Monique F. Smeets

RECQL4, a RecQ family helicase, is essential for DNA replication and genome stability. Mutations in RECQL4 cause severe human disorders yet we do not fully understand its functions, particularly regarding ATP-dependent helicase activity. To understand RECQL4’s functions further, we performed a genome-wide forward genetic screen using a murine model harbouring patient-like RECQL4 mutations. We identify KLHDC3, a substrate-binding subunit of the Cullin-RING ligase E3 complex, loss as the most significant rescue allele. KLHDC3 loss restores proliferation and replication in RECQL4-deficient cells by stabilizing trace levels of a truncated RECQL4 fragment containing the N-terminal 480 amino acids, lacking the helicase and C-terminal regions. This RECQL4 fragment forms after Cre-mediated recombination of the Recql4 ^fl allele and contains a neo-degron sequence specific for KLHDC3. Although this mechanism does not apply to human mutations, it demonstrates that minimal RECQL4 levels, without any ATPase domain/activity, are sufficient to support DNA replication. This demonstrates that RECQL4 is an essential and non-redundant regulator of DNA replication and cell viability and that this activity does not require its ATP-dependent helicase activity.

Background: Protein ubiquitination is a key post-translational modification that governs protein stability and cellular homeostasis. KLHDC3 is a substrate recognition receptor in the recently identified C-terminal degron-mediated DesCEND ubiquitination pathway. It selectively binds proteins with C-terminal RxxxG motifs, targeting them for degradation. While N-terminal degron pathways are well-characterized, the physiological roles of C-terminal degrons remain poorly understood. To explore KLHDC3’s function in a physiological context, we generated mice deficient in the Klhdc3 gene. Results: Klhdc3 -deficient mice exhibited sub-Mendelian birth rates and progressive postnatal lethality, with a median survival of 136 days and a maximum lifespan of approximately one year. Surviving mice showed early growth retardation followed by normalization of body mass, and later developed pronounced obesity, with some individuals reaching fat mass levels exceeding 50% of total body weight. Transcriptomic and proteomic analyses of Klhdc3 ^−/− embryonic fibroblasts revealed significant changes in protein expression with minimal alterations in transcript levels, consistent with KLHDC3’s role in post-translational regulation. Among the upregulated proteins, HINT1 was identified as a novel KLHDC3 substrate containing a C-terminal degron motif. Protein stability assays and immunoblotting confirmed HINT1 as a candidate target of KLHDC3. Conclusions: This study establishes a physiological role for the DesCEND pathway in vivo and identifies KLHDC3 as a critical regulator of development, survival, and adiposity in mice. The identification of HINT1 as a putative KLHDC3 substrate expands our understanding of C-terminal degron-mediated protein regulation and suggests broader implications for developmental and metabolic processes.

ADAR1 edits adenosine to inosine (A-to-I) in double-stranded RNA to prevent MDA5 sensing of cellular transcripts, its key physiological role. However, editing-independent functions of ADAR1 remain poorly understood. Using a series of Adar1 mutant mice rescued by loss of MDA5 and PKR, we investigated isoform-specific, editing-independent roles of ADAR1 in vivo . We found that the cytoplasmic ADAR1p150 isoform is essential for maintaining peripheral T cell numbers and differentiation of hematopoietic stem and progenitor cells (HSPCs). In bone marrow transplants, ADAR1p150 protein, but not its editing activity, was crucial for T cell regeneration and HSPC repopulation, demonstrating a cell-intrinsic function in hematopoiesis. Experiments with IFNβ-treatment of purified HSPCs in vitro and in vivo IFNAR1 neutralization revealed hypersensitivity to tonic type I interferon (IFN) in the absence of ADAR1p150. Using cell lines, we demonstrate that type I IFN activates the OAS-RNaseL pathway, leading to cell death. This study shows that tonic type I IFN induces immunogenic cellular RNAs in sterile conditions. ADAR1p150 suppresses immune sensing of self-dsRNAs through both editing-dependent (MDA5) and editing-independent (PKR, OAS-RNaseL) mechanisms. Thus, ADAR1p150 protein levels and activity combine to set the threshold for tolerance to self-derived dsRNAs.

Protein ubiquitination is a critical post-translational modification that regulates protein stability and cellular homeostasis. KLHDC3 is a substrate recognition receptor within the recently identified C-terminal degron-mediated DesCEND ubiquitination pathway. It has been characterised to selectively bind proteins with C-terminal RxxxG motifs, targeting them for degradation. Unlike the well-characterised N-terminal degron pathway, the physiological roles of C-terminal degrons remain poorly understood. To investigate KLHDC3 function in vivo , we generated Klhdc3-deficient ( Klhdc3 ^-/- ) mice. These mice exhibited sub-Mendelian birth rates and progressive postnatal lethality, with a median survival of 136 days and maximum survival observed of ∼1 year of age. Early growth retardation was apparent, followed by a normalisation of body mass with age. However, as Klhdc3 ^-/- mice aged they developed pronounced obesity at the expense of lean mass, with some individuals reaching fat mass exceeding 50% of total body weight. Combined transcriptomic and proteomic analyses of Klhdc3 ^-/- embryonic fibroblasts revealed significant changes in protein expression with minimal impact on transcript levels, consistent with KLHDC3’s role in post-translational regulation. Among the upregulated proteins, HINT1 was identified as a novel KLHDC3 substrate, possessing a C-terminal degron motif. Protein stability assays and immunoblotting confirmed HINT1 as a direct target of KLHDC3. These findings establish a new in vivo physiological role for the DesCEND pathway and highlight KLHDC3 as a key regulator of development, survival, and adiposity in mice.

RECQL4 is a member of the RecQ family of helicases, playing essential roles in DNA replication and maintaining genome integrity. Mutations in RECQL4 are linked to severe human diseases, including Rothmund-Thomson Syndrome, RAPIDALINO Syndrome, and Baller-Gerold Syndrome. However, we still do not fully understand its functions and genetic interactions. The role of the ATP-dependent helicase activity in RECQL4 remains controversial. To understand RECQL4’s functions further, we conducted a genome-wide forward genetic screen using murine models that closely mimic the RECQL4 mutations found in patients with Rothmund-Thomson syndrome. Our goal was to identify loss-of-function alleles that could rescue the proliferation and viability defects associated with RECQL4 mutation. From our screening we identified the loss of KLHDC3, a substrate-binding subunit of the Cullin-RING ligase (CRL) E3, as the most significant rescue allele. KLHDC3 facilitates the ubiquitin-mediated destruction of proteins with specific C-terminal degron motifs. Its loss normalized cell proliferation and DNA replication rates in cells with mutated RECQL4. Further analysis revealed that the loss of KLHDC3 led to the stabilization of minute levels of a truncated RECQL4 protein. This RECQL4 fragment contained a neo-degron sequence specific for KLHDC3, formed after Cre-mediated recombination of the Recql4 ^fl allele. Although this rescue mechanism does not apply to human RECQL4 mutations, it shows that very low chromatin-bound levels of a truncated RECQL4 protein—comprising only the N-terminal 480 amino acids, including its Sld2-like domain but lacking the ATP-dependent helicase domain and the entire C-terminal portion—are sufficient to support DNA replication in mammalian cells. These results demonstrate that the ATPase activity and helicase domain of RECQL4 are not essential for DNA replication in mammals. Furthermore, our findings suggest that there are unlikely to be monogenic loss-of-function alleles that can rescue RECQL4 mutations. This demonstrates that RECQL4 is an essential and non-redundant regulator of DNA replication and cell viability and that this activity does not require the ATP dependent helicase activity.

Recurrent mutations in RNA splicing proteins and epigenetic regulators contribute to the development of myelodysplastic syndrome (MDS) and related myeloid neoplasms. In chronic myelomonocytic leukemia (CMML), SRSF2 mutations occur in ~50% of patients and TET2 mutations in ~60%. Clonal analysis indicates that either mutation can arise as the founder lesion. Based on human cancer genetics we crossed an inducible Srsf2^P95H/+ mutant model with Tet2^fl/fl mice to mutate both concomitantly in hematopoietic stem cells. At 20–24 weeks post mutation induction, we observed subtle differences in the Srsf2/Tet2 mutants compared to either single mutant. Under conditions of native hematopoiesis with aging, we see a distinct myeloid bias and monocytosis in the Srsf2/Tet2 mutants. A subset of the compound Srsf2/Tet2 mutants display an increased granulocytic and distinctive monocytic proliferation (myelomonocytic hyperplasia), with increased immature promonocytes and monoblasts and binucleate promonocytes. Exome analysis of progressed disease demonstrated mutations in genes and pathways similar to those reported in human CMML. Upon transplantation, recipients developed leukocytosis, monocytosis, and splenomegaly. We reproduce Srsf2/Tet2 co-operativity in vivo, yielding a disease with core characteristics of CMML, unlike single Srsf2 or Tet2 mutation. This model represents a significant step toward building high fidelity and genetically tractable models of CMML.

Recurrent mutations in two pathways - the RNA spliceosome (eg. SRSF2, SF3B1, U2AF1 ) and epigenetic regulators (eg. DNMT3, TET2 ) – contribute to the development of myelodysplastic syndrome (MDS) and related myeloid neoplasms. In chronic myelomonocytic leukemia (CMML), SRSF2 mutations occur in ∼50% of patients and TET2 mutations in ∼60%, representing two of the most frequent mutations in these cancers. Clonal analysis has indicated that either mutation can arise as the founder lesion, however, our understanding of the basis for the co-operativity of these mutations in the evolution of CMML is limited. Based on human cancer genetics we crossed an inducible Srsf2 ^P95H/+ mutant model with Tet2 ^fl/fl mice to mutate both concomitantly (or individually) in hematopoietic stem cells. At 20-24 weeks post gene mutation, we observed subtle differences in the Srsf2 / Tet2 mutants compared to either single mutant. Under conditions of native hematopoiesis with aging, we see a distinct myeloid bias and monocytosis in the Srsf2 / Tet2 mutants. A subset of the compound Srsf2 / Tet2 mutants display an increased granulocytic and distinctive monocytic proliferation (myelo-monocytic hyperplasia), with increased immature promonocytes and monoblasts (∼10-15% total nucleated cells), and evidence of binucleate promonocytes. Exome analysis of progressed disease demonstrates mutations in genes and pathways similar to those reported in human CMML. Upon transplantation, recipients developed leukocytosis, monocytosis and splenomegaly. This demonstrates we can reproduce Srsf2/Tet2 co-operativity in vivo , yielding a disease with core characteristics of CMML, unlike single Srsf2 or Tet2 mutation. This model represents a significant step toward building high fidelity and genetically tractable models of CMML. Key points: Srsf2 ^P95H/+ co-operates with Tet2 ^-/- to initiate CMML in a murine model Srsf2 ^P95H and Tet2 null mutations synergize in the development of monocytosis

Current strategies to target RNA splicing mutant myeloid cancers proposes targeting the remaining splicing apparatus. This approach has only been modestly sensitizing and is also toxic to non-mutant-bearing wild-type cells. To explore potentially exploitable genetic interactions with spliceosome mutations, we combined data mining and functional screening for synthetic lethal interactions with an Srsf2P95H/+ mutation. Analysis of missplicing events in a series of both human and murine SRSF2P95H mutant samples across multiple myeloid diseases (acute myeloid leukemia, myelodysplastic syndromes, chronic myelomonocytic leukemia) was performed to identify conserved missplicing events. From this analysis, we identified that the cell-cycle and DNA repair pathways were overrepresented within the conserved misspliced transcript sets. In parallel, to functionally define pathways essential for survival and proliferation of Srsf2P95H/+ cells, we performed a genome-wide Clustered regularly interspaced short palindromic repeat loss-of-function screen using Hoxb8 immortalized R26-CreERki/+Srsf2P95H/+ and R26-CreERki/+Srsf2+/+ cell lines. We assessed loss of single guide RNA representation at 3 timepoints: immediately after Srsf2P95H/+ activation, and at 1 week and 2 weeks after Srsf2P95H/+ mutation. Pathway analysis demonstrated that the cell-cycle and DNA damage response pathways were among the top synthetic lethal pathways with Srsf2P95H/+ mutation. Based on the loss of guide RNAs targeting Cdk6, we identified that palbociclib, a CDK6 inhibitor, showed preferential sensitivity in Srsf2P95H/+ cell lines and in primary nonimmortalized lin−cKIT+Sca-1+ cells compared with wild-type controls. Our data strongly suggest that the cell-cycle and DNA damage response pathways are required for Srsf2P95H/+ cell survival, and that palbociclib could be an alternative therapeutic option for targeting SRSF2 mutant cancers.

Rothmund-Thomson Syndrome (RTS) is an autosomal recessive disorder characterized by defects in the skeletal system such as bone hypoplasia, short stature, low bone mass, and an increased incidence of osteosarcoma. RTS type 2 patients have germline compound bi-allelic protein-truncating mutations of RECQL4. As existing murine models employ Recql4 null alleles, we have attempted to more accurately model RTS by generating mice with patient-mimicking truncating Recql4 mutations. Truncating mutations impaired the stability and subcellular localization of RECQL4, and resulted in homozygous embryonic lethality and a haploinsufficient low bone mass phenotype. Combination of a truncating mutation with a conditional Recql4 null allele demonstrated that the skeletal defects were intrinsic to the osteoblast lineage. However, the truncating mutations did not promote tumorigenesis. We utilized murine Recql4 null cells to assess the impact of human RECQL4 mutations using an in vitro complementation assay. While some mutations created unstable protein products, others altered subcellular localization of the protein. Interestingly, the severity of the phenotypes correlated with the extent of protein truncation. Collectively, our results reveal that truncating RECQL4 mutations in mice lead to an osteoporosis-like phenotype through defects in early osteoblast progenitors and identify RECQL4 gene dosage as a novel regulator of bone mass.

Rothmund-Thomson Syndrome (RTS) is an autosomal recessive disorder characterized by poikiloderma, sparse or absent hair, and defects in the skeletal system such as bone hypoplasia, short stature, low bone mass, and an increased incidence of osteosarcoma. RTS type 2 patients typically present with germline compound bi-allelic protein-truncating mutations of RECQL4 . As existing murine models predominantly employ Recql4 null alleles, we have here attempted to more accurately model the mutational spectrum of RTS by generating mice with patient-mimicking truncating Recql4 mutations. We found that truncating mutations impaired stability and subcellular localization of RECQL4, which translated to a homozygous embryonic lethality and haploinsufficient low bone mass and reduced cortical bone thickness phenotypes. Combination of a truncating mutation with a conditional Recql4 null allele demonstrated that these defects were intrinsic to the osteoblast lineage. However, the truncating mutations did not promote tumorigenesis, even after exposure to irradiation. We also utilized murine Recql4 null cells to assess the impact of a wider range of human RECQL4 mutations using an in vitro complementation assay. We found differential effects of distinct RECQL4 mutations. While some created unstable protein products, others altered subcellular localization of the protein. Interestingly, the severity of the phenotypes correlated with the extent of protein truncation. Collectively, our results reveal that truncating RECQL4 mutations lead to the development of an osteoporosis-like phenotype through defects in early osteoblast progenitors in mice and identify RECQL4 gene dosage as a novel regulator of bone mass.