic congenital heart disease. Indeed, Down Syndrome is a major cause of congenital heart defects in humans. Endocardial cushion defect is the predominant cardiac abnormality in Down Syndrome, leading to characteristic atrioventricular septal or heart valve lesions (6). Previous work from laboratories and others demonstrates that the calcineurin/NFAT signalling pathway is essential for the development of endocardial cushions and heart valves, and that perturbations of the pathway lead to many features of Down syndrome. Activation of this evolutionally conserved DSCR1 regulatory sequence requires calcineurin and NFAT signalling in the endocardium(6). NFAT proteins bind to the regulatory sequence and trigger its enhancer activity. NFAT is sufficient to induce the expression of DSCR1 in cells that normally have undetectable or minimal NFAT or DSCR1 (6). Although recent work has shown that the expression of DSCR1 in neurons and the heart is induced by the NFAT pathway, it is not clear how DSCR1 interacts with NFAT within the endocardium during heart valve development. The role of calcineurin in regulating DSCR1 expression in the heart remains undefined. Nor is there unambiguous evidence that DSCR1 is a direct transcriptional target of NFAT in the heart (6). Also the calcineurin/NFAT signalling pathway also plays a part in vascular smooth muscle development, it has been shown to induce vascular smooth muscle cell proliferation and migration in response to receptor tyrosine kinase (RTK) and G-protein-coupled receptor (GPCR) agonists, respectively (9). Blocking Calcineurin/NFAT signalling in vivo suppresses experimental balloon injury-induced neointimal hyperplasia, suggesting Calcineurin/NFAT activity is involved in smooth muscle cell phenotypic modulation. With over expression of DSCR1 caused by triosomy of chromosome 21, this will block the calcineurin/NFAT signalling pathway and so suppress vascular smooth muscle cells which in turn causes underdevelopment of the heart (9).
Down Syndrome Critical Region Control on Brain Development
The regulator of calcineurin proteins (RCANs) are a family of small, highly conserved proteins that can bind and inhibit the calcium-regulated protein phosphatase calcineurin. Calcineurin is intimately involved in many facets of synaptic plasticity and memory formation because of its ability to directly dephosphorylate critical targets in both the presynaptic and postsynaptic compartments of neurons. Calcineurin also can indirectly promote dephosphorylation of many other proteins by increasing protein phosphatase 1 (PP1) activity (7). Calcineurin accomplishes this through dephosphorylation of the PP1 inhibitors inhibitor-1 (I-1) and dopamine and cAMP-regulated phosphoprotein-32 (DARPP-32). Behavioural and electro physiological studies using both genetic and pharmacological methods to alter calcineurin activity indicate that calcineurin is likely a negative regulator of learning, memory, and synaptic plasticity (7). Finally, dysregulation of calcineurin activity is associated with many forms of brain disease and injury, including Alzheimer's disease and excitotoxic ischemia. Because of the importance of calcineurin in regulating diverse neuronal processes, there is growing interest in the potential for trisomy of RCAN1, also known as DSCR1 to contribute to the plethora of phenotypes associated with Down syndrome through its interaction with calcineurin (7). Thus, understanding how RCAN1 contributes normally to mechanisms of learning and memory formation is critical to understanding its role in the deficits observed in Down syndrome and Alzheimer's patients(7).
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Other Genes and Transcription Factors That Can Contribute to the Development of Down Syndrome
Even though calcineurin/NFAT signalling pathway has a major role in the development of Down Syndrome phenotypes, there are also other transcription factors that are involved in the development of Down Syndrome. Prep1 belongs to the TALE class of homeodomain proteins and is essential for embryonic development (8). Prep1 is a transcription factor that, in combination with its major partners, Pbx proteins, regulates the overall size of the organism and individual organs as well as major developmental pathways. PREP1 maps on chromosome 21 (21;q22.3) in humans and chromosome 17 in mice and is over expressed 1.5-fold in brain tissues of Down Syndrome patients. The presence of PREP1 in the Down Syndrome Critical Region of chromosome 21 suggests that Prep1 might be involved in the phenotype of Down Syndrome (8). With over expression in brain tissue this could cause patients with Down Syndrome the characteristic learning difficulties. Another gene is DYRK1 which is localized within the Down syndrome critical region (DSCR) of human chromosome 21, and with DSCR1 gene synergistically reduce the nuclear occupancy of the NFAT proteins. DYRK1 is a kinase that promotes NFAT nuclear export. Apparently a 1.5-fold over expression of these two proteins leads to the dysregulation of calcineurin/NFAT signalling, resulting in a constellation of Down Syndrome features (8).
In conclusion Down Syndrome Critical Region has an important role in the development of the features and phenotype of Down Syndrome via the calcineurin/NFAT signalling pathway. This pathway has been shown to be involved in the underdevelopment of organs particularly the heart which contributes to the congenital heart disease, which a large proportion of Down Syndrome patients suffer with and also the brain where regulator of calcineurin proteins inhibit calcineurin and therefore inhibit the calcineurin/NFAT pathway which prevents normal neuronal and brain development. With knowledge on how features of Down Syndrome are brought about by what genes, proteins and pathways, new drugs or therapies could be developed to either block these pathways, preventing inhibition resulting in normal development, or by gene deletion so there is less expression of Down Syndrome Critical Region Proteins.