Aetiology of childhood leukaemia

The acute leukaemias account for about 30% of all malignancy seen in childhood across the Western world. A peak incidence of precursor B cell ALL has emerged as socio-economic conditions have improved in countries worldwide. From twin studies and the use of neonatal blood spots it has been possible to back track the first initiating genetic events within critical haemopoietic cells to foetal development in utero for most precursor B cell ALL and some cases of AML. These events may occur as part of normal foetal development. Whether other factors (environmental or constitutional) are involved to increase the chance of these first genetic changes happening is unclear. For some leukaemias (e.g. infant MLL positive ALL) the first event appears adequate to create a malignant clone but for the majority of ALL and AML further 'genetic' changes are required, probably postnatal. Many environmental factors have been proposed as causative for leukaemia but only ionising irradiation and certain chemicals, e.g. benzene and cytotoxics (alkylators and topoisomerase II inhibitors) have been confirmed and then principally for acute myeloid leukaemia. It appears increasingly likely that delayed, dysregulated responses to 'common' infectious agents play a major part in the conversion of pre-leukaemic clones into overt precursor B cell ALL, the most common form of childhood leukaemia. Constitutional polymorphic alleleic variants in immune response genes (especially the HLA Class II proteins) and cytokines may play a role in determining the type of immune response. High penetrance germ-line mutations are involved in only about 5% of childhood leukaemias (more in AML than ALL). There is little evidence to support any role of viral transformation in causation, unlike in animals. Other environmental factors for which some evidence exists include non-ionising electromagnetic radiation and electric fields, although their mode of action in leukaemogenesis remains unclear. There is no single cause for childhood leukaemia and for most individuals a combination of factors appears to be necessary; all involving gene-environment interactions. To date few clear preventative measures have emerged, except the complete avoidance of first trimester X-rays in pregnancy; a healthy diet with adequate oral folic acid intake both preconception and early in pregnancy; and the early exposure of children to other children outside the home to facilitate stimulation and maturation of the natural immune system. Here then are clear echoes of the "hygiene hypothesis" regarding the initiation of allergies, autoimmune disease and type I diabetes mellitus in children and young people.

Perinatal and reproductive factors: a report on haematological malignancies from the UKCCS

The United Kingdom Childhood Cancer Study was designed to examine the potential aetiological role of a range of perinatal and reproductive factors. Our use of clinical records permitted a more exact characterisation of reproductive events than is possible in investigations that rely on self-reporting; and the increased specificity with which antecedent events were measured produced more precise risk estimates, albeit ones based on progressively smaller numbers. Information on the conduct of this component of the study and results for 1485 children with haematological malignancies and 4864 controls are presented. The 'find' rate for obstetric records was high at 86% for cases, with 81% having information on both matched controls. Associations were seen for severe hyperemesis (Odds Ratio=3.6, 95%Confidence Interval=1.3-10.1, for all leukaemias), polyhydramnios (OR=4.0, 95%CI=1.5-10.3, for acute myeloid leukaemia (AML)), anaemia (haemoglobin <10 g, OR=2.6, 95%CI=1.7-4.1, for AML), and pre-eclampsia (OR=1.7, 95%CI=1.1-2.7, for non-Hodgkin's lymphoma). Babies who developed leukaemia were heavier at birth (>4000 g, OR=1.2, 95%CI=1.0-1.4), as were their older siblings (>4000 g, OR=1.4, 95%1.0-1.9). Mothers' whose children developed common B-cell precursor acute lymphoblastic leukaemia (ALL) were more likely to have had a previous molar pregnancy (OR=5.2, 95%CI=1.9-14.7). Gender-specific analysis revealed that findings often differed markedly for boys and girls; and, in common with other reports, strong associations with Down's syndrome were seen for both ALL and AML.