cancers Review The Mutational Landscape of Myeloid Leukaemia in Down Syndrome Carini Picardi Morais de Castro 1, Maria Cadefau 1,2 and Sergi Cuartero 1,2,* 1 Josep Carreras Leukaemia Research Institute (IJC), Campus Can Ruti, 08916 Badalona, Spain; [email protected] (C.P.M.d.C); [email protected] (M.C.) 2 Germans Trias i Pujol Research Institute (IGTP), Campus Can Ruti, 08916 Badalona, Spain * Correspondence: [email protected] Simple Summary: Leukaemia occurs when specific mutations promote aberrant transcriptional and proliferation programs, which drive uncontrolled cell division and inhibit the cell’s capacity to differentiate. In this review, we summarize the most frequent genetic lesions found in myeloid leukaemia of Down syndrome, a rare paediatric leukaemia specific to individuals with trisomy 21. The evolution of this disease follows a well-defined sequence of events and represents a unique model to understand how the ordered acquisition of mutations drives malignancy. Abstract: Children with Down syndrome (DS) are particularly prone to haematopoietic disorders. Paediatric myeloid malignancies in DS occur at an unusually high frequency and generally follow a well-defined stepwise clinical evolution. First, the acquisition of mutations in the GATA1 transcription factor gives rise to a transient myeloproliferative disorder (TMD) in DS newborns. While this condition spontaneously resolves in most cases, some clones can acquire additional mutations, which trigger myeloid leukaemia of Down syndrome (ML-DS). These secondary mutations are predominantly found in chromatin and epigenetic regulators—such as cohesin, CTCF or EZH2—and Citation: de Castro, C.P.M.; Cadefau, in signalling mediators of the JAK/STAT and RAS pathways. Most of them are also found in non-DS M.; Cuartero, S. The Mutational myeloid malignancies, albeit at extremely different frequencies. Intriguingly, mutations in proteins Landscape of Myeloid Leukaemia in involved in the three-dimensional organization of the genome are found in nearly 50% of cases. How Down Syndrome. Cancers 2021, 13, the resulting mutant proteins cooperate with trisomy 21 and mutant GATA1 to promote ML-DS is not 4144. https://doi.org/10.3390/ fully understood. In this review, we summarize and discuss current knowledge about the sequential cancers13164144 acquisition of genomic alterations in ML-DS. Academic Editor: Ilaria Iacobucci Keywords: myeloid leukaemia; Down syndrome; trisomy 21; acute megakaryoblastic leukaemia Received: 3 July 2021 (AMKL); transcription; chromatin; signalling Accepted: 11 August 2021 Published: 18 August 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in Trisomy 21 is the most common chromosomal disorder in humans and is the genetic published maps and institutional affil- basis of Down syndrome (DS) [1]. This multisystem disorder results in numerous pheno- iations. typic features, including craniofacial abnormalities and cognitive impairment [2]. Although solid tumours are less frequent in the DS than the non-DS population [3–5], individuals with DS have a higher risk of developing haematopoietic disorders [4]. DS neonates have a variety of haematological abnormalities such as high haemoglobin concentration, large Copyright: © 2021 by the authors. mean cell volume, erythroblastosis, high leukocyte count and low frequency of platelets [6]. Licensee MDPI, Basel, Switzerland. DS children are at greater risk of developing acute lymphoblastic leukaemia (ALL) and This article is an open access article acute myeloid leukaemia (AML), by an estimated factor of 27-fold and 150-fold compared distributed under the terms and with the general population, respectively [4,7]. AML is a heterogeneous group of myeloid conditions of the Creative Commons leukaemias that originate from clones of haematopoietic stem and progenitor cells (HSPCs) Attribution (CC BY) license (https:// and myeloid lineage precursors carrying genetic mutations that alter cell proliferation and creativecommons.org/licenses/by/ compromise differentiation. In myeloid leukaemia of Down syndrome (ML-DS), most 4.0/). Cancers 2021, 13, 4144. https://doi.org/10.3390/cancers13164144 https://www.mdpi.com/journal/cancers Cancers 2021, 13, x 2 of 19 Cancers 2021, 13, 4144 2 of 18 ter cell proliferation and compromise differentiation. In myeloid leukaemia of Down syn- drome (ML-DS), most cases phenotypically reflect acute megakaryoblastic leukaemia cases phenotypically reflect acute megakaryoblastic leukaemia (AMKL), a rare subtype of (AMKL), a rare subtype of AML in which mutations compromise megakaryocytic maturation. AML in which mutations compromise megakaryocytic maturation. MLML-DS-DS is is characterized characterized by by a a distinctive distinctive multi multi-step-step evolution evolution in in which which it it is is always always precededpreceded by by a a pre pre-leukaemic-leukaemic condition condition known known as as transient transient abnormal abnormal myelopoiesis myelopoiesis (TAM) (TAM) oror transient transient myeloproliferative myeloproliferative disorder disorder (TMD) (TMD) [6,8] [6,8] (Figure (Figure 11).). AllAll TMDTMD casescases harbourharbour mutationsmutations in in the the haematopoietic haematopoietic transcription transcription factor factor GATA1 GATA1 [9,10] [9,10].. The The disorder disorder occurs occurs inin approximately approximately 10% 10% of of DS DS newborns newborns and and is is usually usually diagnosed diagnosed 2 2 months months after after birth birth by by thethe identification identification of of a a high high number number of of immature immature blasts blasts in in the the circu circulatinglating blood blood [8,11,12] [8,11,12].. However,However, the the presence presence of of GATA1 GATA1 mutations in in up up to 30% of DS newborns indicates indicates an an additionaladditional 20% 20% of of undetected undetected or or silent silent TMD TMD cases cases [13] [13].. Children Children with with TMD TMD may may develop develop severesevere symptoms symptoms such such as as thrombocytopaenia, thrombocytopaenia, leukocytosis, leukocytosis, anaemia, anaemia, lymphocytosis lymphocytosis and and liverliver failure, failure, and and approximately approximately 20% 20% of of patients patients do do not not survive survive [13 [13–17]–17. In]. Inthe the majority majority of TMDof TMD and andsilent silent TMD TMD cases, cases,the patient the patient goes into goes permanent into permanent remission remission without any without treatment. any However,treatment. 20 However,–30% of children 20–30% with of childrenTMD develop with ML TMD-DS develop before 5 ML-DSyears of beforeage [18 5–20] years. ML of- DSage is [ 18characterized–20]. ML-DS by isa low characterized number of by white a low blood number cells and of white high concentrations blood cells and of high im- matureconcentrations blasts and of dysplastic immature blastsmyeloid and cells. dysplastic Around myeloid 80% of children cells. Around with ML 80%-DS of respond children wellwith to ML-DS chemotherapy respond welland tosurvive, chemotherapy but the other and survive,20% suffers but thefrom other relapse 20% [ suffers8,17], high- from lightingrelapse [the8,17 need], highlighting of new therapeutic the need ofapproaches. new therapeutic approaches. FigureFigure 1. 1. TheThe sequential sequential acquisition acquisition of of mutations mutations in in ML ML-DS.-DS. Trisomy Trisomy 21 21 alters alters foetal foetal haematopoiesis haematopoiesis causingcausing an an expansion expansion of of haematopoietic haematopoietic stem stem cells cells (HSCs) (HSCs) with with skewing skewing towards towards megakaryocyte megakaryocyte-- erythroiderythroid progenitors progenitors (MEPs). (MEPs). A A total total of of 30% 30% of of Down Down syndrome syndrome (DS) (DS) neonates neonates acquire acquire mutations mutations inin GATA1GATA1 whichwhich lead lead to to the the exc exclusivelusive production production of of the the short short isoform, isoform, GATA1s, GATA1s, which which promotes promotes a transient myeloproliferative disorder (TMD). A total of 20% of children with TMD will develop a transient myeloproliferative disorder (TMD). A total of 20% of children with TMD will develop myeloid leukaemia of Down syndrome (ML-DS) following the acquisition of secondary mutations. myeloid leukaemia of Down syndrome (ML-DS) following the acquisition of secondary mutations. TheThe molecular molecular mechanisms mechanisms involved involved in inthe the progression progression from from TMD TMD to ML to- ML-DSDS are not are fullynotfully understood. understood. Large Large-scale-scale sequencing sequencing studies studies have identified have identified the most the frequent most frequent muta- tionsmutations acquired acquired in ML in-DS ML-DS in addition in addition to GATA1 to GATA1 mutationsmutations [9,10,21] [9,10. Strikingly,,21]. Strikingly, approxi- ap- matelyproximately half of half the cases of the have cases mutations have mutations in cohesin in cohesinor CTCF or, theCTCF two, main the two drivers main of drivers three- dimensionalof three-dimensional (3D) genome (3D) folding. genome This folding. high Thisfrequency high frequency is unexpected, is unexpected, especially especiallyfor CTCF whichfor CTCF is rarelywhich mutated is rarely in mutated AML or inother AML myeloid or other malignancies. myeloid malignancies. Moreover, chromatin Moreover, modifchromatiniers such modifiers as EZH2 such are as moreEZH2 frequentlyare more mutated frequently