Structure and synthesis

The NT-proBNP molecule traces its origins to a parent prohormone called proBNP, which is secreted by cardiomyocytes as a response to stress, cardiac pressure, or ventricular expansion ^{Reference Rørth, Jhund and Yilmaz25,Reference Rodeheffer26} Additional causes like myocardial ischaemia and endocrine or paracrine interference by other hormones and cytokines trigger the release of proBNP ^{Reference Weber and Hamm27} This glycosylated prohormone is comprised of 108 amino acids, and it is subsequently de-glycosylated and cleaved by the proNP convertases (corin and furin) into the inactive 76-amino acid NT-proBNP and an active 32-amino acid C-terminal brain natriuretic peptide. ^{Reference Kerkelä, Ulvila and Magga28,Reference Semenov, Tamm and Seferian29}

The human gene encoding brain natriuretic peptide is localised on the p arm of chromosome 1, and the messenger RNA encoding brain natriuretic peptide contains a repeat Thymine-Adenine-Thymine-Thymine-Thymine-Adenine-Thymine sequence that is considered unstable ^{Reference Arden, Viars and Weiss30,Reference Cao, Jia and Zhu31} The data suggest that brain natriuretic peptide is regulated by post-translational processing, meaning that its gene product can rapidly increase when under proper stimuli ^{Reference Maalouf and Bailey32,Reference Vodovar, Séronde and Laribi33} The transcription of brain natriuretic peptide messenger RNA and the synthesis and secretion of the brain natriuretic peptide protein take place in an eruptive manner, where they are quickly released into surrounding tissues instead of being stored in the normal physiological cardiac tissue ^{Reference Cao, Jia and Zhu31} In pathological conditions, this unstable messenger RNA synthesises a pre-proBNP precursor composed of 134 amino acids, which once released into circulation, is further split into a signal peptide made up of 26 amino acids, and the 108 amino acid proBNP, which subsequently yields NT-proBNP, as can be seen in Figure 1. ^{Reference Hadzović-Dzuvo, Kucukalić-Selimović and Nakas-Ićindić34-Reference Palazzuoli, Gallotta and Quatrini36} Reference Fu, Ping and Zhu ³⁷

Figure 1. The pathways of brain natriuretic peptide and NT-proBNP synthesis. Represented are the aetiologies of the increased synthesis of pre-proBNP and the subsequent cleavage of its signal sequence to yield a C-terminal proBNP that also gets cleaved into the inactive NT-proBNP measured by blood tests and bioactive brain natriuretic peptide that plays a crucial role in maintaining homeostasis by performing the functions listed above.

Degradation and clearance

The three known natriuretic peptide receptors in mammals are natriuretic peptide receptor-A, natriuretic peptide receptor-B, and natriuretic peptide receptor-C, and the first two represent two of the five transmembrane guanylyl cyclases found in humans ^{Reference Potter, Abbey-Hosch and Peptides40} Natriuretic peptide receptor-A is a receptor for both atrial natriuretic peptide and brain natriuretic peptide, and it is abundant in the vascular endothelial system, in addition to the brain, kidneys, adrenal glands, and lungs ^{Reference Maalouf and Bailey32,Reference Nagase, Katafuchi and Hirose41} Natriuretic peptide receptor-B is a receptor for the third member of the natriuretic peptide family, the C-type natriuretic peptide, which aids in lowering blood pressure and in preventing the development of atherogenesis and aneurysms ^{Reference Moyes and Chu42} The third receptor, natriuretic peptide receptor-C, is also known as the clearance receptor, and its primary function is to clear circulating natriuretic peptides via receptor-mediated internalisation and degradation ^{Reference Potter, Yoder and Flora43,Reference Pandey44,Reference Jansen, Mackasey and Moghtadaei45} Of note is that natriuretic peptides can also be cleared by neutral endopeptidase, insulin-degrading enzyme, dipeptidyl peptidase-4, or renal excretion. ^{Reference Fu, Ping and Wang46}

Since NT-proBNP is not an active natriuretic peptide, it cannot be cleared by natriuretic peptide receptor-C. Data have shown that there was no difference in plasma NT-proBNP levels between the aortic root and the peripheral veins, which means that this biomarker is not cleared in the systemic circulation ^{Reference Tsutamoto, Sakai and Yamamoto47} Studies have also shown that it is mainly cleared by the kidneys, ^{Reference Jafri, Kashif and Tai48–Reference Rosner50} and this means that renal dysfunction could potentially lead to the elevation of plasma NT-proBNP. In addition, studies have demonstrated that impaired glomerular filtration and renal clearance in diseases like chronic kidney disease can cause the accumulation of NT-proBNP ^{Reference Tsutamoto, Sakai and Yamamoto47,Reference Palmer and Richards51} However, in patients with mild renal impairment, there seems to be no significant correlation between the glomerular filtration rate and the plasma levels of NT-proBNP, ^{Reference Daniels and Maisel52} which opens the door for the possibility of other modes of clearance for NT-proBNP.

In the adult population

As for the studies regarding adult mortality, they all converged on the fact that the cohort of deceased persons had higher levels of NT-proBNP when compared to those who survived the disease, as listed in Table 2. NT-proBNP levels are determined by a variety of different immunoassays that employ antibodies directed to distinct epitopes, and such assays notably include the Elecsys proBNP II and the Superflex NT-proBNP assays. ^{Reference Li, Semenov and Feygina99} A study by Ferrari et al. depicted the highest mean (6,295.8 pg/ml) recorded in the deceased group, with an equally high standard deviation (17,527.6). ^{Reference Ferrari, Seveso and Sabetta100} Almost all of the remaining studies found mean/median NT-proBNP levels in the order of thousands in the deceased group, but not one of them found this result in the non-deceased cohort, which fortifies the association between NT-proBNP levels and mortality post-COVID-19 incidence. Furthermore, five studies ^{Reference D’Alto, Marra and Severino101–Reference Zhang, Dong and Li105} administered an echocardiography test, and some of the parameters assessed were left ventricular ejection fraction, tricuspid annulus plane systolic excursion, pulmonary artery systolic pressure, right ventricular dilation, and several left ventricle variables.

Table 2. Study characteristics and baseline NT-proBNP levels in adult alive versus deceased COVID-19 cohorts

A = alive; D = deceased; CK = creatine kinase; CK-MB = creatine kinase isoenzyme-MB; MYO = myoglobin; cTnI = cardiac troponin I; LDH = lactate dehydrogenase; Hs-TnT = high-sensitivity troponin T; Hs-TnI = high-sensitivity troponin I. Data are reported as N, or median (interquartile ranges), or mean ± standard deviation when appropriate.

In the study by D’Alto et al., ^{Reference D’Alto, Marra and Severino101} early and pronounced right ventricular arterial uncoupling was revealed by trans-thoracic echocardiography. The tricuspid annulus plane systolic excursion/pulmonary artery systolic pressure ratio also aided in clarifying the prognostic relevance of an indicator for lung severity (arterial partial pressure of oxygen/fraction of inspired O2). In the same study, NT-proBNP levels were significantly increased in the deceased cohort when compared to the survivors, as per Table 2. In another study by Rath et al., ^{Reference Rath, Petersen-Uribe and Avdiu102} trans-thoracic echocardiography revealed a significantly better left ventricular function in the survivors when compared to the non-survivors, and the NT-proBNP level was significantly lower in the survivors. As for Liu et al. ^{Reference Liu, Xie and Gao103} and Sun et al., ^{Reference Sun, Ning and Tao104} a significant difference arose when comparing tricuspid annulus plane systolic excursion values between the alive and dead cohorts. The same applies when it comes to the NT-proBNP levels. We can therefore notice that there are significant correlations between some cardiac parameters (obtained from echocardiography) and survivorship. Similarly, significant correlations were evident when comparing NT-proBNP levels between the two groups. Therefore, direct associations between NT-proBNP and some echocardiographic parameters should be assessed for future research.

In the paediatric population

Until now, the NT-proBNP studies discussed were mainly focused on adult severe versus non-severe COVID-19 cohorts and deceased versus non-deceased COVID-19 cohorts. However, the literature also depicts novel findings about the value of NT-proBNP in the paediatric population affected by COVID-19, as shown in Table 3. In most studies dealing with paediatric COVID-19 patients, the main comparison groups established were Multisystem Inflammatory Syndrome in Children versus non-CMultisystem Inflammatory Syndrome in Children patients. In short, Multisystem Inflammatory Syndrome in Children, sometimes referred to as Pediatric Inflammatory Multisystem Syndrome is a complication that can follow a COVID-19 infection. ^{Reference Henderson and Yeung80} Clinically, it appears as a severe condition with specific features such as the inflammation of several systems. ^{Reference Henderson and Yeung80} However, cardiac dysfunctions are more commonly observed in Multisystem Inflammatory Syndrome in Children patients. ^{Reference Henderson and Yeung80,Reference Wu and Campbell106} A study by Wu and Campbell ^{Reference Wu and Campbell106} indicates that cardiac manifestations may be present in up to 80% of paediatric patients diagnosed with Multisystem Inflammatory Syndrome in Children. Based on multiple sources of the literature, increased NT-proBNP concentrations are indeed associated with disease severity. In fact, most of the studies showed that children in the Multisystem Inflammatory Syndrome in Children group had significantly greater levels of circulating NT-proBNP compared to those in the non-Multisystem Inflammatory Syndrome in Children cohort (Table 3). A retrospective study led by Abrams et al. ^{Reference Abrams, Oster and Godfred-Cato78} further strengthens the discussed correlation, given that they divided 1,080 Multisystem Inflammatory Syndrome in Children patients into three groups: patients admitted to the ICU (on the same day of hospitalisation versus days after hospitalisation) and patients not admitted to the ICU. The median NT-proBNP level of the first cohort (admitted to the ICU on the same day) was 4,796 pg/ml, significantly greater than that of the third group (patients not admitted to the ICU) which was 558 pg/ml. ^{Reference Abrams, Oster and Godfred-Cato78} These results are crucial, as they indicate that patients requiring critical care during hospitalisation had great elevations of NT-proBNP, where the median was in the order of thousands. Other studies in Table 3 also point to similar findings, where Multisystem Inflammatory Syndrome in Children patients requiring intensive care had higher levels of NT-proBNP than those who did not (p < 0.05). ^{Reference Corwin, Sartori and Chiotos107–Reference Abdel-Haq, Asmar and Leon110} It is important to note that Multisystem Inflammatory Syndrome in Children must be temporally associated with a Severe Acute Respiratory Syndrome Coronavirus 2 infection and is considered a post-infectious condition. ^{Reference Henderson and Yeung80} One study showed that children diagnosed with Multisystem Inflammatory Syndrome in Children without evidence of a Severe Acute Respiratory Syndrome Coronavirus 2 infection had significantly lower levels of NT-proBNP compared to those with evidence of infection (via a positive Severe Acute Respiratory Syndrome Coronavirus 2 polymerase chain reaction or immunoglobulin G serology results), with respective NT-proBNP concentration medians of eleven pg/ml and 1,140 pg/ml. ^{Reference Whittaker, Bamford and Kenny111}

Table 3. Study characteristics and major NT-proBNP findings in paediatric patients

Further, elevated circulating concentrations of NT-proBNP were also associated with a spectrum of cardiac manifestations in the paediatric COVID-19 and Multisystem Inflammatory Syndrome in Children patients. Children with elevated cardiac biomarkers (troponin, NT-proBNP, and/or Creatine Kinase-MB) had higher chances of being diagnosed with myocarditis, coronary artery aneurysm, and other cardiac injuries. ^{Reference Pouletty, Borocco and Ouldali81,Reference Whittaker, Bamford and Kenny111} In fact, most studies in Table 3 suggest probing for cardiac injury upon testing high levels of NT-proBNP in a paediatric Multisystem Inflammatory Syndrome in Children or COVID-19 patient. Interestingly, the study by Abrams et al. ^{Reference Abrams, Oster and Godfred-Cato78} studied the association between clinical cardiac manifestations and different markers tested (fibrinogen, D-dimer, troponin, brain natriuretic peptide, NT-proBNP, C-reactive protein, and others). Odds ratio analyses showed that elevated concentrations of brain natriuretic peptide and NT-proBNP were most closely correlated with decreased cardiac function and myocarditis. Moreover, NT-proBNP and interleukin-6 were the only two markers correlated with coronary artery abnormalities, and not troponin or brain natriuretic peptide. ^{Reference Abrams, Oster and Godfred-Cato78} Although NT-proBNP and brain natriuretic peptide are both robust monitors of cardiac function, NT-proBNP seems to be more sensitive in detecting heart failure. Indeed, in a 2022 French study on paediatric patients with positive Severe Acute Respiratory Syndrome Coronavirus 2 tests, Raynor et al. ^{Reference Raynor, Vallée and Belkarfa83} suggest that due to kinetic differences between the two molecules, NT-proBNP is to be favoured over brain natriuretic peptide in detecting heart failure, at least in Multisystem Inflammatory Syndrome in Children patients.

In most studies presented in Table 2, patients were retrospectively grouped on an outcome-related basis, generally between mild and severe cases, and ICU admission instances were considered moderate-to-severe. The parameter of interest, NT-proBNP, marked some interesting findings, where in almost all of the studies, its levels were consistently higher in accordance with severity. This means that for the most part, the more severe the health status of a cohort, be it a Multisystem Inflammatory Syndrome in Children, Kawasaki Disease, or any other cohort, the higher the NT-proBNP levels. Most studies did not report the actual change of NT-proBNP levels in patients over the course of the disease, but only retrospectively noted its value upon admission and compared it to the severity of the illness. However, according to the main findings, we can only conjecture that clinically, a sudden increase in NT-proBNP levels has to be correlated to greater severity of the cardiac manifestation in question, as the data suggest.

Unlike the case of adults, there is limited information pertaining to paediatric NT-proBNP COVID-19 cohorts, and therefore more studies need to be published. Also, additional biomarkers like troponin should be investigated, and more studies on paediatric COVID-19 and/or Multisystem Inflammatory Syndrome in Children populations should be carried out.