key: cord-342396-n3txsvf7 authors: Ciaglia, Elena; Vecchione, Carmine; Puca, Annibale Alessandro title: COVID-19 Infection and Circulating ACE2 Levels: Protective Role in Women and Children date: 2020-04-23 journal: Front Pediatr DOI: 10.3389/fped.2020.00206 sha: doc_id: 342396 cord_uid: n3txsvf7 nan that up to 67% of the phenotypic variation in circulating ACE2 could be accounted for by genetic factors (12) . Among different polymorphisms, it has been speculated that ACE2 rs2106809 might exhibit primary effects on the ACE2 levels. The circulating ACE2 levels tend to be greater in CC or CT genotype compared with that in the TT genotype. One possible mechanism can be mediated by microRNA, which could modulate endothelial function via translational repression and/or posttranscriptional degradation. Furthermore, several significant differences in the frequency of distribution of ACE2 variants among different racial and ethnic lines have been described. A recent single-cell RNAsequencing (RNA-seq) analysis indicated that Asian males may have a higher expression of tissue ACE2 (13) . In another case control study conducted in the north eastern Chinese Han population, the serum ACE2 activity negatively correlated with body mass index (BMI), pulse pressure, and estrogen levels in female EH (essential hypertension) patients (14) . These observations point both to a cardiovascular protective effect of circulating levels of ACE2 and simultaneously prove that estrogens participate in the upregulation of ACE2 expression and activity levels (15) . This might explain the relative protection of female vs. male in COVID-19 infection. Taken together, this evidence seem to indicate that the putative sex predisposition to COVID-19, with men being more susceptible, might be reflective of a peculiar ACE plasma profile. A putative trend toward this kind of association was also seen in children. Children generally have higher levels of ACE2 than adults (16) . For example, ACE levels in children (6 months to 17 years of age) are 13-100 U/l compared with 9-67 U/l in adults when using an FAPGG-based enzymatic activity assay. Of note is the fact that children with confirmed COVID-19 have generally presented with mild symptoms. Cases of coronavirus disease 2019 (COVID-19) among children in China have been less severe than those in adults, according to a new study. In a study of 1,099 patients in China, just 0.9 percent of the confirmed cases were under the age of nine, while only 1.2 percent were between 10 and 19 years old (17) . A similar phenomenon in a mouse study in North Carolina was registered by Baric et al.-although SARS-CoV can replicate fairly well, younger animals are really resistant to infection in terms of the disease. When older animals were tested, the severity of SARS illnesses rose (18) . In our opinion, the explanation for the correlation between age and COVID-19 disease severity might be related not only to the immune decline of an aged immune system (termed immunesenescence) but also to a peculiar ACE plasma profile that may characterize children from birth. Indeed in mid to late pregnancy in women, an increase in urine and plasma levels of ACE2 were found as well as an increase in local placental/uterine production and activity of ACE2, suggesting a systemic hemodynamic role in the enhancement of placentalfetal blood flow and rapid fetal growth (19) . ACE can pass through the placenta, enabling the mother to transfer to baby her immunity and other kinds of protective soluble factors. Epidemiological characteristics and transmission patterns of pediatric patients with COVID-19 in China revealed that, contrary to adults, there was no significant gender difference in young patients (20) ; this is probably due to the influence of the degree of sexual maturation in children and adolescents. Indeed, not only estradiol, via the ER, is a known modulator of the ACE/ACE2 and AT1/AT2 receptor, but ACE is also connected to male reproduction. Catalytic activity of testis ACE contains only the carboxy-terminal domain of ACE, which has exhibited unknown effects on a substrate other than angiotensin I (21) . The reason why the disease is less robust in extremely young animals or humans than in older ones may therefore lie not only in some "cross-immunity" offered by previous infection to "common cold" viruses experienced by children, nor does it lie exclusively in a powerful immune system that, as a result, is not affected by the senescence process; it is probably also affected by an unique ACE2 plasma profile that need to be dissected. By a buffering effect, and much like neutralizing antibodies, soluble ACE2 may help children and asymptomatic people to better counteract virus spreading to a cell target. On one hand, this could help them to contain infection. On the other hand, this could also let these carriers be an important reservoir of circulating virus, and so this deserve much of our attention in the near future. Answering questions about coronavirus in children and in people who develop less severe symptoms could reverberate well-beyond this escaper population. It could shed light on the reasons why some patients are most at risk and why others could better counteract the spreading of the virus. Furthermore, studying the physiology of those who are less affected could be of help in the development of treatment and a vaccine. In the last years, the ACE2 activity level has been a potential biomarker for the variations of blood pressure, providing useful information for the prediction and prevention of cardiac dysfunction. Now, circulating level of ACE2 may have prognostic effect in monitoring COVID-infection, and the genetic analysis of ACE2 polymorphisms might be a key element of individualized care for its prevention, diagnosis, and treatment. In this context, an ELISA-based accurate quantification of human soluble ACE2, not only in serum and EDTA plasma but also in more accessible human body fluids (e.g., saliva, urine, tears, and milk), should be proposed as a first rapid test screening. To be noted, a standardized protocol for sampling, transport, and storage before its dosage, must be rigorously followed to ensure the accuracy and reliability of inter-and intra-individual quantitation during the course of pathology. Furthermore, correct tests should be carried out in simultaneously in aged-matched healthy volunteers for comparisons. If the current hypothesis is correct, ACE2 determination, by both ELISA and more sensitive HPLC-MS methods, may represent a less extensive and time-consuming means to monitor COVID-19 infection both at pre-clinical and clinical levels. With the rapid progress that has been made with diagnostic reagents (e.g., nucleic acid and IgM or IgG detection or both), drug repurposing (e.g., remdesivir and chloroquine), immunotherapeutic approaches (e.g., Tocilizumab), and vaccine production as a consequence of the outbreak of novel COVID-19, we thought that it is timely to shed light on the putative link between circulating ACE2 and disease severity. Indeed, as discussed, it may represent a rapidly emerging field of study for therapeutic intervention in the context of COVID-19 infection. Concerning this, as Penniger JM and colleagues declared in the last days, the availability of recombinant ACE2 (rhACE2; APN01, GSK2586881), its safety profile, and the anti-inflammatory effects (mainly linked to its ability to reduce IL-6 plasma levels) will be the impetus to rapidly launch a pilot trial of rhACE2 as a hopeful treatment option for patients with severe COVID-19 (clinical trials.gov#NCT04287686). 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This work was supported by: research funding from Cariplo Foundation (no. 2016-0874) to AP and CV; PRIN-20157ATSLF_009 to AP and CV; EC was supported by a fellowship from Fondazione Umberto Veronesi (FUV 2019cod.2198). Funding/financial support was obtained also from the Italian Ministry of Health, RicercaCorrente to the IRCCS MultiMedica. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.Copyright © 2020 Ciaglia, Vecchione and Puca. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. 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