t(1;7)(p36;q32): a new recurring abnormality in primary myelodysplastic syndrome

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.

Chromosome 7 and Haematological Malignancies

Abnormalities of chromosome 7 are the most common clonal chromosomal changes observed in myelodysplasia (MDS) and the second most frequent in acute myeloid leukaemia (AML) [1-5]. These changes may consist of long arm deletion (7q-) or total loss of the whole chromosome (monosomy 7) from bone marrow cells [1, 4, 6-24] and was first reported in association with myeloid disease in 1964 with the report of 3 cases of refractory anaemia, granulocytic hyperplasia [25]. The association between chromosome 7 alterations, MDS and AML in children and adults is clear, however, a rare association with lymphoid malignancies has also been recently reported. The abnormalities may occur in de novo MDS/AML, secondary cases following exposure to drugs, radiotherapy and toxins and in addition in a range of constitutional disorders including Fanconi's anaemia, congenital neutropenia and neurofibromatosis type 1 (NF1). The broad spread of conditions in which this consistent genetic change can occur leads one to speculate that there is an underlying instability in chromosome 7 and that genes on this chromosome play a role in the development of malignancy. The loss of DNA associated with malignant progression suggests the presence of a tumour suppressor gene (or genes) [26, 27]. Patients with monosomy 7 usually present as classical MDS with abnormal erythroid, megakaryocyte and myeloid differentiation [7, 28]. From a mechanistic perspective, increased cell proliferation and apoptosis is a common feature possibly induced by the failure of normal haematopoietic maturation. In all groups the presence of chromosome 7 abnormalities defines a poor prognostic group [29]. The majority of patients with MDS transform to a form of acute leukaemia resistant to therapy, including bone marrow transplantation (BMT). Although fluorescence in situ hybridization (FISH) has accelerated the study of these disorders at the cytogenetic and molecular levels, [4, 30, 31, 32, 33] no gene has been clearly implicated. A few candidate genes are under investigation. While the loss of chromsome 7 material is crutial in the malignant process it is almost certainly not the primary molecular abnormality. An initiating event genetic event predisposing to chromosome breakage and loss probably occurs in haematopoietic cells permitting chromosome 7 loss and progression to clonal malignancy as a secondary event.

Identification of a new murine runt domain-containing gene, Cbfa3, and localization of the human homolog, CBFA3, to chromosome 1p35-pter

Core binding factor (CBF) is a heterodimeric transcription factor composed of two distinct subunits. The monomeric beta subunit is ubiquitously expressed, whereas expression of the three alpha subunits isolated previously seems to be restricted mainly to hematopoietic tissues. To isolate additional alpha genes, degenerate oligonucleotides derived from the runt domain--a region shared by all alpha genes--were used for screening cDNA libraries. A 228-bp fragment was isolated from a mouse thymus cDNA library, which showed 82 and 76% DNA sequence identity to the previously isolated murine alpha genes, Cbfa1 and Cbfa2. This novel alpha gene was named Cbfa3. The corresponding sequence from the human homolog CBFA3 was obtained by cosmid cloning and sequencing of the appropriate restriction fragment. The corresponding regions of mouse Cbfa3 and human CBFA3 show 91% nucleotide identity and 100% protein identity. In situ hybridization and physical mapping of somatic cell hybrids localized CBFA3 to chromosome 1p35-pter.