A new study led by researchers at Dana-Farber Cancer Institute advances the understanding of the sex bias associated with blastic plasmacytoid dendritic cell neoplasm (BPDCN) and also the mechanisms that cause the rare and aggressive form of leukemia, which is 3 times as likely to develop in individuals with only one X chromosome, primarily men.
Detailing the study findings in Cancer Discovery, researchers led by Andrew Lane, MD, PhD, Director of the BPDCN Center at Dana-Farber, show how they connected X chromosome mutations in the ZRSR2 gene to "an extremely male-predominant leukemia"(2022; doi: 10.1158/2159-8290.CD-20-1513).
"Aberrant RNA splicing induced by ZRSR2 mutation impairs dendritic cell inflammatory signaling, interferon production, and apoptosis, revealing a sex- and lineage-related tumor suppressor pathway," the investigators reported.
The ZRSR2 gene mutations account for at least 50 percent of the one X-chromosome-predominant pattern in BPDCN in the study. Other causes may have included undetectable mutations of ZRSR2 or possibly mutations in other genes.
BPDCN develops in precursors of plasmacytoid dendritic cells (pDCs), which produce type 1 interferons in response to the bacteria and viruses. An X chromosome gene encoding a splicing factor, ZRSR2, removes segments of messenger RNA called introns to enable conversion into a protein usable by a cell.
While using whole-exome sequencing to study primary BPDCN and disease-associated mutations, the researchers discovered that loss-of-function mutations in ZRSR2 impair the activation of apoptosis after inflammatory stimuli associated with intron retention and the inability to up-regulate the type 1 interferon regulatory factor 7 (IRF7).
"We linked acquired alterations in RNA splicing to abnormal pDC development and inflammatory response through toll-like receptors," the researchers reported. They added that, in vivo, BPDCN-associated mutations were observed to promote pDC expansion and signatures of decreased activation.
Together, the study data "support a model in which male-biased mutation in hematopoietic progenitors alters pDC function and confers protection from apoptosis, which may impair immunity and predispose to leukemic transformation."
Identifying the EXITS
While the alteration of messenger RNA by ZRSR2 is required for the conversion of DNA instructions into a protein that immune cells need to function properly, it often results in "mis-splicing" when ZRSR2 is mutated, according to the researchers.
"We found that when ZRSR2 is mutated, many genes are 'mis-spliced,' and this includes genes important to how the immune system responds to infection or inflammation," Lane stated upon the publication of the study results.
This mis-splicing not only prevents the pDCs from performing their primary role of sensing inflammation, but also stops them from dying immediately afterward as programmed, he noted. As a result, pDCs "hang around longer" and in too great a number, which the researchers believe "makes them more likely to acquire additional mutations that can turn them into a leukemia cell."
Drugs that modulate RNA splicing currently being tested in other blood cancers may be effective for treating BPDCN as well. Another strategy is to block pathways that inhibit cell death using a BCL-2 inhibitor, such as venetoclax, which the researchers are currently testing in combination with other drugs in a clinical trial.
Meanwhile, because the high frequency of ZRSR2 loss-of-function mutations in BPDCN is found exclusively in male patients, it "nominates it as a lineage-specific EXITS gene," or an escape from X inactivation tumor suppressor gene, previously discovered by Lane and colleagues. EXITS genes afford additional protection to women from cancer development.
ZRSR2 loss-of-function mutations were not observed as EXITS genes in their earlier study because it is most often mutated in BPDCN, myelodysplastic syndromes, and other myeloid neoplasms like chronic myelomonocytic leukemia that were not included in large-scale sequencing efforts such as The Cancer Genome Atlas, the researchers reported: "therefore, this suggests that additional sex-biased cancer genes remain discoverable in understudied diseases."
The TLR-IRF Pathway
Further research will examine the effect of other mutations in BPDCN on the TLR-IRF pathway, and whether they contribute to similar phenotypes.
The current study findings, meanwhile, "illustrate how identifying functional consequences of a sex-biased mutation may uncover more generalized tumor biology that is relevant even in cases without the specific mutation," the researchers reported.
While previous studies found sex bias in nonmalignant pDC function and that females have more robust pDC activation and type 1 interferon production than males, it was attributed to higher female expression of the X chromosome or autosomal genes.
"Our data nominate ZRSR2 as another candidate gene that might render male pDCs less susceptible to activation-induced cell death and could contribute to transformation even in the absence of a mutation," according to the researchers.
The effect of ZRSR2 loss, meanwhile, was much more dramatic in TLR-stimulated cells than at baseline. While the fact that IRF7 is regulated by delayed splicing during inflammation "again points to the TLR-IRF pathway as a common node that may contribute to BPDCN."
The Dana-Farber investigators previously found that IRF7 was among the key pDC genes down-regulated in BPDCN using single-cell RNA-seq, which suggested that suppression of the pathway "might be a common feature of pDC transformation even in the absence of a splicing factor mutation."
Noting that genetically engineered mouse models have yet to develop spontaneous BPDCN in vivo, the researchers concluded, "Distinct acquired alterations in pDCs caused by mutations may converge on TLR signaling to confer a clonal advantage by evasion of activation-induced apoptosis, which could explain why we observed decreased activation after TLR stimulation across BPDCN genotypes. This suggests optimal modeling of transformation by BPDCN-associated mutations may need to be performed in the presence of inflammation."
Chuck Holt is a contributing writer.
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