Telomerase and the benign and malignant megakaryoblastic leukemias of Down syndrome

Transient leukemia of Down syndrome

Childhood leukemia is mostly a "developmental accident" during fetal hematopoiesis and may require multiple prenatal and postnatal "hits". The World Health Organization defines transient leukemia of Down syndrome (DS) as increased peripheral blood blasts in neonates with DS and classifies this type of leukemia as a separate entity. Although it was shown that DS predisposes children to myeloid leukemia, neither the nature of the predisposition nor the associated genetic lesions have been defined. Acute myeloid leukemia of DS is a unique disease characterized by a long pre-leukemic, myelodysplastic phase, unusual chromosomal findings and a high cure rate. In the present manuscript, we present a comprehensive review of the literature about clinical and biological findings of transient leukemia of DS (TL-DS) and link them with the genetic discoveries in the field. We address the manuscript to the pediatric generalist and especially to the next generation of pediatric hematologists.

Down syndrome, RASopathies, and other rare syndromes

In this article we discuss the occurrence of myeloid neoplasms in patients with a range of syndromes that are due to germline defects of the RAS signaling pathway and in patients with trisomy 21. Both RAS mutations and trisomy 21 are common somatic events contributing to leukemogenis. Thus, the increased leukemia risk observed in children affected by these conditions is biologically highly plausible. Children with myeloid neoplasms in the context of these syndromes require different treatments than children with sporadic myeloid neoplasms and provide an opportunity to study the role of trisomy 21 and RAS signaling during leukemogenesis and development.

Transient Abnormal Myelopoiesis: A Varied Spectrum of Clinical Presentation

Transient myeloproliferative disorder (TMD) is a condition seen almost exclusively in newborns with Down syndrome (DS). It can have a spectrum of clinical presentation ranging from being asymptomatic with incidental detection to a stormy course and fatal outcome. We describe three cases of TMD having different clinical presentation, course, complications and outcome. All but one had Down’s phenotype; one of them had a severe disease with tumor lysis syndrome and died of liver failure, while the other one had pericardial effusion and cardiac failure but survived. The third patient had a very benign course of illness requiring only supportive care. Newborns with DS should be screened for TMD by a complete blood count during their first month of life, irrespective of symptoms. With increasing knowledge about the natural history and management guidelines, the prognosis of this rare and unique entity has improved in recent years.

Myeloid leukemia in Down syndrome

Although adults with Down syndrome (DS) show a decreased incidence of cancer compared to individuals without DS, children with DS are at an increased risk of leukemia. Nearly half of these childhood leukemias are classified as acute megakaryoblastic leukemia (AMKL), a relatively rare subtype of acute myeloid leukemia (AML). Here, we summarize the clinical features of myeloid leukemia in DS, review recent research on the mechanisms of leukemogenesis, including the roles of GATA1 mutations and trisomy 21, and discuss treatment strategies. Given that trisomy 21 is a relatively common event in hematologic malignancies, greater knowledge of how the genes on chromosome 21 contribute to DS-AMKL will increase our understanding of a broader class of patients with leukemia.

Telomeres in trisomy 21 amniocytes

Individuals with trisomy 21 have an increased risk of developing leukemia and premature dementia. They also have a higher rate of telomere loss. The aim of the study was to compare telomere length and the hTERC gene copy number, which encodes the telomerase RNA subunit, in amniocytes of trisomy 21 conceptions and normal pregnancies. A quantitative fluorescence-in-situ protocol (Q-FISH) was used to compare telomere length in amniocytes cultured from 11 trisomy 21 conceptions and from 14 normal pregnancies. Quantification was conducted using novel computer software. Fluorescence in situ hybridization (FISH) was used to assess the percentage of cells with additional copies of hTERC. We found that the immunofluorescence intensity, which represents telomere length, was significantly lower in amniocytes from trisomy 21 conceptions compared to the control group. The trisomy 21 group had a higher number of cells with additional copies of hTERC. This observation could be one of the cytogenetic parameters that represent a state of genetic instability and might play a role in the pathomechanism of typical features of Down syndrome, such as dementia and malignancy.

Unbalanced chromosome 1 abnormalities leading to partial trisomy 1q in four infants with Down syndrome and acute megakaryocytic leukemia

Background

Children with Down syndrome (DS) have an increased risk of childhood acute leukemia, especially acute megakaryoblastic leukemia (AMKL) also called acute myeloid leukemia (AML) type M7. Here four yet unreported infants with such malignancies are reported.

Results

An unbalanced translocation involving chromosome 1 was identified by GTG banding in all cases. These were characterized in more detail by molecular cytogenetic approaches. Additional molecular analysis revealed in three of the four cases mutations in exon 2 of the GATA binding protein 1 (globin transcription factor 1), located in Xp11.23.

Conclusion

Our results corroborate that abnormalities of chromosome 1 are common in DS-associated AMKL. Whether this chromosomal region contains gene(s) involved in hematopoietic malignant transformation remains to be determined.

Transient myeloproliferative disorder in neonates with and without Down syndrome: a tale of 2 syndromes

Transient myeloproliferative disorder (TMD) in newborns with Down syndrome (DS) has been well described. We report 4 newborns, 2 with DS and 2 without DS, who developed TMD. One newborn with DS developed multiorgan failure and died despite treatment with low-dose cytarabine. In 3 newborns, the TMD resolved spontaneously. Two of these patients, 1 with and 1 without DS developed leukemia on subsequent follow-up and were treated successfully. We reviewed the clinical and laboratory data on 14 non-DS infants with TMD reported in the literature. According to limited data, these patients are more likely to develop leukemia than DS patients, however their outcome is better.

Hydrops fetalis in a neonate with down syndrome, transient myeloproliferative disorder and hepatic fibrosis

Transient myeloproliferative disorder is a self limiting disorder characterized by leukocytosis with the presence of megakaryoblasts in the peripheral blood and bone marrow, anemia, thrombocytopenia, and organomegaly. It occurs in approximately 10% of newborn infants with Down syndrome. Hepatic fibrosis is seen in the severe form of transient myeloproliferative disorder with Down syndrome that is characterized by diffuse intralobular sinusoidal fibrosis and extramedullary hematopoesis. We describe a patient with hydrops fetalis, Down syndrome, and transient myeloproliferative disorder. We suggest that patients with the severe form of transient myeloproliferative disorder should be examined for hepatic fibrosis.

The role of telomere maintenance in the spontaneous growth arrest of pediatric low-grade gliomas

Spontaneous tumor regression of pediatric low-grade gliomas (PLGG). We speculated that lack of telomere maintenance is responsible for this behavior. We first looked for evidence of telomerase activity and alternative lengthening of telomeres (ALT) in 56 PLGG. Telomerase activity was observed in 0 of 11 PLGG, in contrast to 10 of 13 high-grade pediatric brain tumors. There was no ALT in 45 of 45 samples. We then applied Q-FISH to eight patients whose indolent PLGG underwent two metachronous biopsies over a lag of several years. Telomere shortening was observed in the second biopsy in all tumors, but not in normal brain control (P < .0001), indicating that lack of telomere maintenance is associated with continuous telomere erosion. Based on these observations, we found that younger PLGG patients, who exhibit more aggressive and frequently recurrent tumors, had significantly longer telomeres than older ones (P = .00014). Tumors with a terminal restriction fragment length <7.5 did not recur, whereas the presence of longer telomeres (>8.0) conferred a high likelihood of late recurrences in PLGG. Our findings provide a plausible biologic mechanism to explain the tendency of PLGG to exhibit growth arrest and spontaneous regression. Telomere maintenance may therefore represent the first known biologic prognostic marker in PLGG.

Recent insights into the mechanisms of myeloid leukemogenesis in Down syndrome

GATA-1 is the founding member of a transcription factor family that regulates growth and maturation of a diverse set of tissues. GATA-1 is expressed primarily in hematopoietic cells and is essential for proper development of erythroid cells, megakaryocytes, eosinophils, and mast cells. Although loss of GATA-1 leads to differentiation arrest and apoptosis of erythroid progenitors, absence of GATA-1 promotes accumulation of immature megakaryocytes. Recently, we and others have reported that mutagenesis of GATA1 is an early event in Down syndrome (DS) leukemogenesis. Acquired mutations in GATA1 were detected in the vast majority of patients with acute megakaryoblastic leukemia (DS-AMKL) and in nearly every patient with transient myeloproliferative disorder (TMD), a "preleukemia" that may be present in as many as 10% of infants with DS. Although the precise pathway by which mutagenesis of GATA1 contributes to leukemia is unknown, these findings confirm that GATA1 plays an important role in both normal and malignant hematopoiesis. Future studies to define the mechanism that results in the high frequency of GATA1 mutations in DS and the role of altered GATA1 in TMD and DS-AMKL will shed light on the multistep pathway in human leukemia and may lead to an increased understanding of why children with DS are markedly predisposed to leukemia.

Lack of circulating megakaryoblasts in newborn peripheral blood: development and validation of a sensitive flow cytometric detection method

It is currently thought that approximately 1% of children with Down syndrome will develop a "premalignant" syndrome known as transient myeloproliferative disorder (TMD). Prospective, population-based studies of the incidence of TMD in Down syndrome infants is lacking. Although most cases of TMD resolve by 1 year of age, data suggest that 10% to 20% of Down syndrome patients with TMD develop AML-M7 (megakaryoblastic leukemia). To identify the true incidence of TMD in the Down syndrome population, a sensitive, rapid, and cost-effective method of quantifying circulating megakaryoblasts in large numbers of patients was needed. In this pilot study, the authors tested the hypothesis that there are fewer than 1% megakaryoblasts of nucleated cells circulating in the blood of normosomic infants. Four-antigen flow cytometry was used to establish the percentage of megakaryoblasts present in each of 100 cord blood samples collected blindly from "normosomic" live births. There was a mean percentage of 0.017% megakaryoblasts in 100 cord blood samples from normosomic infants. Flow cytometry proved to be a sensitive, rapid, and reproducible method for the quantification of megakaryoblasts. Less than 1% of circulating nucleated cells in the blood of newborn infants are megakaryoblasts, providing a comparison population for the authors' larger proposed incidence study.

Fetal and neonatal leukemia

The biological and clinical characteristics of perinatal leukemia differ significantly from those of leukemia in older children, and the prognosis is generally bleak. Once complete remission is achieved, neonates with acute myelocytic leukemia (AML) fare much better than those with acute lymphocytic leukemia (ALL). The results of this study suggest that age, sex, type of leukemia, and cytogenetic findings have a strong influence on outcome. Neonates, particularly females, with pre-B ALL have a much worse prognosis than neonates and older children with this disease. Transient leukemia in the Down syndrome neonate is associated with significant morbidity; close follow-up is recommended for at least the first 3 years of life because of the potential of developing acute leukemia, particularly AMKL (M7). The purpose of this review is to focus on the fetus and neonate in an attempt to determine the various ways leukemia differs clinically and morphologically from the disease occurring in older infants and children and to demonstrate that certain types of leukemia have a poor prognosis compared with those occurring in older children.

Cytogenetic and molecular genetic aspects of childhood myeloproliferative/myelodysplastic disorders

Of the myeloproliferative/myelodysplastic disorders (MPD/MDS) that occur in childhood most, regarding the cytogenetic and molecular genetic basis, is known about the two purely paediatric disorders: juvenile myelomonocytic leukaemia (JMML) and transient myeloproliferative disorder (TMD). Although much has been published about these two disorders, their aetiology is by no means fully established. It would appear, however, that in this paediatric subset of MPDs a stage/developmentally specific vulnerability for proliferation and transformation exists. The study of the molecular basis of many other MPD-like syndromes that also occur in childhood, has been greatly accelerated by the identification of rare, but recurring, cytogenetic abnormalities involving 8p11 and 5q31-33. Good collaborative studies could result in similar progress being made in the understanding of JMML and TMD.