The Gremlin1 cis-regulatory landscape: a paradigm to study enhancer cooperation in regulation of transcription dynamics

The transcriptional regulation of developmental genes expression patterns in time, space and levels, is governed by cis-regulatory modules (CRMs). The activity of CRMs is controlled by transcription factor complexes that act as downstream mediators of signaling inputs. CRMs are associated with their target genes in chromatin domains with enhanced contact frequency, the so-called topologically associating domains (TADs). The incoming signaling cues are integrated into specific transcriptional outputs, which orchestrate development and differentiation. Limb bud development is one of the main molecular and cellular paradigms to study the roles of gene expression regulation during embryonic development. Limbs are external organs, easily accessible, largely dispensable for embryonic and postnatal survival and have adapted to numerous specific functions during vertebrate evolution, resulting in the high level of morphological diversity among vertebrates. The molecular pathways and morphogenetic events that govern limb patterning are largely conserved, reflecting their crucial roles in gene regulation during limb development. Our group previously identified and functionally analyzed the SHH/GREM1/AER-FGF epithelial-mesenchymal (e-m) self-regulatory signaling system that controls early limb bud outgrowth and patterning. The BMP antagonist Gremlin1 (Grem1) is one of the functionally most essential nodes in this system. Its spatio-temporal expression is regulated by the converging trans-acting inputs of the major limb bud signaling pathways. These inputs are integrated into the dynamic regulation of Grem1 expression by its 310 kb cis-regulatory landscape. For my Ph.D. research, I used the mouse Grem1 cis-regulatory landscape as a paradigm to study gene transcriptional regulation in the context of embryonic limb bud development. I identified and genetically analyzed the functionally relevant Grem1-associated CRMs. To this end, I initially used reporter assays in transgenic mouse embryos to assess their potential enhancer activity. CRMs with established enhancer activities were then functionally studied by generating CRISPR/Cas9-engineered loss of enhancer function mutant mice. This, in combination with molecular analysis, was used to assess their role(s) in the Grem1 transcriptional regulation. In addition, I used 4C-seq assays to study the physical interactions among CRMs and the Grem1 promoter, in wild-type and mutant mouse limb buds. I also addressed the question of the downstream consequences of enhancer deletions on limb bud development by tracking apoptosis and quantifying limb buds’ cellular proliferation. My studies revealed that the enhancer redundancy and diversity that regulates the Grem1 expression dynamics during mouse limb bud development was much more complex than the one-to-one correlation often described by others. None of the CRMs characterized was essential on its own for limb development. The transcriptional activities of different CRMs were additive in levels and partially redundant in regulating the spatial and temporal dynamics of the Grem1 expression. The spatio-temporal changes in Grem1 expression levels, caused by the loss of different enhancers alone, were not sufficient to explain the observed phenotypes. Therefore, additional mouse strains lacking several CRMs were generated and analyzed. In light of these results, I performed a comparative molecular analysis of key genes in the self-regulatory SHH/GREM1/AER-FGF signaling system, which provided a better molecular understanding of how these cis-regulatory alterations affect the limb bud outgrowth and patterning. This analysis showed that the cis-regulatory alterations affecting levels and spatio-temporal kinetics of the Grem1 expression are accompanied by specific changes in the self-regulatory feedback loops in mutant limb buds. In addition, I investigated potential effects on the structure of the Grem1 TAD and revealed that alterations in the interactions among CRMs and the Grem1 promoter contributed to the transcriptional regulation of Grem1 expression. This extensive genetic analysis led to the following major conclusion: the control of transcript levels by the Grem1-associated CRMs is additive, while they function in a cooperative manner to regulate the spatial dynamics of the Grem1 expression in mouse limb buds. In particular, deleting several of the CRMs that regulate spatial aspects of the Grem1 expression disrupts this cooperativity. This, in turn, weakens the robustness of the limb patterning system and results in the loss of pentadactyly. It appears that the observed limb skeletal deformity phenotypes strongly correlate with reduced cell proliferation. Structural analyses reveal that intra-TAD rearrangements play a major role in the robustness of the Grem1 expression

Spatial regulation by multiple Gremlin1 enhancers provides digit development with cis-regulatory robustness and evolutionary plasticity.

Precise cis-regulatory control of gene expression is essential for normal embryogenesis and tissue development. The BMP antagonist Gremlin1 (Grem1) is a key node in the signalling system that coordinately controls limb bud development. Here, we use mouse reverse genetics to identify the enhancers in the Grem1 genomic landscape and the underlying cis-regulatory logics that orchestrate the spatio-temporal Grem1 expression dynamics during limb bud development. We establish that transcript levels are controlled in an additive manner while spatial regulation requires synergistic interactions among multiple enhancers. Disrupting these interactions shows that altered spatial regulation rather than reduced Grem1 transcript levels prefigures digit fusions and loss. Two of the enhancers are evolutionary ancient and highly conserved from basal fishes to mammals. Analysing these enhancers from different species reveal the substantial spatial plasticity in Grem1 regulation in tetrapods and basal fishes, which provides insights into the fin-to-limb transition and evolutionary diversification of pentadactyl limbs

SMAD4 target genes are part of a transcriptional network that integrates the response to BMP and SHH signaling during early limb bud patterning

ABSTRACT SMAD4 regulates gene expression in response to BMP and TGFβ signal transduction and is required for diverse morphogenetic processes, but its target genes have remained largely elusive. Here, we use an epitope-tagged Smad4 allele for ChIP-seq analysis together with transcriptome analysis of wild-type and mouse forelimb buds lacking Smad4 in the mesenchyme. This analysis identifies the SMAD4 target genes during establishment of the feedback signaling system and establishes that SMAD4 predominantly mediates BMP signal-transduction during early limb bud development. Unexpectedly, the initial analysis reveals that the expression of cholesterol biosynthesis enzymes is precociously down-regulated and intracellular cholesterol levels reduced in Smad4 -deficient limb bud mesenchymal progenitors. The SMAD4 target GRNs includes genes, whose expression in the anterior limb bud is up-regulated by interactions of SMAD4 complexes with enhancers active in the anterior mesenchyme. This reveals a predominant function of SMAD4 in up-regulating target gene expression in the anterior limb bud mesenchyme. Analysis of differentially expressed genes that are shared between Smad4 - and Shh -deficient limb buds corroborates the positive role of SMAD4 in transcriptional regulation of anterior genes and reveals a repressive effect on posterior genes that are positively regulated by SHH signaling. This analysis uncovers the overall opposing effects of SMAD4-mediated BMP and SHH signalling on transcriptional regulation during early limb bud development. In summary, this analysis indicates that during early digit patterning and limb bud outgrowth, the anterior/proximal and proximo/distal expression dynamics of co-regulated genes are controlled by distinct and contrasting trans-regulatory inputs from SHH and SMAD4-mediated BMP signal transduction

SMAD4 target genes are part of a transcriptional network that integrates the response to BMP and SHH signaling during early limb bud patterning.

SMAD4 regulates gene expression in response to BMP and TGFβ signal transduction, and is required for diverse morphogenetic processes, but its target genes have remained largely elusive. Here, we identify the SMAD4 target genes in mouse limb buds using an epitope-tagged Smad4 allele for ChIP-seq analysis in combination with transcription profiling. This analysis shows that SMAD4 predominantly mediates BMP signal transduction during early limb bud development. Unexpectedly, the expression of cholesterol biosynthesis enzymes is precociously downregulated and intracellular cholesterol levels are reduced in Smad4-deficient limb bud mesenchymal progenitors. Most importantly, our analysis reveals a predominant function of SMAD4 in upregulating target genes in the anterior limb bud mesenchyme. Analysis of differentially expressed genes shared between Smad4- and Shh-deficient limb buds corroborates this function of SMAD4 and also reveals the repressive effect of SMAD4 on posterior genes that are upregulated in response to SHH signaling. This analysis uncovers opposing trans-regulatory inputs from SHH- and SMAD4-mediated BMP signal transduction on anterior and posterior gene expression during the digit patterning and outgrowth in early limb buds

Spatial regulation by multiple Gremlin1 enhancers provides digit development with cis-regulatory robustness and evolutionary plasticity

Precise cis-regulatory control of gene expression is essential for normal embryogenesis and tissue development. The BMP antagonist Gremlin1 (Grem1) is a key node in the signalling system that coordinately controls limb bud development. Here, we use mouse reverse genetics to identify the enhancers in the Grem1 genomic landscape and the underlying cis-regulatory logics that orchestrate the spatio-temporal Grem1 expression dynamics during limb bud development. We establish that transcript levels are controlled in an additive manner while spatial regulation requires synergistic interactions among multiple enhancers. Disrupting these interactions shows that altered spatial regulation rather than reduced Grem1 transcript levels prefigures digit fusions and loss. Two of the enhancers are evolutionary ancient and highly conserved from basal fishes to mammals. Analysing these enhancers from different species reveal the substantial spatial plasticity in Grem1 regulation in tetrapods and basal fishes, which provides insights into the fin-to-limb transition and evolutionary diversification of pentadactyl limbs.This research was initiated with support from the Bonus-of-Excellence SNF grant 310030B_166685 (to A.Z. and R.Z.) and then supported by the ERC advanced grant INTEGRAL ERC-2015-AdG; Project ID 695032 (to R.Z.) and the University of Basel provided core funding (to A.Z. and R.Z.). Additional funding support was provided by the National Institutes of Health grant R01 GM124251 (to K.A.P.). The research of J.L.R. is supported by MICINN grants BFU2017-82974-P and MDM-2016-0687. K.O. is supported by the Special Postdoctoral Researcher Program of RIKEN

Paediatric COVID-19 mortality: a database analysis of the impact of health resource disparity

Background The impact of the COVID-19 pandemic on paediatric populations varied between high-income countries (HICs) versus low-income to middle-income countries (LMICs). We sought to investigate differences in paediatric clinical outcomes and identify factors contributing to disparity between countries.Methods The International Severe Acute Respiratory and Emerging Infections Consortium (ISARIC) COVID-19 database was queried to include children under 19 years of age admitted to hospital from January 2020 to April 2021 with suspected or confirmed COVID-19 diagnosis. Univariate and multivariable analysis of contributing factors for mortality were assessed by country group (HICs vs LMICs) as defined by the World Bank criteria.Results A total of 12 860 children (3819 from 21 HICs and 9041 from 15 LMICs) participated in this study. Of these, 8961 were laboratory-confirmed and 3899 suspected COVID-19 cases. About 52% of LMICs children were black, and more than 40% were infants and adolescent. Overall in-hospital mortality rate (95% CI) was 3.3% [=(3.0% to 3.6%), higher in LMICs than HICs (4.0% (3.6% to 4.4%) and 1.7% (1.3% to 2.1%), respectively). There were significant differences between country income groups in intervention profile, with higher use of antibiotics, antivirals, corticosteroids, prone positioning, high flow nasal cannula, non-invasive and invasive mechanical ventilation in HICs. Out of the 439 mechanically ventilated children, mortality occurred in 106 (24.1%) subjects, which was higher in LMICs than HICs (89 (43.6%) vs 17 (7.2%) respectively). Pre-existing infectious comorbidities (tuberculosis and HIV) and some complications (bacterial pneumonia, acute respiratory distress syndrome and myocarditis) were significantly higher in LMICs compared with HICs. On multivariable analysis, LMIC as country income group was associated with increased risk of mortality (adjusted HR 4.73 (3.16 to 7.10)).Conclusion Mortality and morbidities were higher in LMICs than HICs, and it may be attributable to differences in patient demographics, complications and access to supportive and treatment modalities

Respiratory support in patients with severe COVID-19 in the International Severe Acute Respiratory and Emerging Infection (ISARIC) COVID-19 study: a prospective, multinational, observational study

Background: Up to 30% of hospitalised patients with COVID-19 require advanced respiratory support, including high-flow nasal cannulas (HFNC), non-invasive mechanical ventilation (NIV), or invasive mechanical ventilation (IMV). We aimed to describe the clinical characteristics, outcomes and risk factors for failing non-invasive respiratory support in patients treated with severe COVID-19 during the first two years of the pandemic in high-income countries (HICs) and low middle-income countries (LMICs). Methods: This is a multinational, multicentre, prospective cohort study embedded in the ISARIC-WHO COVID-19 Clinical Characterisation Protocol. Patients with laboratory-confirmed SARS-CoV-2 infection who required hospital admission were recruited prospectively. Patients treated with HFNC, NIV, or IMV within the first 24 h of hospital admission were included in this study. Descriptive statistics, random forest, and logistic regression analyses were used to describe clinical characteristics and compare clinical outcomes among patients treated with the different types of advanced respiratory support. Results: A total of 66,565 patients were included in this study. Overall, 82.6% of patients were treated in HIC, and 40.6% were admitted to the hospital during the first pandemic wave. During the first 24 h after hospital admission, patients in HICs were more frequently treated with HFNC (48.0%), followed by NIV (38.6%) and IMV (13.4%). In contrast, patients admitted in lower- and middle-income countries (LMICs) were less frequently treated with HFNC (16.1%) and the majority received IMV (59.1%). The failure rate of non-invasive respiratory support (i.e. HFNC or NIV) was 15.5%, of which 71.2% were from HIC and 28.8% from LMIC. The variables most strongly associated with non-invasive ventilation failure, defined as progression to IMV, were high leukocyte counts at hospital admission (OR [95%CI]; 5.86 [4.83-7.10]), treatment in an LMIC (OR [95%CI]; 2.04 [1.97-2.11]), and tachypnoea at hospital admission (OR [95%CI]; 1.16 [1.14-1.18]). Patients who failed HFNC/NIV had a higher 28-day fatality ratio (OR [95%CI]; 1.27 [1.25-1.30]). Conclusions: In the present international cohort, the most frequently used advanced respiratory support was the HFNC. However, IMV was used more often in LMIC. Higher leucocyte count, tachypnoea, and treatment in LMIC were risk factors for HFNC/NIV failure. HFNC/NIV failure was related to worse clinical outcomes, such as 28-day mortality. Trial registration This is a prospective observational study; therefore, no health care interventions were applied to participants, and trial registration is not applicable

Respiratory support in patients with severe COVID-19 in the International Severe Acute Respiratory and Emerging Infection (ISARIC) COVID-19 study: a prospective, multinational, observational study

Background: Up to 30% of hospitalised patients with COVID-19 require advanced respiratory support, including high-flow nasal cannulas (HFNC), non-invasive mechanical ventilation (NIV), or invasive mechanical ventilation (IMV). We aimed to describe the clinical characteristics, outcomes and risk factors for failing non-invasive respiratory support in patients treated with severe COVID-19 during the first two years of the pandemic in high-income countries (HICs) and low middle-income countries (LMICs). Methods: This is a multinational, multicentre, prospective cohort study embedded in the ISARIC-WHO COVID-19 Clinical Characterisation Protocol. Patients with laboratory-confirmed SARS-CoV-2 infection who required hospital admission were recruited prospectively. Patients treated with HFNC, NIV, or IMV within the first 24 h of hospital admission were included in this study. Descriptive statistics, random forest, and logistic regression analyses were used to describe clinical characteristics and compare clinical outcomes among patients treated with the different types of advanced respiratory support. Results: A total of 66,565 patients were included in this study. Overall, 82.6% of patients were treated in HIC, and 40.6% were admitted to the hospital during the first pandemic wave. During the first 24 h after hospital admission, patients in HICs were more frequently treated with HFNC (48.0%), followed by NIV (38.6%) and IMV (13.4%). In contrast, patients admitted in lower- and middle-income countries (LMICs) were less frequently treated with HFNC (16.1%) and the majority received IMV (59.1%). The failure rate of non-invasive respiratory support (i.e. HFNC or NIV) was 15.5%, of which 71.2% were from HIC and 28.8% from LMIC. The variables most strongly associated with non-invasive ventilation failure, defined as progression to IMV, were high leukocyte counts at hospital admission (OR [95%CI]; 5.86 [4.83–7.10]), treatment in an LMIC (OR [95%CI]; 2.04 [1.97–2.11]), and tachypnoea at hospital admission (OR [95%CI]; 1.16 [1.14–1.18]). Patients who failed HFNC/NIV had a higher 28-day fatality ratio (OR [95%CI]; 1.27 [1.25–1.30]). Conclusions: In the present international cohort, the most frequently used advanced respiratory support was the HFNC. However, IMV was used more often in LMIC. Higher leucocyte count, tachypnoea, and treatment in LMIC were risk factors for HFNC/NIV failure. HFNC/NIV failure was related to worse clinical outcomes, such as 28-day mortality. Trial registration This is a prospective observational study; therefore, no health care interventions were applied to participants, and trial registration is not applicable

Thrombotic and hemorrhagic complications of COVID-19 in adults hospitalized in high-income countries compared with those in adults hospitalized in low- and middle-income countries in an international registry

Background: COVID-19 has been associated with a broad range of thromboembolic, ischemic, and hemorrhagic complications (coagulopathy complications). Most studies have focused on patients with severe disease from high-income countries (HICs). Objectives: The main aims were to compare the frequency of coagulopathy complications in developing countries (low- and middle-income countries [LMICs]) with those in HICs, delineate the frequency across a range of treatment levels, and determine associations with in-hospital mortality. Methods: Adult patients enrolled in an observational, multinational registry, the International Severe Acute Respiratory and Emerging Infections COVID-19 study, between January 1, 2020, and September 15, 2021, met inclusion criteria, including admission to a hospital for laboratory-confirmed, acute COVID-19 and data on complications and survival. The advanced-treatment cohort received care, such as admission to the intensive care unit, mechanical ventilation, or inotropes or vasopressors; the basic-treatment cohort did not receive any of these interventions. Results: The study population included 495,682 patients from 52 countries, with 63% from LMICs and 85% in the basic treatment cohort. The frequency of coagulopathy complications was higher in HICs (0.76%-3.4%) than in LMICs (0.09%-1.22%). Complications were more frequent in the advanced-treatment cohort than in the basic-treatment cohort. Coagulopathy complications were associated with increased in-hospital mortality (odds ratio, 1.58; 95% CI, 1.52-1.64). The increased mortality associated with these complications was higher in LMICs (58.5%) than in HICs (35.4%). After controlling for coagulopathy complications, treatment intensity, and multiple other factors, the mortality was higher among patients in LMICs than among patients in HICs (odds ratio, 1.45; 95% CI, 1.39-1.51). Conclusion: In a large, international registry of patients hospitalized for COVID-19, coagulopathy complications were more frequent in HICs than in LMICs (developing countries). Increased mortality associated with coagulopathy complications was of a greater magnitude among patients in LMICs. Additional research is needed regarding timely diagnosis of and intervention for coagulation derangements associated with COVID-19, particularly for limited-resource settings

Characteristics and outcomes of an international cohort of 600 000 hospitalized patients with COVID-19

Background: We describe demographic features, treatments and clinical outcomes in the International Severe Acute Respiratory and emerging Infection Consortium (ISARIC) COVID-19 cohort, one of the world's largest international, standardized data sets concerning hospitalized patients. Methods: The data set analysed includes COVID-19 patients hospitalized between January 2020 and January 2022 in 52 countries. We investigated how symptoms on admission, co-morbidities, risk factors and treatments varied by age, sex and other characteristics. We used Cox regression models to investigate associations between demographics, symptoms, co-morbidities and other factors with risk of death, admission to an intensive care unit (ICU) and invasive mechanical ventilation (IMV). Results: Data were available for 689 572 patients with laboratory-confirmed (91.1%) or clinically diagnosed (8.9%) SARS-CoV-2 infection from 52 countries. Age [adjusted hazard ratio per 10 years 1.49 (95% CI 1.48, 1.49)] and male sex [1.23 (1.21, 1.24)] were associated with a higher risk of death. Rates of admission to an ICU and use of IMV increased with age up to age 60 years then dropped. Symptoms, co-morbidities and treatments varied by age and had varied associations with clinical outcomes. The case-fatality ratio varied by country partly due to differences in the clinical characteristics of recruited patients and was on average 21.5%. Conclusions: Age was the strongest determinant of risk of death, with a ∼30-fold difference between the oldest and youngest groups; each of the co-morbidities included was associated with up to an almost 2-fold increase in risk. Smoking and obesity were also associated with a higher risk of death. The size of our international database and the standardized data collection method make this study a comprehensive international description of COVID-19 clinical features. Our findings may inform strategies that involve prioritization of patients hospitalized with COVID-19 who have a higher risk of death