Cytogenetics of childhood preleukemia

Childhood myeloproliferative disorders

Disorders classified as paediatric myeloproliferative disorders (MPD), such as juvenile chronic myeloid leukaemia (JCML), and as paediatric myelodysplastic syndrome (MDS), are essentially diseases characterized by abnormal myeloproliferation and they share similar genetic events on chromosome 7. As such, the abnormalities of increased myeloproliferation in childhood (AIMC) should be considered under the same heading. Constitutional and other genetic factors play an essential role in children and include the NF1 gene, whereas toxic exposure is of greater importance in adults. The most common cytogenetic alteration is that of monosomy or deletion of the long arm of chromosome 7. Critical regions have been identified and mapped by fluorescence in situ hybridization (FISH). It appears that the similar critical regions on chromosome 7 are involved, and suggests that these regions may contain genes important in the pathogenesis of AIMC.

Monosomy 7 and 7q--associated with myeloid malignancy

An association between the complete or partial loss of chromosome 7 and preleukaemic myelodysplasia or acute myeloid leukaemia has been recognized from the early days of tumour cytogenetic analysis. Detection of such abnormalities usually heralds a poor prognosis. The loss of DNA on chromosome 7 has led to speculation that tumour-suppressor genes may play a significant role in this form of leukaemogenesis, although it may be part of a multistep process. A further association with leukaemia secondary to carcinogen exposure including previous chemotherapy or a number of congenital anaemias has increased the interest in discovering the gene or genes on chromosome 7. Banded chromosome analysis has suggested that there are two broad critical regions on the long arm of chromosome 7 at bands 7q22 and 7q34-q36 that may contain the relevant genes. Initial molecular analysis has confirmed these two regions to be of significance. The advent of fluorescence in-situ hybridization techniques has facilitated some definition of the 7q22 region, with identification of candidate genes for further functional analysis. It is becoming clear that there will be more than one gene on chromosome 7 involved in the leukaemic process and with the definition of these genes it may be possible to look for associations with different phenotypes and prognosis. As for the reason for chromosome 7 showing a particular predisposition to total or partial loss we may speculate that the DNA sequence and structure may confer a 'fragility' on the chromosome. A greater understanding of the DNA structure of the long arm may provide real insight into the mechanisms of leukaemia. We would like to speculate in the long term that this could lead to the ability to screen for leukaemia susceptibility and avoidance of 'inducers' in those at risk.

Involvement of 11p15 and 3q21q26 in therapy-related myeloid leukemia (t-ML) in children. Case reports and review of the literature

The cytogenetic findings of therapy-related myeloid leukemia (t-ML) in three children are presented. These included one male patient with acute lymphoblastic leukemia (ALL) who underwent bone marrow transplantation and developed therapy-related myeloproliferative disease (t-MPD) in the female-donor hematopoietic cells 2.5 years after receiving radiation and epipodophyllotoxin therapy for ALL testicular relapse. Bone marrow leukemic cell karyotype revealed 46,XX,add (11)(p15) and a normal female karyotype in the peripheral blood lymphocytes. The other two children, one with ALL and one with ganglioneuroblastoma, developed fatal t-MPD and therapy-related acute myeloblastic leukemia (t-AML) preceded by myelodysplastic syndrome (t-MDS), respectively, 5 years after diagnosis, following administration of alkylating agents and irradiation. Monosomy 7 was present in both, and was combined with inv(3)(q21q26) in the second patient. Our review of the cytogenetic findings in 91 previously reported pediatric patients with t-ML suggested that the involvement of 11p15 and 3q21-->23, 3q24-q26 with or without a combination of translocation 11q23 and -7/7q-, respectively, are nonrandom aberrations of t-ML in children. Comparison of the chromosomal changes in t-ML between the pediatric and an adult series revealed some differences which may result from differences in treatment modalities and which, in addition, may indicate a possible role of genetic and/or age-dependent factors in the pathogenesis of therapy-related leukemogenesis in children.

Chromosome 21 rearrangement in acute biphenotypic leukemia

A patient with myelodysplastic syndrome (MDS) and a 47,XY,+21 karyotype at diagnosis, was documented to have a clonal chromosome 21 rearrangement, i(21q), four months before transformation to acute biphenotypic leukemia. For 4 months after transformation, isochromosome 21 persisted while the patient was receiving treatment with zidovudine. Vitamin D3 was added to zidovudine for an additional month, during which time the trisomy 21 clone reappeared as the predominant cell population. The unique aspects of this patient are the atypical evolution of chromosome 21, the transformation to biphenotypic leukemia, and the occurrence of i(21q) associated with biphenotypic leukemia evolving from an MDS.

Myelodysplastic syndrome with partial deletion of the long arm of chromosome 5: first report of a case in a child

A childhood case of myelodysplastic syndrome (MDS) with a deletion of the long arm of chromosome 5 (5q-) is reported. The patient was an 8 year old boy who has recurrent angina. Laboratory evaluation revealed the following: hemoglobin 8.1 gm/dl, white blood cell count 4.9 x 10(3)/l with 3% atypical lymphocytes, and platelet count 17.7 x 10(4)/l. A bone marrow aspirate revealed 20% blast cells and dysmyelopoietic changes involving all three marrow cell lines. Karyotype analysis of marrow cells revealed 46,XY,5q- in 100% of the metaphases. These findings led to a diagnosis of MDS with 5q-, which is most commonly found in adult MDS. This case seems to represent an exceedingly rare childhood case of MDS with 5q-.

Chromosome 7 abnormalities in children with Down syndrome and preleukemia

Three children, two boys and one girl, with Down syndrome (DS) who presented with preleukemia and loss of all or part of chromosome 7 were studied. Initial presentation, with cytopenias and less than 25% blasts in the bone marrow, was between 13 and 30 months of age. Progression to acute nonlymphocytic leukemia occurred 1-8 months after initial presentation. The morphologic type was megakaryoblastic in two, and undifferentiated in one. Two children achieved remission with intensive therapy, and one continues in remission off therapy; the other child died in remission of accidental causes. The third child died of respiratory distress and leukemia after no intervention was chosen. These cases represent the first examples of chromosome 7 abnormalities associated with DS and leukemia, and suggest differences from the "monosomy 7" syndrome seen in children without DS.

Monosomy 7 syndrome. Clinical heterogeneity in children and adolescents

Bone marrow monosomy 7 is the most frequent karyotypic abnormality found in patients with chronic myeloproliferative disorders. To a review of 46 previously reported pediatric patients we add three additional cases. Clinical presentation is usually dependent upon which cell lines are most perturbed in this pluripotent stem cell disorder. Sixteen (35%) children presented by their first birthday and 35 (76%) by their sixth birthday. Distinctive differences in presentation exist between infants, children, and adolescents. Younger patients were more symptomatic and had greater degree of hepatosplenomegaly and leukocytosis. The prognosis is very poor and death usually occurs within two years from complications attributable to cytopenias, cellular dysfunction, or transformation to acute nonlymphocytic leukemia. Implications for therapy are discussed.

Translocations and deletions with breakpoint on 21q are nonrandomly associated with treatment-related acute nonlymphocytic leukemia and preleukemia

Six of 70 (8.6%) consecutive cases with therapy-related acute nonlymphocytic leukemia (ANLL) or preleukemia had a translocation or deletion with a breakpoint on 21q. Such aberrations were seen in only one of 200 (0.5%) consecutive cases of de novo ANLL examined at our laboratory. The figures reflect a 17.1-fold increased incidence of 21q aberrations in therapy-related ANLL or preleukemia, compared with ANLL de novo. The difference is highly significant (p = 0.003). The increased incidence of 21q aberrations in therapy-related myelodysplastic syndromes was confirmed by literature studies. Band 21q22 was most often involved. Cases with t(8;21), which is strongly associated with the M2 variant of ANLL, or cases with i(21q), which is supposedly due to a centromeric misdivision, were not included in the count. It is concluded that the 21q aberrations are associated with treatment-related ANLL or preleukemia with at least the same degree of specificity as aberrations of #5 and #7.

Presence of isochromosomes in hematologic diseases

Several different structural chromosome aberrations have been observed in human neoplasias. In this report we describe the isochromosomes found in nine patients with hematologic malignancies: five with leukemia, one with sideroblastic anemia, and three with malignant lymphomas. The isochromosomes i(7q), i(11q), i(17q), and i(21q) were detected in these patients. We suggest that the presence of isochromosomes permits us to speak of a gene-dosage effect and that this mechanism may play a role in malignant transformation.

Frequency of constitutional chromosome alterations in patients with hematologic neoplasias

From 1978 to 1985 cytogenetic studies were performed on 718 patients with different hematologic diseases. Nine (1.25%) had a constitutional chromosome alteration. One patient had trisomy 21, four had balanced translocations and four had sex chromosome anomalies. Although the frequency of constitutional alterations was twice that seen in the newborn population, an analysis of these data and also from the literature shows a random association between constitutional chromosome alterations and hematologic neoplasias, except for patients with Down's syndrome.

Cytogenetic evidence for involvement of erythroid progenitors in a child with therapy linked myelodysplasia

A 6-year-old male with prior metastatic retinoblastoma developed a therapy linked myelodysplastic syndrome. Whole bone marrow cytogenetics showed monosomy 7 and a marker chromosome. To determine the progenitor level of origin of the malignant clone, we studied the karyotypes of marrow erythroid and granulocyte/macrophage colonies grown in methyl cellulose. All erythroid and granulocyte/macrophage colonies had an abnormal karyotype with 45 chromosomes (monosomy 7) and several colonies contained the marker chromosome. These findings give direct evidence that this patient's myelodysplastic syndrome involved an early stem cell which was capable of both erythroid and granulocyte differentiation.

Acute nonlymphocytic leukemia

The authors review the advances of the last decade in bone marrow culture cytogenetics and immunology that have aided in understanding the pathophysiology of acute nonlymphocytic leukemia and they also discuss the major strides in treatment that have occurred.

Sequential development of distinct clonal chromosome abnormalities in a patient with preleukaemia

Preleukaemia has been identified as a clonal haemopathy in which progression to acute leukaemia involves conservation of the preleukaemic karyotype in the blast cells or the development of new abnormalities superimposed on the original clone. In this report, a case of childhood preleukaemia is presented in which two cytogenetically distinct clones developed over 2 years in a dysplastic marrow that was initially karyotypically normal. One clone with 47 chromosomes (47,XY,+21), disappeared without therapy. Predominance of the cytogenetically abnormal clone, 45,XY,-12,-17,t(12;17)(p11;q11) was associated with the development of acute myelogenous leukaemia and myelofibrosis. The development of independent clonal abnormalities in the unstable preleukaemic marrow may occur more commonly than has been previously recognized. Implications of the progression of the karyotypic abnormalities are discussed.

Cellular homologs of the avian erythroblastosis virus erb-A and erb-B genes are syntenic in mouse but asyntenic in man

Avian erythroblastosis virus, a retrovirus that causes erythroblastosis and sarcomas in infected birds, possesses two host cell-derived genes [viral (v) erb-A and erb-B]. Although v-erb-B seems to be responsible for oncogenic transformation, v-erb-A might have an enhancing effect on transformation. In chickens, the natural host for avian erythroblastosis virus, cellular (c) erb-A and erb-B genes appear to be unlinked, but their chromosomal locations in other species are unknown. To ascertain the chromosomal location of c-erb genes in man and mouse, we analyzed interspecies somatic cell and microcell hybrids by Southern filter hybridization techniques using specific v-erb-A and v-erb-B probes. We found c-erb-A sequences on human chromosome 17 (17p11----qter) and located c-erb-B on human chromosome 7 (7pter----q22). In contrast, both c-erb-A and c-erb-B reside on mouse chromosome 11.