Down syndrome and leukemia: An insight into the disease biology and current treatment options.

Children with Down syndrome (DS) have a 10- to 20-fold greater predisposition to develop acute leukemia compared to the general population, with a skew towards myeloid leukemia (ML-DS). While ML-DS is known to be a subtype with good outcome, patients who relapse face a dismal prognosis. Acute lymphocytic leukemia in DS (DS-ALL) is considered to have poor prognosis. The relapse rate is high in DS-ALL compared to their non-DS counterparts. We have a better understanding about the mutational spectrum of DS leukemia. Studies using animal, embryonic stem cell- and induced pluripotent stem cell-based models have shed light on the mechanism by which these mutations contribute to disease initiation and progression. In this review, we list the currently available treatment strategies for DS-leukemias along with their outcome with emphasis on challenges with chemotherapy-related toxicities in children with DS. We focus on the mechanisms of initiation and progression of leukemia in children with DS and highlight the novel molecular targets with greater success in preclinical trials that have the potential to progress to the clinic.

The impact of Down syndrome-specific non-malignant hematopoietic regeneration in the bone marrow on the detection of leukemic measurable residual disease.

BACKGROUND: Detection of measurable residual disease detection (MRD) by flow cytometry after the first course of chemotherapy is a standard measure of early response in patients with acute myeloid leukemia (AML). Myeloid leukemia associated with Down Syndrome (ML-DS) is a distinct form of AML. Differences in steady-state and regenerating hematopoiesis between patients with or without DS are not well understood. This understanding is essential to accurately determine the presence of residual leukemia in patients with ML-DS. METHODS: A standardized antibody panel defined quantitative antigen expression in 115 follow-up bone marrow (BM) aspirates from 45 patients following chemotherapy for ML-DS or DS precursor B-cell acute lymphoblastic leukemia (B-ALL-DS) with the "difference from normal (ΔN)" technique. When possible, FISH and SNP/CGH microarray studies were performed on sorted cell fractions. RESULTS: 93% of BM specimens submitted post chemotherapy had a clearly identifiable CD34+ CD56+ population present between 0.06% and 2.6% of total non-erythroid cells. An overlapping CD34+ HLA-DRheterogeneous population was observed among 92% of patients at a lower frequency (0.04%-0.8% of total non-erythroid cells). In B-ALL-DS patients, the same CD34+ CD56+ HLA-DRheterogeneous expression was observed. FACS-FISH/Array studies demonstrated no residual genetic clones in the DS-specific myeloid progenitor cells. CONCLUSIONS: Non-malignant myeloid progenitors in the regenerating BM of patients who have undergone chemotherapy for either ML-DS or B-ALL-DS express an immunophenotype that is different from normal BM of non-DS patients. Awareness of this DS-specific non-malignant myeloid progenitor is essential to the interpretation of MRD by flow cytometry in patients with ML-DS.

A sensitive and inexpensive high-resolution melting-based testing algorithm for diagnosis of transient abnormal myelopoiesis and myeloid leukemia of Down syndrome.

Patients with Down syndrome (DS) are commonly affected by a pre-leukemic disorder known as transient abnormal myelopoiesis (TAM). This condition usually undergoes spontaneous remission within the first 2 months after birth; however, in children under 5, 20%-30% of cases evolve to myeloid leukemia of Down syndrome (ML-DS). TAM and ML-DS are caused by co-operation between trisomy 21 and acquired mutations in the GATA1 gene. Currently, only next-generation sequencing (NGS)-based methodologies are sufficiently sensitive for diagnosis in samples with small GATA1 mutant clones (≤10% blasts). Alternatively, this study presents research on a new, fast, sensitive, and inexpensive high-resolution melting (HRM)-based diagnostic approach that allows the detection of most cases of GATA1 mutations, including silent TAM. The algorithm first uses flow cytometry for blast count, followed by HRM and Sanger sequencing to search for mutations on exons 2 and 3 of GATA1. We analyzed 138 samples of DS patients: 110 of asymptomatic neonates, 10 suspected of having TAM, and 18 suspected of having ML-DS. Our algorithm enabled the identification of 33 mutant samples, among them five cases of silent TAM (5/110) and seven cases of ML-DS (7/18) with blast count ≤10%, in which GATA1 alterations were easily detected by HRM. Depending on the type of genetic variation and its location, our methodology reached sensitivity similar to that obtained by NGS (0.3%) at a considerably reduced time and cost, thus making it accessible worldwide.

Increased risk of leukaemia in children with Down syndrome: a somatic evolutionary view.

Children show a higher incidence of leukaemia compared with young adolescents, yet their cells are less damaged because of their young age. Children with Down syndrome (DS) have an even higher risk of developing leukaemia during the first years of life. The presence of a constitutive trisomy of chromosome 21 (T21) in DS acts as a genetic driver for leukaemia development, however, additional oncogenic mutations are required. Therefore, T21 provides the opportunity to better understand leukaemogenesis in children. Here, we describe the increased risk of leukaemia in DS during childhood from a somatic evolutionary view. According to this idea, cancer is caused by a variation in inheritable phenotypes within cell populations that are subjected to selective forces within the tissue context. We propose a model in which the increased risk of leukaemia in DS children derives from higher rates of mutation accumulation, already present during fetal development, which is further enhanced by changes in selection dynamics within the fetal liver niche. This model could possibly be used to understand the rate-limiting steps of leukaemogenesis early in life.

Molecular Mechanisms of the Genetic Predisposition to Acute Megakaryoblastic Leukemia in Infants With Down Syndrome.

Individuals with Down syndrome are genetically predisposed to developing acute megakaryoblastic leukemia. This myeloid leukemia associated with Down syndrome (ML-DS) demonstrates a model of step-wise leukemogenesis with perturbed hematopoiesis already presenting in utero, facilitating the acquisition of additional driver mutations such as truncating GATA1 variants, which are pathognomonic to the disease. Consequently, the affected individuals suffer from a transient abnormal myelopoiesis (TAM)-a pre-leukemic state preceding the progression to ML-DS. In our review, we focus on the molecular mechanisms of the different steps of clonal evolution in Down syndrome leukemogenesis, and aim to provide a comprehensive view on the complex interplay between gene dosage imbalances, GATA1 mutations and somatic mutations affecting JAK-STAT signaling, the cohesin complex and epigenetic regulators.

GATA1 mutations in a cohort of Malaysian children with Down syndrome-associated myeloid disorder.

INTRODUCTION: Children with Down syndrome (DS) are at increased risk of developing distinctive clonal myeloid disorders, including transient abnormal myelopoiesis (TAM) and myeloid leukaemia of DS (ML-DS). TAM connotes a spontaneously resolving congenital myeloproliferative state observed in 10%-20% of DS newborns. Following varying intervals of apparent remission, a proportion of children with TAM progress to develop ML-DS in early childhood. Therefore, TAM and ML-DS represent a biological continuum. Both disorders are characterised by recurring truncating somatic mutations of the GATA1 gene, which are considered key pathogenetic events. METHODS: We herein report, to our knowledge, the first observation on the frequency and nature of GATA1 gene mutations in a cohort of Malaysian children with DS-associated TAM (n = 9) and ML-DS (n = 24) encountered successively over a period of five years at a national referral centre. RESULTS: Of the 29 patients who underwent GATA1 analysis, GATA1 mutations were observed in 15 (51.7%) patients, including 6 (75.0%) out of 8 patients with TAM, and 9 (42.9%) of 21 patients with ML-DS. All identified mutations were located in exon 2 and the majority were sequence-terminating insertions or deletions (66.7%), including several hitherto unreported mutations (12 out of 15). CONCLUSION: The low frequency of GATA1 mutations in ML-DS patients is unusual and potentially indicates distinctive genomic events in our patient cohort.

Hematologic Response to Vorinostat Treatment in Relapsed Myeloid Leukemia of Down Syndrome.

Children with Down syndrome are at high risk to develop myeloid leukemia (ML-DS). Despite their excellent prognosis, children with ML-DS particularly suffer from severe therapy-related toxicities and for relapsed ML-DS the cure rates are very poor. Here we report the clinical course of one child with ML-DS treated with the histone deacetylase (HDAC) inhibitor vorinostat (suberoylanilide hydroxamic acid) after second relapse. The child had previously received conventional chemotherapy and stem cell transplantation, yet showed a remarkable clinical and hematologic response. Thus, HDAC inhibitor may represent an effective class of drugs for the treatment of ML-DS.

The impact of trisomy 21 on foetal haematopoiesis.

The high frequency of a unique neonatal preleukaemic syndrome, transient abnormal myelopoiesis (TAM), and subsequent acute myeloid leukaemia in early childhood in patients with trisomy 21 (Down syndrome) points to a specific role for trisomy 21 in transforming foetal haematopoietic cells. N-terminal truncating mutations in the key haematopoietic transcription factor GATA1 are acquired during foetal life in virtually every case. These mutations are not leukaemogenic in the absence of trisomy 21. In mouse models, deregulated expression of chromosome 21-encoded genes is implicated in leukaemic transformation, but does not recapitulate the effects of trisomy 21 in a human context. Recent work using primary human foetal liver and bone marrow cells, human embryonic stem cells and iPS cells shows that prior to acquisition of GATA1 mutations, trisomy 21 itself alters human foetal haematopoietic stem cell and progenitor cell biology causing multiple abnormalities in myelopoiesis and B-lymphopoiesis. The molecular basis by which trisomy 21 exerts these effects is likely to be extremely complex, to be tissue-specific and lineage-specific and to be dependent on ontogeny-related characteristics of the foetal microenvironment.