Infectious Origins of Childhood Leukemia
Authors: Klemm Lars , Swaminathan Srividya , Ford Anthony M , Schwarz Klaus , Schatz David G. , Lieber Michael R. , Greaves Melvin F , Muschen Markus 
Published: October 2019

Abstract
Abstract 751

Background:
In most cases, childhood acute lymphoblastic leukemia can be retraced to a recurrent genetic lesion in utero, which establishes a pre-leukemic clone. The TEL-AML1 fusion gene, for instance, arises prenatally and defines the most frequent subtype of childhood ALL. Strikingly, ∼1 of 100 healthy newborns carry a TEL-AML1 pre-leukemic clone, but only <1% of these children will eventually develop leukemia. Encounter of infectious antigen in B cell typically leads to activation of the mutator enzyme AID. While AID is required for class switch recombination and somatic hypermutation of immunoglobulin genes during affinity maturation of germinal center B cells, its premature activation may be deleterious. The underlying questions for this project were (1) how are B cells during their early development safeguarded from pre-mature AID expression and (2) whether pre-mature expression of AID in early B cell development is deleterious in the sense that it pre-disposes to the clonal evolution of a pre-leukemic B cell clone in the bone marrow.

Results:
We performed a comprehensive analysis of human B cell development in bone marrow samples from two children carrying deleterious mutations of the IL7RA gene encoding one chain of the human IL7 receptor. As opposed to normal human pre-B cells, pre-B cells from IL7RA-mutant patients carried somatically mutated immunoglobulin genes consistent with aberrant expression of AID in these cells. This led to the hypothesis that signaling via IL7Ra suppresses premature activation of AID-dependent hypermutation. To test this hypothesis, we stimulated mouse pre-B cells with LPS in the presence or absence of IL7, which is normally abundantly present in the bone marrow. While pre-B cells did not respond to LPS in the presence of IL7, IL7 withdrawal dramatically sensitized pre-B cells to LPS exposure: in the absence of IL7, LPS-stimulation of pre-B cells resulted in similar AID protein levels as in splenic germinal center B cells, where AID is normally active. We confirmed these observations studying pre-B cells from an AID-GFP reporter transgenic mouse strain. While LPS resulted in ∼2% AID-GFP+ cells in the presence of IL7, the fraction of AID-GFP+ cells increased to ∼45% when IL7 was removed. Since IL7Ra signaling involves Stat5 phosphorylation, we studied inducible Cre-mediated deletion of Stat5, which had the same effect as IL7 withdrawal and led to transcriptional de-repression of AID. IL7Ra/Stat5 signaling likely involves negative regulation of FoxO3A via AKT since expression of a constitutively active FoxO3A mutant potentiated AID expression in pre-B cells. We next searched for a normal pre-B cell subset, in which loss of IL7Ra/Stat5 signaling occurs naturally. Since inducible activation of pre-B cell receptor signaling results in downregulation of IL7Ra surface expression, we tested pre-B cell receptor-positive stages of B cell development. Interestingly, AID mRNA levels were increased by >10-fold at the transition from IL7Ra-positive Fraction C' pre-B cells to IL7Ra-negative Fraction D pre-B cells.

Conclusion:
AID is a tightly controlled mutator enzyme in mature germinal center B cells. The factors that prevent premature expression of AID during early B cell development were not known. Here, we here we report a novel, IL7Ra/Stat5-dependent mechanism by which pre-B cells are rendered non-responsive to antigen-dependent upregulation of AID. Attenuation of the IL7Ra/Stat5 signal occurs naturally in Fraction D pre-B cells. As a consequence, Fraction D pre-B cells express significant levels of AID for a short time. We propose that Fraction D pre-B cells represent a subset of increased genetic vulnerability in the natural history of childhood ALL.

Disclosures:
No relevant conflicts of interest to declare.