Mechanisms of clonal evolution in childhood acute lymphoblastic leukemia

Childhood acute lymphoblastic leukemia (ALL) can often be traced to a pre-leukemic clone carrying a prenatal genetic lesion. Postnatally acquired mutations then drive clonal evolution toward overt leukemia. The enzymes RAG1-RAG2 and AID, which diversify immunoglobulin-encoding genes, are strictly segregated in developing cells during B lymphopoiesis and peripheral mature B cells, respectively. Here we identified small pre-BII cells as a natural subset with increased genetic vulnerability owing to concurrent activation of these enzymes. Consistent with epidemiological findings on childhood ALL etiology, susceptibility to genetic lesions during B lymphopoiesis at the transition from the large pre-BII cell stage to the small pre-BII cell stage was exacerbated by abnormal cytokine signaling and repetitive inflammatory stimuli. We demonstrated that AID and RAG1-RAG2 drove leukemic clonal evolution with repeated exposure to inflammatory stimuli, paralleling chronic infections in childhood.

Identification and characterization of OSTL (RNF217) encoding a RING-IBR-RING protein adjacent to a translocation breakpoint involving ETV6 in childhood ALL

Genomic aberrations involving ETV6 on band 12p13 are amongst the most common chromosomal abnormalities in human leukemia. The translocation t(6;12)(q23;13) in a childhood B-cell acute lymphoblastic leukemia (ALL) cell line fuses ETV6 with the putative long non-coding RNA gene STL. Linking STL properties to leukemia has so far been difficult. Here, we describe a novel gene, OSTL (annotated as RNF217 in Genbank), which shares the first exon and a CpG island with STL but is transcribed in the opposite direction. Human RNF217 codes for a highly conserved RING finger protein and is mainly expressed in testis and skeletal muscle with different splice variants. RNF217 shows regulated splicing in B cell development, and is expressed in a number of human B cell leukemia cell lines, primary human chronic myeloid leukemia, acute myeloid leukemia with normal karyotype and acute T-ALL samples. Using a yeast two-hybrid screen, we identified the anti-apoptotic protein HAX1 to interact with RNF217. This interaction could be mapped to the C-terminal RING finger motif of RNF217. We propose that some of the recurring aberrations involving 6q might deregulate the expression of RNF217 and result in imbalanced apoptosis signalling via HAX1, promoting leukemia development.

A heterozygous S32I mutation in IκBα that causes EDID results in defective development of lymphoid organs and impaired B cell function (68.13)

Autosomal ectodermal dysplasia with immunodeficiency (EDID) is caused by mutations in the inhibitor of NFκB α (IκBα), which is phosphorylated and degraded in response to several immune signaling pathways. We generated a mouse model of EDID by replacing one IκBα allele with a non-phosphorylatable IκBα, which resulted in decreased NFκB signaling. These mice have dysmorphic hair and teeth, as well as decreased serum immunoglobulins, and a severe decrease in their specific antibody response to T-dependent and T-independent antigens. The mice lack lymph nodes (LNs) and Peyers patches (PPs), and have a disrupted splenic architecture, with no marginal zone and fail to develop germinal centers (GCs). T cell function is intact but B cell function is deficient in vitro. Rag2 bone marrow chimeras formed proper lymphoid organs and had normal cutaneous hapten sensitivity but did not produce specific antibodies. This mouse model shows that autosomal EDID results in failure to develop LNs and PPs, disorganized splenic architecture, failure to develop GGs and an intrinsic B cell defect.