Multiple myeloma (MM) is the second most common hematologic malignancy in the United States. Chromosome 13 deletions are found in approximately one half (50%) of all patient samples and is associated with worse outcomes. The retinoblastoma gene (Rb1) has been implicated as a candidate tumor suppressor at 13q14, however, unmutated Rb1 remains expressed from retained alleles in the majority of cases. We sought to explore the role of Rb1 in plasma cells by generating mice with deficiency of Rb1 targeted to late B-cell development.
The Rb family member p107 can compensate for Rb1 function, so we crossed Rb1Flox/Flox and P107−/−mouse strains with Cγ1-Cre knock-in mice that express the Cre recombinase specifically in germinal center (GC) B-cells to generate mice with tissue-specific deletion of Rb in germinal center cells (Cg1-Rb-KO), and we aged these mice to assess disease development.
In Cg1-Rb-KO mice, we demonstrated the Rb1 locus was successfully recombined in splenic germinal center cells (B220+, GL7+ and IgG1+), as well as in post-germinal center cells (B220-, CD138+). Recombination was also detectable in bone marrow B-cell but not myeloid cell progenitors. Cg1-Rb-KO mice developed normally but became ill starting at 40 weeks of age. Sick mice demonstrated weight loss, hypoplastic spleens, osteopenia and significant lymphopenia. Thorough analysis of bone marrow and spleen hematopoietic progenitor cell subsets by multi-parameter flow cytometry at the time of death revealed dramatic reductions in marrow B-cell (Hardy fractions C-D, from 3.18×103to 40 cells and from 1.4×106 to 7.5×104cells respectively), suggesting an effective collapse of the B-cell lineage in mice lacking Rb. Examination of Cg1-Rb-KO mice prior to becoming sick (age 30–35 weeks) demonstrated increased numbers of B-cell progenitors in the bone marrow, consistent with Rb's known role in cell cycle regulation. Together, our results suggest Cg1-Rb-KO mice develop benign B-cell hyperplasia followed by a lethal collapse of the B-cell lineage. To help explain this B-cell exhaustion phenotype, we measured reactive oxygen species (ROS) in the B-cell of young Cg1-Rb-KO mice. We observed significantly increased ROS in Cg1-Rb-KO mice at baseline and following LPS stimulation ex vivo and in vivo compared to p107-KO control mice, suggesting that B-cells from Cg1-Rb-KO mice have elevated levels of oxidative stress.
Rb1 is important for maintenance of B-cell homeostasis and protects B-lineage progenitor cells from oxidative stress and exhaustion. Rb1 deletion was not sufficient for B-cell transformation and additional mutations are likely required for MM development in the context of complete RB1 loss.
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Asterisk with author names denotes non-ASH members.