Jürgen Groet

Acquired mutations in GATA1 in neonates with Down's syndrome with transient myeloid disorder

Transient myeloid disorder is a unique self-regressing neoplasia specific to Downs syndrome. The transcription factor GATA1 is needed for normal growth and maturation of erythroid cells and megakaryocytes. Mutations in GATA1 have been reported in acute megakaryoblastic leukaemia in Downs syndrome. We aimed to investigate changes in GATA1 in patients with Downs syndrome and either transient myeloid disorder (n=10) or acute megakaryoblastic leukaemia (n=6). We recorded mutations eliminating exon 2 from GATA1 in all patients with transient myeloid disorder (age 0-24 days) and in all with acute megakaryoblastic leukaemia (age 14-38 months). The range of mutations did not differ between patients with each disorder. Patients with transient myeloid disorder with mutations in GATA1 can regress spontaneously to complete remission, and mutations do not necessarily predict later acute megakaryoblastic leukaemia.

Microarray transcript profiling distinguishes the transient from the acute type of megakaryoblastic leukaemia (M7) in Down's syndrome, revealing PRAME as a specific discriminating marker

Transient myeloproliferative disorder (TMD) is a unique, spontaneously regressing neoplasia specific to Downs syndrome (DS), affecting up to 10% of DS neonates. In 20–30% of cases, it reoccurs as progressive acute megakaryoblastic leukaemia (AMKL) at 2–4 years of age. The TMD and AMKL blasts are morphologically and immuno‐phenotypically identical, and have the same acquired mutations in GATA1. We performed transcript profiling of nine TMD patients comparing them with seven AMKL patients using Affymetrix HG‐U133A microarrays. Similar overall transcript profiles were observed between the two conditions, which were only separable by supervised clustering. Taqman analysis on 10 TMD and 10 AMKL RNA samples verified the expression of selected differing genes, with statistical significance (P < 0·05) by Students t‐test. The Taqman differences were also reproduced on TMD and AMKL blasts sorted by a fluorescence‐activated cell sorter. Among the significant differences, CDKN2C, the effector of GATA1‐mediated cell cycle arrest, was increased in AMKL but not TMD, despite the similar level of GATA1. In contrast, MYCN (neuroblastoma‐derived oncogene) was expressed in TMD at a significantly greater level than in AMKL. MYCN has not previously been described in leukaemogenesis. Finally, the tumour antigen PRAME was identified as a specific marker for AMKL blasts, with no expression in TMD. This study provides markers discriminating TMD from AMKL‐M7 in DS. These markers have the potential as predictive, diagnostic and therapeutic targets. In addition, the study provides further clues into the pathomechanisms discerning self‐regressive from the progressive phenotype.

Tumorigenesis in Down's syndrome: big lessons from a small chromosome

If assessed by a number of criteria for cancer predisposition, Downs syndrome (DS) should be an overwhelmingly cancer-prone condition. Although childhood leukaemias occur more frequently in DS, paradoxically, individuals with DS have a markedly lower incidence of most solid tumours. Understanding the mechanisms that are capable of overcoming such odds could potentially open new routes for cancer prevention and therapy. In this Opinion article, we discuss recent reports that suggest unique and only partially understood mechanisms behind this paradox, including tumour repression, anti-angiogenic effects and stem cell ageing and availability.

Loss‐of‐function JAK3 mutations in TMD and AMKL of Down syndrome

Acquired mutations activating Janus kinase 3 (jak3) have been reported in Down syndrome (DS) and non‐DS patients with acute megakaryoblastic leukaemia (AMKL). This highlighted jak3‐activation as an important event in the pathogenesis of AMKL, and predicted inhibitors of jak3 as conceptual therapeutics for AMKL. Of 16 DS‐transient myeloproliferative disorder (TMD)/AMKL patients tested, seven showed JAK3 mutations. Three mutations deleted the kinase (JH1) domain, abolishing the main function of jak3. Another patient displayed a mutation identical to a previously reported inherited loss‐of‐function causing severe combined immunodeficiency. Our data suggest that both gain‐, and loss‐of function mutations of jak3 can be acquired in DS‐TMD/AMKL.

Trisomic dose of several chromosome 21 genes perturbs haematopoietic stem and progenitor cell differentiation in Down's syndrome

Children with Downs syndrome (DS) have 20–50-fold higher incidence of all leukaemias (lymphoid and myeloid), for reasons not understood. As incidence of many solid tumours is much lower in DS, we speculated that disturbed early haematopoietic differentiation could be the cause of increased leukaemia risk. If a common mechanism is behind the risk of both major leukaemia types, it would have to arise before the bifurcation to myeloid and lymphoid lineages. Using the transchromosomic system (mouse embryonic stem cells (ESCs)) bearing an extra human chromosome 21 (HSA21)) we analyzed the early stages of haematopoietic commitment (mesodermal colony formation) in vitro. We observed that trisomy 21 (T21) causes increased production of haemogenic endothelial cells, haematopoietic stem cell precursors and increased colony forming potential, with significantly increased immature progenitors. Transchromosomic colonies showed increased expression of Gata-2, c-Kit and Tie-2. A panel of partial T21 ESCs allowed us to assign these effects to HSA21 sub-regions, mapped by 3.5 kbp-resolution tiling arrays. The Gata-2 increase on one side, and c-Kit and Tie-2 increases on the other, could be attributed to two different, non-overlapping HSA21 regions. Using human-specific small interfering RNA silencing, we could demonstrate that an extra copy of RUNX1, but not ETS-2 or ERG, causes an increase in Tie-2/c-Kit levels. Finally, we detected significantly increased levels of RUNX1, C-KIT and PU.1 in human foetal livers with T21. We conclude that overdose of more than one HSA21 gene contributes to the disturbance of early haematopoiesis in DS, and that one of the contributors is RUNX1. As the observed T21-driven hyperproduction of multipotential immature precursors precedes the bifurcation to lymphoid and myeloid lineages, we speculate that this could create conditions of increased chance for acquisition of pre-leukaemogenic rearrangements/mutations in both lymphoid and myeloid lineages during foetal haematopoiesis, contributing to the increased risk of both leukaemia types in DS.

The importance of understanding individual differences in Down syndrome

In this article, we first present a summary of the general assumptions about Down syndrome (DS) still to be found in the literature. We go on to show how new research has modified these assumptions, pointing to a wide range of individual differences at every level of description. We argue that, in the context of significant increases in DS life expectancy, a focus on individual differences in trisomy 21 at all levels—genetic, cellular, neural, cognitive, behavioral, and environmental—constitutes one of the best approaches for understanding genotype/phenotype relations in DS and for exploring risk and protective factors for Alzheimer’s disease in this high-risk population.

Frequent cases of RAS-mutated Down syndrome acute lymphoblastic leukaemia lack JAK2 mutations

Children with Down syndrome (DS) and acute lymphoblastic leukaemia (ALL) have poorer survival and more relapses than non-DS children with ALL, highlighting an urgent need for deeper mechanistic understanding of DS-ALL. Here, using full-exome or cancer genes-targeted sequencing of 42 ALL samples from 39 DS patients, we uncover driver mutations in RAS, (KRAS and NRAS) recurring to a similar extent (15/42) as JAK2 (12/42) mutations or P2RY8-CRLF2 fusions (14/42). RAS mutations are almost completely mutually exclusive with JAK2 mutations (P=0.016), driving a combined total of two-thirds of analysed cases. Clonal architecture analysis reveals that both RAS and JAK2 drove sub-clonal expansions primarily initiated by CRLF2 rearrangements, and/or mutations in chromatin remodellers and lymphocyte differentiation factors. Remarkably, in 2/3 relapsed cases, there is a switch from a primary JAK2- or PTPN11-mutated sub-clone to a RAS-mutated sub-clone in relapse. These results provide important new insights informing the patient stratification strategies for targeted therapeutic approaches for DS-ALL.

Brief report : isogenic induced pluripotent stem cell lines from an adult with mosaic down syndrome model accelerated neuronal ageing and neurodegeneration

Trisomy 21 (T21), Down Syndrome (DS) is the most common genetic cause of dementia and intellectual disability. Modeling DS is beginning to yield pharmaceutical therapeutic interventions for amelioration of intellectual disability, which are currently being tested in clinical trials. DS is also a unique genetic system for investigation of pathological and protective mechanisms for accelerated ageing, neurodegeneration, dementia, cancer, and other important common diseases. New drugs could be identified and disease mechanisms better understood by establishment of well‐controlled cell model systems. We have developed a first nonintegration‐reprogrammed isogenic human induced pluripotent stem cell (iPSC) model of DS by reprogramming the skin fibroblasts from an adult individual with constitutional mosaicism for DS and separately cloning multiple isogenic T21 and euploid (D21) iPSC lines. Our model shows a very low number of reprogramming rearrangements as assessed by a high‐resolution whole genome CGH‐array hybridization, and it reproduces several cellular pathologies seen in primary human DS cells, as assessed by automated high‐content microscopic analysis. Early differentiation shows an imbalance of the lineage‐specific stem/progenitor cell compartments: T21 causes slower proliferation of neural and faster expansion of hematopoietic lineage. T21 iPSC‐derived neurons show increased production of amyloid peptide‐containing material, a decrease in mitochondrial membrane potential, and an increased number and abnormal appearance of mitochondria. Finally, T21‐derived neurons show significantly higher number of DNA double‐strand breaks than isogenic D21 controls. Our fully isogenic system therefore opens possibilities for modeling mechanisms of developmental, accelerated ageing, and neurodegenerative pathologies caused by T21. Stem Cells 2015;33:2077–2084

Intracerebral haemorrhage in Down syndrome: protected or predisposed?

Down syndrome (DS), which arises from trisomy of chromosome 21, is associated with deposition of large amounts of amyloid within the central nervous system. Amyloid accumulates in two compartments: as plaques within the brain parenchyma and in vessel walls of the cerebral microvasculature. The parenchymal plaque amyloid is thought to result in an early onset Alzheimer’s disease (AD) dementia, a phenomenon so common amongst people with DS that it could be considered a defining feature of the condition. The amyloid precursor protein ( APP) gene lies on chromosome 21 and its presence in three copies in DS is thought to largely drive the early onset AD. In contrast, intracerebral haemorrhage (ICH), the main clinical consequence of vascular amyloidosis, is a more poorly defined feature of DS. We review recent epidemiological data on stroke (including haemorrhagic stroke) in order to make comparisons with a rare form of familial AD due to duplication (i.e. having three copies) of the APP region on chromosome 21, here called ‘dup-APP’, which is associated with more frequent and severe ICH. We conclude that although people with DS are at increased risk of ICH, this is less common than in dup-APP, suggesting the presence of mechanisms that act protectively. We review these mechanisms and consider comparative research into DS and dup-APP that may yield further pathophysiological insight.

Transchromosomic cell model of Down syndrome shows aberrant migration, adhesion and proteome response to extracellular matrix

BackgroundDown syndrome (DS), caused by trisomy of human chromosome 21 (HSA21), is the most common genetic birth defect. Congenital heart defects (CHD) are seen in 40% of DS children, and >50% of all atrioventricular canal defects in infancy are caused by trisomy 21, but the causative genes remain unknown.ResultsHere we show that aberrant adhesion and proliferation of DS cells can be reproduced using a transchromosomic model of DS (mouse fibroblasts bearing supernumerary HSA21). We also demonstrate a deacrease of cell migration in transchromosomic cells independently of their adhesion properties. We show that cell-autonomous proteome response to the presence of Collagen VI in extracellular matrix is strongly affected by trisomy 21.ConclusionThis set of experiments establishes a new model system for genetic dissection of the specific HSA21 gene-overdose contributions to aberrant cell migration, adhesion, proliferation and specific proteome response to collagen VI, cellular phenotypes linked to the pathogenesis of CHD.