In Vivo Modeling of CLL Transformation to Richter Syndrome Reveals Convergent Evolutionary Paths and Therapeutic Vulnerabilities

Transformation to aggressive disease histologies generates formidable clinical challenges across cancers, but biological insights remain few. We modeled the genetic heterogeneity of chronic lymphocytic leukemia (CLL) through multiplexed in vivo CRISPR-Cas9 B-cell editing of recurrent CLL loss-of-function drivers in mice and recapitulated the process of transformation from indolent CLL into large cell lymphoma [i.e., Richter syndrome (RS)]. Evolutionary trajectories of 64 mice carrying diverse combinatorial gene assortments revealed coselection of mutations in Trp53, Mga, and Chd2 and the dual impact of clonal Mga/Chd2 mutations on E2F/MYC and interferon signaling dysregulation. Comparative human and murine RS analyses demonstrated tonic PI3K signaling as a key feature of transformed disease, with constitutive activation of the AKT and S6 kinases, downmodulation of the PTEN phosphatase, and convergent activation of MYC/PI3K transcriptional programs underlying enhanced sensitivity to MYC/mTOR/PI3K inhibition. This robust experimental system presents a unique framework to study lymphoid biology and therapy.

SIGNIFICANCE

Mouse models reflective of the genetic complexity and heterogeneity of human tumors remain few, including those able to recapitulate transformation to aggressive disease histologies. Herein, we model CLL transformation into RS through multiplexed in vivo gene editing, providing key insight into the pathophysiology and therapeutic vulnerabilities of transformed disease. This article is highlighted in the In This Issue feature, p. 101.

Activation of Notch and Myc Signaling via B-cell-Restricted Depletion of Dnmt3a Generates a Consistent Murine Model of Chronic Lymphocytic Leukemia

Chronic lymphocytic leukemia (CLL) is characterized by disordered DNA methylation, suggesting these epigenetic changes might play a critical role in disease onset and progression. The methyltransferase DNMT3A is a key regulator of DNA methylation. Although DNMT3A somatic mutations in CLL are rare, we found that low DNMT3A expression is associated with more aggressive disease. A conditional knockout mouse model showed that homozygous depletion of Dnmt3a from B cells results in the development of CLL with 100% penetrance at a median age of onset of 5.3 months, and heterozygous Dnmt3a depletion yields a disease penetrance of 89% with a median onset at 18.5 months, confirming its role as a haploinsufficient tumor suppressor. B1a cells were confirmed as the cell of origin of disease in this model, and Dnmt3a depletion resulted in focal hypomethylation and activation of Notch and Myc signaling. Amplification of chromosome 15 containing the Myc gene was detected in all CLL mice tested, and infiltration of high-Myc-expressing CLL cells in the spleen was observed. Notably, hyperactivation of Notch and Myc signaling was exclusively observed in the Dnmt3a CLL mice, but not in three other CLL mouse models tested (Sf3b1-Atm, Ikzf3, and MDR), and Dnmt3a-depleted CLL were sensitive to pharmacologic inhibition of Notch signaling in vitro and in vivo. Consistent with these findings, human CLL samples with lower DNMT3A expression were more sensitive to Notch inhibition than those with higher DNMT3A expression. Altogether, these results suggest that Dnmt3a depletion induces CLL that is highly dependent on activation of Notch and Myc signaling. SIGNIFICANCE: Loss of DNMT3A expression is a driving event in CLL and is associated with aggressive disease, activation of Notch and Myc signaling, and enhanced sensitivity to Notch inhibition.

A hotspot mutation in transcription factor IKZF3 drives B cell neoplasia via transcriptional dysregulation

Hotspot mutation of IKZF3 (IKZF3-L162R) has been identified as a putative driver of chronic lymphocytic leukemia (CLL), but its function remains unknown. Here, we demonstrate its driving role in CLL through a B cell-restricted conditional knockin mouse model. Mutant Ikzf3 alters DNA binding specificity and target selection, leading to hyperactivation of B cell receptor (BCR) signaling, overexpression of nuclear factor κB (NF-κB) target genes, and development of CLL-like disease in elderly mice with a penetrance of ~40%. Human CLL carrying either IKZF3 mutation or high IKZF3 expression was associated with overexpression of BCR/NF-κB pathway members and reduced sensitivity to BCR signaling inhibition by ibrutinib. Our results thus highlight IKZF3 oncogenic function in CLL via transcriptional dysregulation and demonstrate that this pro-survival function can be achieved by either somatic mutation or overexpression of this CLL driver. This emphasizes the need for combinatorial approaches to overcome IKZF3-mediated BCR inhibitor resistance.

Studies on the regenerative recovery of long-term denervated muscle in rats

Denervated extensor digitorum longus (EDL) muscles in rats rapidly lose mass and contractile force. After two months of denervation, mass and maximum tetanic force have fallen to 31% and 2% of the values of contralateral control muscles. Our purpose was to determine if grafting a long-term denervated muscle into an innervated site provides an effective means of restoring its structure and function. EDL muscles that had been denervated for periods of 2-12 months were freely grafted into innervated sites of EDL muscles in 4-month inbred host animals. Contralateral normally innervated EDL muscles from the same donors were implanted into the opposite legs of the same hosts. Two months after grafting, the muscles were removed and measurements were made in vitro of isometric contractile properties. The grafts were then prepared for morphological analysts. In all cases, the maximum forces generated by innervated grafts of denervated muscles were greater than those generated by denervated muscles. However, when compared with grafts of control muscles in the contralateral limb, grafts of previously denervated muscles showed a steady decline in structural and functional recovery corresponding to the time of previous denervation. The decline was especially pronounced for muscles denervated between 2 and 7 months prior to grafting. Grafts of 7-month denervated muscles were restored to only 17% of the maximum tetanic force of contralateral control grafts compared with 83% for grafts of 2-month denervated muscles. The longer a muscle had been denervated prior to grafting, the higher proportion of thin atrophic muscle fibers it contained. We conclude that grafting into an innervated site improves the mass and maximum force of a muscle over the denervated state, but the longer the period of prior denervation the poorer the recovery of the grafted muscles.

Confocal analysis of the dystrophin protein complex in muscular dystrophy

The dystrophin protein complex (DPC), composed of at least 10 proteins that associate with dystrophin, is critical for the maintenance of normal muscle fiber structure and physiology. In this study, we used immunohistochemistry and confocal microscopy to examine the relative abundance and distribution of several of these proteins in muscle biopsies taken from patients with various muscular dystrophies. The optical sectioning capability of confocal microscopy allowed us to comprehensively analyze the semiquantitative expression of components of the DPC. Alpha-sarcoglycan-deficient patients displayed a marked reduction in membrane immunostaining of the sarcoglycan complex. Gamma-sarcoglycan-deficient patients showed variable decreased immunostaining of the sarcoglycan complex proteins. When beta-sarcoglycan was expressed appropriately at the sarcolemma of gamma-sarcoglycan-deficient patients, intracellular labeling of beta-sarcoglycan was also present. Beta-sarcoglycan-deficient patients showed poor localization of extracellular matrix proteins in addition to a complete absence of the sarcoglycans. Merosin-deficient patients showed relatively normal immunostaining levels of all other members of the DPC. Finally, dystrophin-deficient patients showed little or no change in the expression of extracellular matrix proteins; however, some sarcoglycans were significantly decreased. These data allowed us to suggest unique fundamental interactions between the members of the DPC.

Plasma membrane cytoskeleton of muscle: a fine structural analysis

The discovery of dystrophin and its definition as the causative molecule in Duchenne Muscular Dystrophy has led to a renewed interest in the molecular structure of the muscle fiber plasma membrane and its association with the extracellular basal lamina. The original identification of dystrophin gave credence to the possibility that the plasma membrane of the muscle fiber may be highly organized and involved in maintaining appropriate homeostasis in this actively contracting cellular system. In this review, we examine the currently known members of the muscle fiber plasma membrane cytoskeleton and the interactions that occur between the different members of this complex using histological, electron microscopic, and confocal methods. From our studies and others cited in this review, it is clear that the dystrophin cytoskeletal complex is not completely understood and component molecules continue to be discovered. Perhaps equally importantly, currently defined molecules (such as alpha-actinin or neuronal nitric oxide synthase) are being recognized as being specifically associated with the complex. What is striking from all of the studies, to date, is that while we are able to identify members of the dystrophin cytoskeletal complex and while we are able to associate mutations of individual molecules with disease(s), we are still unable to truly define the roles of each of the molecules in maintaining the normal physiology of the muscle fiber.

The recovery of long-term denervated rat muscles after Marcaine treatment and grafting

Disruption of the nerve supply results in the rapid loss of mass and contractile force in skeletal muscles. These losses are reversible to a high degree in short-term denervated muscles with grafting and nerve implantation. However, return is much poorer in long-term denervated muscles. This study examined the basis for the differences in the recovery of non-denervated and 7-month denervated rat extensor digitorum longus (EDL) muscles after grafting and nerve implantation. We found that the level of recovery is related to the ability of muscle fibers to degenerate and regenerate after grafting. Fibers within long-term denervated muscles do not degenerate and regenerate as well as those within muscles which are not denervated prior to grafting. The functional recovery of the denervated muscles is significantly improved when their fibers are induced to degenerate with the myotoxic anesthetic, Marcaine, Degeneration of these fibers is followed by massive regeneration. The finding that denervated muscles are capable of being restored to a significant level by inducing regeneration may be useful in the clinical treatment of denervated muscles.