Acute Promyelocytic Leukemia, Retinoic Acid, and Arsenic: A Tale of Dualities

Acute promyelocytic leukemia (APL) is driven by the promyelocytic leukemia (PML)/retinoic acid receptor α (RARA) fusion oncoprotein. Over the years, it has emerged as a model system to understand how this simple (and sometimes sole) genetic alteration can transform hematopoietic progenitors through the acquisition of dominant-negative properties toward both transcriptional control by nuclear receptors and PML-mediated senescence. The fortuitous identification of two drugs, arsenic trioxide (ATO) and all-trans-retinoic acid (ATRA), that respectively bind PML and RARA to initiate PML/RARA degradation, has allowed an unprecedented dissection of the cellular and molecular mechanisms involved in patients' cure by the ATO/ATRA combination. This analysis has unraveled the dual and complementary roles of RARA and PML in both APL initiation and cure by the ATRA/ATO combination. We discuss how some of the features unraveled by APL studies may be more broadly applicable to some other forms of leukemia. In particular, the functional synergy between drugs that promote differentiation and those that initiate apoptosis/senescence to impede self-renewal could pave the way to novel curative combinations.

Creg1 Regulates Erythroid Development via TGF-β/Smad2-Klf1 Axis in Zebrafish

Understanding the regulation of normal erythroid development will help to develop new potential therapeutic strategies for disorders of the erythroid lineage. Cellular repressor of E1A-stimulated genes 1 (CREG1) is a glycoprotein that has been implicated in the regulation of tissue homeostasis. However, its role in erythropoiesis remains largely undefined. In this study, it is found that CREG1 expression increases progressively during erythroid differentiation. In zebrafish, creg1 mRNA is preferentially expressed within the intermediate cell mass (ICM)/peripheral blood island (PBI) region where primitive erythropoiesis occurs. Loss of creg1 leads to anemia caused by defective erythroid differentiation and excessive apoptosis of erythroid progenitors. Mechanistically, creg1 deficiency results in reduced activation of TGF-β/Smad2 signaling pathway. Treatment with an agonist of the Smad2 pathway (IDE2) could significantly restore the defective erythroid development in creg1-/- mutants. Further, Klf1, identified as a key target gene downstream of the TGF-β/Smad2 signaling pathway, is involved in creg1 deficiency-induced aberrant erythropoiesis. Thus, this study reveals a previously unrecognized role for Creg1 as a critical regulator of erythropoiesis, mediated at least in part by the TGF-β/Smad2-Klf1 axis. This finding may contribute to the understanding of normal erythropoiesis and the pathogenesis of erythroid disorders.

Znf687 recruits Brd4-Smrt complex to regulate gfi1aa during neutrophil development

Neutrophils are key component of the innate immune system in vertebrates. Diverse transcription factors and cofactors act in a well-coordinated manner to ensure proper neutrophil development. Dysregulation of the transcriptional program triggering neutrophil differentiation is associated with various human hematologic disorders such as neutropenia, neutrophilia, and leukemia. In the current study we show the zinc finger protein Znf687 is a lineage-preferential transcription factor, whose deficiency leads to an impaired neutrophil development in zebrafish. Mechanistically, Znf687 functions as a negative regulator of gfi1aa, a pivotal modulator in terminal granulopoiesis, to regulate neutrophil maturation. Moreover, we found BRD4, an important epigenetic regulator, directly interacts with ZNF687 in neutrophils. Deficiency of brd4 results in similar defective neutrophil development as observed in znf687 mutant zebrafish. Biochemical and genetic analyses further reveal that instead of serving as a canonical transcriptional coactivator, Brd4 directly interacts and bridges Znf687 and Smrt nuclear corepressor on gfi1aa gene's promoter to exert transcription repression. In addition, the ZNF687-BRD4-SMRT-GFI1 transcriptional regulatory network is evolutionary conserved in higher vertebrate. Overall, our work indicates Znf687 and Brd4 are two novel master regulators in promoting terminal granulopoiesis.

History of Developing Acute Promyelocytic Leukemia Treatment and Role of Promyelocytic Leukemia Bodies

The story of acute promyelocytic leukemia (APL) discovery, physiopathology, and treatment is a unique journey, transforming the most aggressive form of leukemia to the most curable. It followed an empirical route fueled by clinical breakthroughs driving major advances in biochemistry and cell biology, including the discovery of PML nuclear bodies (PML NBs) and their central role in APL physiopathology. Beyond APL, PML NBs have emerged as key players in a wide variety of biological functions, including tumor-suppression and SUMO-initiated protein degradation, underscoring their broad importance. The APL story is an example of how clinical observations led to the incremental development of the first targeted leukemia therapy. The understanding of APL pathogenesis and the basis for cure now opens new insights in the treatment of other diseases, especially other acute myeloid leukemias.

Structural Basis of PML-RARA Oncoprotein Targeting by Arsenic Unravels a Cysteine Rheostat Controlling PML Body Assembly and Function

PML nuclear bodies (NB) are disrupted in PML-RARA-driven acute promyelocytic leukemia (APL). Arsenic trioxide (ATO) cures 70% of patients with APL, driving PML-RARA degradation and NB reformation. In non-APL cells, arsenic binding onto PML also amplifies NB formation. Yet, the actual molecular mechanism(s) involved remain(s) elusive. Here, we establish that PML NBs display some features of liquid-liquid phase separation and that ATO induces a gel-like transition. PML B-box-2 structure reveals an alpha helix driving B2 trimerization and positioning a cysteine trio to form an ideal arsenic-binding pocket. Altering either of the latter impedes ATO-driven NB assembly, PML sumoylation, and PML-RARA degradation, mechanistically explaining clinical ATO resistance. This B2 trimer and the C213 trio create an oxidation-sensitive rheostat that controls PML NB assembly dynamics and downstream signaling in both basal state and during stress response. These findings identify the structural basis for arsenic targeting of PML that could pave the way to novel cancer drugs.

SIGNIFICANCE

Arsenic curative effects in APL rely on PML targeting. We report a PML B-box-2 structure that drives trimer assembly, positioning a cysteine trio to form an arsenic-binding pocket, which is disrupted in resistant patients. Identification of this ROS-sensitive triad controlling PML dynamics and functions could yield novel drugs. See related commentary by Salomoni, p. 2505. This article is featured in Selected Articles from This Issue, p. 2489.

The PML hub: An emerging actor of leukemia therapies

PML assembles into nuclear domains that have attracted considerable attention from cell and cancer biologists. Upon stress, PML nuclear bodies modulate sumoylation and other post-translational modifications, providing an integrated molecular framework for the multiple roles of PML in apoptosis, senescence, or metabolism. PML is both a sensor and an effector of oxidative stress. Emerging data has demonstrated its key role in promoting therapy response in several hematological malignancies. While these membrane-less nuclear hubs can enforce efficient cancer cell clearance, their downstream pathways deserve better characterization. PML NBs are druggable and their known modulators may have broader clinical utilities than initially thought.

Primary cells from patients with adult T cell leukemia/lymphoma depend on HTLV-1 Tax expression for NF-κB activation and survival

Adult T cell leukemia/lymphoma (ATL) is an aggressive malignancy secondary to chronic infection with human T cell leukemia virus type 1 (HTLV-1). The viral oncoprotein Tax initiates T cell transformation through activation of critical cellular pathways, including NF-κB. Unexpectedly, Tax protein is not detectable in most ATL cells, in contrast to the HTLV-1 HBZ protein which antagonizes Tax effects. Here, we demonstrate that primary ATL cells from patients with acute or chronic ATL express very low levels of Tax mRNA and protein. Critically, survival of these primary ATL cells is dependent on continued Tax expression. Mechanistically, Tax extinction results in reversal of NF-κB activation, P53/PML activation and apoptosis. Tax drives interleukin-10 (IL-10) expression and recombinant IL-10 rescues the survival of tax-depleted primary ATL cells. These results demonstrate the critical role of continued Tax and IL-10 expression for the survival of primary ATL cells, highlighting their relevance as therapeutic targets.

Clonal hematopoiesis driven by chromosome 1q/MDM4 trisomy defines a canonical route toward leukemia in Fanconi anemia

Fanconi anemia (FA) patients experience chromosome instability, yielding hematopoietic stem/progenitor cell (HSPC) exhaustion and predisposition to poor-prognosis myeloid leukemia. Based on a longitudinal cohort of 335 patients, we performed clinical, genomic, and functional studies in 62 patients with clonal evolution. We found a unique pattern of somatic structural variants and mutations that shares features of BRCA-related cancers, the FA-hallmark being unbalanced, microhomology-mediated translocations driving copy-number alterations. Half the patients developed chromosome 1q gain, driving clonal hematopoiesis through MDM4 trisomy downmodulating p53 signaling later followed by secondary acute myeloid lukemia genomic alterations. Functionally, MDM4 triplication conferred greater fitness to murine and human primary FA HSPCs, rescued inflammation-mediated bone marrow failure, and drove clonal dominance in FA mouse models, while targeting MDM4 impaired leukemia cells in vitro and in vivo. Our results identify a linear route toward secondary leukemogenesis whereby early MDM4-driven downregulation of basal p53 activation plays a pivotal role, opening monitoring and therapeutic prospects.

Zbtb14 regulates monocyte and macrophage development through inhibiting pu.1 expression in zebrafish

Macrophages and their precursor cells, monocytes, are the first line of defense of the body against foreign pathogens and tissue damage. Although the origins of macrophages are diverse, some common transcription factors (such as PU.1) are required to ensure proper development of monocytes/macrophages. Here, we report that the deficiency of zbtb14, a transcription repressor gene belonging to ZBTB family, leads to an aberrant expansion of monocyte/macrophage population in zebrafish. Mechanistically, Zbtb14 functions as a negative regulator of pu.1, and SUMOylation on a conserved lysine is essential for the repression activity of Zbtb14. Moreover, a serine to phenylalanine mutation found in an acute myeloid leukemia (AML) patient could target ZBTB14 protein to autophagic degradation. Hence, ZBTB14 is a newly identified gene implicated in both normal and malignant myelopoiesis.

Cystine uptake inhibition potentiates front-line therapies in acute myeloid leukemia

By querying metabolic pathways associated with leukemic stemness and survival in multiple AML datasets, we nominated SLC7A11 encoding the xCT cystine importer as a putative AML dependency. Genetic and chemical inhibition of SLC7A11 impaired the viability and clonogenic capacity of AML cell lines in a cysteine-dependent manner. Sulfasalazine, a broadly available drug with xCT inhibitory activity, had anti-leukemic activity against primary AML samples in ex vivo cultures. Multiple metabolic pathways were impacted upon xCT inhibition, resulting in depletion of glutathione pools in leukemic cells and oxidative stress-dependent cell death, only in part through ferroptosis. Higher expression of cysteine metabolism genes and greater cystine dependency was noted in NPM1-mutated AMLs. Among eight anti-leukemic drugs, the anthracycline daunorubicin was identified as the top synergistic agent in combination with sulfasalazine in vitro. Addition of sulfasalazine at a clinically relevant concentration significantly augmented the anti-leukemic activity of a daunorubicin-cytarabine combination in a panel of 45 primary samples enriched in NPM1-mutated AML. These results were confirmed in vivo in a patient-derived xenograft model. Collectively, our results nominate cystine import as a druggable target in AML and raise the possibility to repurpose sulfasalazine for the treatment of AML, notably in combination with chemotherapy.

Actinomycin D Targets NPM1c-Primed Mitochondria to Restore PML-Driven Senescence in AML Therapy

Acute myeloid leukemia (AML) pathogenesis often involves a mutation in the NPM1 nucleolar chaperone, but the bases for its transforming properties and overall association with favorable therapeutic responses remain incompletely understood. Here we demonstrate that an oncogenic mutant form of NPM1 (NPM1c) impairs mitochondrial function. NPM1c also hampers formation of promyelocytic leukemia (PML) nuclear bodies (NB), which are regulators of mitochondrial fitness and key senescence effectors. Actinomycin D (ActD), an antibiotic with unambiguous clinical efficacy in relapsed/refractory NPM1c-AMLs, targets these primed mitochondria, releasing mitochondrial DNA, activating cyclic GMP-AMP synthase signaling, and boosting reactive oxygen species (ROS) production. The latter restore PML NB formation to drive TP53 activation and senescence of NPM1c-AML cells. In several models, dual targeting of mitochondria by venetoclax and ActD synergized to clear AML and prolong survival through targeting of PML. Our studies reveal an unexpected role for mitochondria downstream of NPM1c and implicate a mitochondrial/ROS/PML/TP53 senescence pathway as an effector of ActD-based therapies.

SIGNIFICANCE

ActD induces complete remissions in NPM1-mutant AMLs. We found that NPM1c affects mitochondrial biogenesis and PML NBs. ActD targets mitochondria, yielding ROS which enforce PML NB biogenesis and restore senescence. Dual targeting of mitochondria with ActD and venetoclax sharply potentiates their anti-AML activities in vivo. This article is highlighted in the In This Issue feature, p. 2945.

Biological Effects of BET Inhibition by OTX015 (MK-8628) and JQ1 in NPM1-Mutated (NPM1c) Acute Myeloid Leukemia (AML)

BET inhibitors (BETi) including OTX015 (MK-8628) and JQ1 demonstrated antileukemic activity including NPM1c AML cells. Nevertheless, the biological consequences of BETi in NPM1c AML were not fully investigated. Even if of better prognosis AML patients with NPM1c may relapse and treatment remains difficult. Differentiation-based therapy by all trans retinoic acid (ATRA) combined with arsenic trioxide (ATO) demonstrated activity in NPM1c AML. We found that BETi, similar to ATO + ATRA, induced differentiation and apoptosis which was TP53 independent in the NPM1c cell line OCI-AML3 and primary cells. Furthermore, BETi induced proteasome-dependent degradation of NPM1c. BETi degraded NPM1c in the cytosol while BRD4 is degraded in the nucleus which suggests that restoration of the NPM1/BRD4 equilibrium in the nucleus of NPM1c cells is essential for the efficacy of BETi. While ATO + ATRA had significant biological activity in NPM1c IMS-M2 cell line, those cells were resistant to BETi. Gene profiling revealed that IMS-M2 cells probably resist to BETi by upregulation of LSC pathways independently of the downregulation of a core BET-responsive transcriptional program. ATO + ATRA downregulated a NPM1c specific HOX gene signature while anti-leukemic effects of BETi appear HOX gene independent. Our preclinical results encourage clinical testing of BETi in NPM1c AML patients.

JAK2V617F myeloproliferative neoplasm eradication by a novel interferon/arsenic therapy involves PML

Interferon α (IFNα) is used to treat JAK2^V617F-driven myeloproliferative neoplasms (MPNs) but rarely clears the disease. We investigated the IFNα mechanism of action focusing on PML, an interferon target and key senescence gene whose targeting by arsenic trioxide (ATO) drives eradication of acute promyelocytic leukemia. ATO sharply potentiated IFNα-induced growth suppression of JAK2^V617F patient or mouse hematopoietic progenitors, which required PML and was associated with features of senescence. In a mouse MPN model, combining ATO with IFNα enhanced and accelerated responses, eradicating MPN in most mice by targeting disease-initiating cells. These results predict potent clinical efficacy of the IFNα+ATO combination in patients and identify PML as a major effector of therapy, even in malignancies with an intact PML gene.

Irf2bp2a regulates terminal granulopoiesis through proteasomal degradation of Gfi1aa in zebrafish

The ubiquitin-proteasome system plays important roles in various biological processes as it degrades the majority of cellular proteins. Adequate proteasomal degradation of crucial transcription regulators ensures the proper development of neutrophils. The ubiquitin E3 ligase of Growth factor independent 1 (GFI1), a key transcription repressor governing terminal granulopoiesis, remains obscure. Here we report that the deficiency of the ring finger protein Interferon regulatory factor 2 binding protein 2a (Irf2bp2a) leads to an impairment of neutrophils differentiation in zebrafish. Mechanistically, Irf2bp2a functions as a ubiquitin E3 ligase targeting Gfi1aa for proteasomal degradation. Moreover, irf2bp2a gene is repressed by Gfi1aa, thus forming a negative feedback loop between Irf2bp2a and Gfi1aa during neutrophils maturation. Different levels of GFI1 may turn it into a tumor suppressor or an oncogene in malignant myelopoiesis. Therefore, discovery of certain drug targets GFI1 for proteasomal degradation by IRF2BP2 might be an effective anti-cancer strategy.

PML/RARA destabilization by hyperthermia: a new model for oncogenic fusion protein degradation?

In this issue, Maimaitiyiming and colleagues demonstrate thermic stress-induced PML/RARA oncogenic fusion protein destabilization driven by corepressor aggregation. Hyperthermia synergizes with PML/RARA degradation by ATO and may circumvent ATO-resistance in historical APL patients. This novel approach could be extended to other corepressor-associated oncogenic fusion proteins.

Interconversion between Tumorigenic and Differentiated States in Acute Myeloid Leukemia

Tumors are composed of phenotypically heterogeneous cancer cells that often resemble various differentiation states of their lineage of origin. Within this hierarchy, it is thought that an immature subpopulation of tumor-propagating cancer stem cells (CSCs) differentiates into non-tumorigenic progeny, providing a rationale for therapeutic strategies that specifically eradicate CSCs or induce their differentiation. The clinical success of these approaches depends on CSC differentiation being unidirectional rather than reversible, yet this question remains unresolved even in prototypically hierarchical malignancies, such as acute myeloid leukemia (AML). Here, we show in murine and human models of AML that, upon perturbation of endogenous expression of the lineage-determining transcription factor PU.1 or withdrawal of established differentiation therapies, some mature leukemia cells can de-differentiate and reacquire clonogenic and leukemogenic properties. Our results reveal plasticity of CSC maturation in AML, highlighting the need to therapeutically eradicate cancer cells across a range of differentiation states.

Inhibition of transcription by dactinomycin reveals a new characteristic of immunogenic cell stress

Chemotherapy still constitutes the standard of care for the treatment of most neoplastic diseases. Certain chemotherapeutics from the oncological armamentarium are able to trigger pre‐mortem stress signals that lead to immunogenic cell death (ICD), thus inducing an antitumor immune response and mediating long‐term tumor growth reduction. Here, we used an established model, built on artificial intelligence to identify, among a library of 50,000 compounds, anticancer agents that, based on their molecular descriptors, were predicted to induce ICD. This algorithm led us to the identification of dactinomycin (DACT, best known as actinomycin D), a highly potent cytotoxicant and ICD inducer that mediates immune‐dependent anticancer effects in vivo. Since DACT is commonly used as an inhibitor of DNA to RNA transcription, we investigated whether other experimentally established or algorithm‐selected, clinically employed ICD inducers would share this characteristic. As a common leitmotif, a panel of pharmacological ICD stimulators inhibited transcription and secondarily translation. These results establish the inhibition of RNA synthesis as an initial event for ICD induction.

Classic and Variants APLs, as Viewed from a Therapy Response

Most acute promyelocytic leukemia (APL) are caused by PML-RARA, a translocation-driven fusion oncoprotein discovered three decades ago. Over the years, several other types of rare X-RARA fusions have been described, while recently, oncogenic fusion proteins involving other retinoic acid receptors (RARB or RARG) have been associated to very rare cases of acute promyelocytic leukemia. PML-RARA driven pathogenesis and the molecular basis for therapy response have been the focus of many studies, which have now converged into an integrated physio-pathological model. The latter is well supported by clinical and molecular studies on patients, making APL one of the rare hematological disorder cured by targeted therapies. Here we review recent data on APL-like diseases not driven by the PML-RARA fusion and discuss these in view of current understanding of "classic" APL pathogenesis and therapy response.

Arsenic trioxide (As2O3) as a maintenance therapy for adult T cell leukemia/lymphoma

Background

Adult T-cell leukemia-lymphoma (ATL) is an aggressive mature lymphoid proliferation associated with poor prognosis. Standard of care includes chemotherapy and/or the combination of zidovudine and interferon-alpha. However, most patients experience relapse less than 6 months after diagnosis. Allogeneic stem cell transplantation is the only curative treatment, but is only feasible in a minority of cases. We previously showed in a mouse model that Arsenic trioxide (As₂O₃) targets ATL leukemia initiating cells.

Results

As₂O₃ consolidation was given in 9 patients with ATL (lymphoma n = 4; acute n = 2; and indolent n = 3), who were in complete (n = 4) and partial (n = 3) remission, in stable (n = 1) and in progressive (n = 1) disease. Patients received up to 8 weeks of As₂O₃ at the dose of 0.15 mg/kg/day intravenously in combination with zidovudine and interferon-alpha. One patient in progression died rapidly. Of the remaining eight patients, three with indolent ATL subtype showed overall survivals of 48, 53 and 97 months, and duration of response to As₂O₃ of 22, 25 and 73 months. The other 5 patients with aggressive ATL subtype had median OS of 36 months and a median duration of response of 10 months. Side effects were mostly hematological and cutaneous (one grade 3) and reversible with dose reduction of AZT/IFN and/or As₂O₃ discontinuation. The virus integration analysis revealed the regression of the predominant malignant clone in one patient with a chronic subtype.

Conclusion

These results suggest that consolidation with As₂O₃ could be an option for patients with ATL in response after induction therapy and who are not eligible for allogeneic stem cell transplantation.

Interferon regulatory factor 2 binding protein 2b regulates neutrophil versus macrophage fate during zebrafish definitive myelopoiesis

Aproper choice of neutrophil-macrophage progenitor cell fate is essential for the generation of adequate myeloid subpopulations during embryonic development and in adulthood. The network governing neutrophil-macrophage progenitor cell fate has several key determinants, such as myeloid master regulators CCAAT enhancer binding protein alpha (C/EBPα) and spleen focus forming virus proviral integration oncogene (PU.1). Nevertheless, more regulators remain to be identified and characterized. To ensure balanced commitment of neutrophil-macrophage progenitors toward each lineage, the interplay among these determinants is not only synergistic, but also antagonistic. Depletion of interferon regulatory factor 2 binding protein 2b (Irf2bp2b), a well-known negative transcription regulator, results in a bias in neutrophil-macrophage progenitor cell fate in favor of macrophages at the expense of neutrophils during the stage of definitive myelopoiesis in zebrafish embryos. Mechanistic studies indicate that Irf2bp2b acts as a downstream target of C/EBPα, repressing PU.1 expression, and that SUMOylation confers the repressive function of Irf2bp2b. Thus, Irf2bp2b is a novel determinant in the choice of fate of neutrophil-macrophage progenitor cells.

A molecule inducing androgen receptor degradation and selectively targeting prostate cancer cells

A new molecule induces AR sumoylation and degradation resulting in selective growth inhibition in AR-dependent prostate cancer cells, but its activity is blunted by interference with proteasomes.

Aberrant androgen signaling drives prostate cancer and is targeted by drugs that diminish androgen production or impede androgen–androgen receptor (AR) interaction. Clinical resistance arises from AR overexpression or ligand-independent constitutive activation, suggesting that complete AR elimination could be a novel therapeutic strategy in prostate cancers. IRC117539 is a new molecule that targets AR for proteasomal degradation. Exposure to IRC117539 promotes AR sumoylation and ubiquitination, reminiscent of therapy-induced PML/RARA degradation in acute promyelocytic leukemia. Critically, ex vivo, IRC117539-mediated AR degradation induces prostate cancer cell viability loss by inhibiting AR signaling, even in androgen-insensitive cells. This approach may be beneficial for castration-resistant prostate cancer, which remains a clinical issue. In xenograft models, IRC117539 is as potent as enzalutamide in impeding growth, albeit less efficient than expected from ex vivo studies. Unexpectedly, IRC117539 also behaves as a weak proteasome inhibitor, likely explaining its suboptimal efficacy in vivo. Our studies highlight the feasibility of AR targeting for degradation and off-target effects’ importance in modulating drug activity in vivo.

TAMM41 is required for heart valve differentiation via regulation of PINK-PARK2 dependent mitophagy

TAMM41, located within the congenital heart diseases (CHD) sensitive region of 3p25 deletion syndrome, is a mitochondrial membrane maintenance protein critical for yeast survival, but its function in higher vertebrates remains unknown. Via in vivo zebrafish model, we found that tamm41 is highly expressed in the developing heart and deficiency of which led to heart valve abnormalities. Molecular mechanistic studies revealed that TAMM41 interacts and modulates the PINK1-PARK2 dependent mitophagy pathway, thereby implicating TAMM41 in heart valve development during zebrafish embryonic cardiogenesis. Furthermore, through screening of the congenital heart diseases (CHD) sensitive region of 3p25 deletion syndrome among 118 sporadic atrioventricular septal defect (AVSD) patients, we identified three cases carrying heterozygous pathogenic intronic variants of TAMM41. All three cases lacked normal full-length TAMM41 transcripts, most likely due to specific expression of the mutant allele. Collectively, our studies highlight essential roles for TAMM41-dependent mitophagy in development of the heart and provide novel insights into the etiology of AVSD.