The relationship between glycosylation of proBNP at threonine 71, BNP, BNP1–32 and obesity in patients with heart failure
Authors: Lewis L.K , Raudsepp S.D , Yandle T.G , Pemberton C.J , Doughty R.N , Richards A.M 
Published: November 2020

Abstract

Background
Heart failure (HF) is a leading cause of morbidity and mortality worldwide. Measurement of BNP and NTproBNP are used in HF for diagnosis and prognosis but levels of these peptides are inappropriately low in obesity, a condition which is associated with increased HF. Cleavage of proBNP to produce BNP and NT-proBNP requires proBNP to be unglycosylated at threonine 71 (T71). Gycosylation at T71 is affected by obesity, resulting in lower plasma NT-proBNP concentrations in patients with higher BMI. However the relationships between BMI, proBNP glycosylation and BNP (particularly the bioactive cardio-protective peptide BNP1-32) have not previously been described.

Methods
Validated in-house assays for BNP, BNP1-32, proBNP, proBNP unglycosylated at T71 (NG-T71) and the commercial Roche assay for NT-proBNP were applied to plasma samples obtained from patients with HF (n=321, PEOPLE study: Prospective Evaluation of Outcome in Patients with Left Ventricular Ejection Fraction).

Results
Median (IQR) concentrations of BNP, BNP1-32, proBNP, NG-T71 and NTproBNP were 10.7 (5–21), 5 (2–9), 27.8 (9–62), 6.2 (3–22) and 217 (104–425) pmol/L respectively. BMI was inversely related to NG-T71, NT-proBNP, BNP and BNP1-32 (r=−0.19, −0.40, −0.36 and −0.34 respectively, all p&lt;0.01) but not proBNP (r=0.11, ns). ProBNP levels in patients with BMI above or below 30 kg/m2 were similar (29.8 (11.2–56.6) and 22.5 (3.9–65) pmol/L, p=0.51), whereas NG-T71, NT-proBNP, BNP and BNP1-32 levels were increased (p&lt;0.001) in patients with BMI &lt;30 (11.6 (3–25.6), 263 (153–486), 13.8 (6.5–25.5) and 6.3 (2.8–10.4)) compared to BMI &gt;30 (3 (1–16), 127 (63–274), 7.8 (3–14) and 3.6 (1.1–7) respectively. The BMI &gt;30 group had increased ProBNP:NT-proBNP, ProBNP:BNP and ProBNP:BNP1-32 ratios (all p&lt;0.001) and proBNP:NG-T71 (p=0.037), whereas ratios of NG-T71 to BNP, BNP1-32 or NT-proBNP were not related to BMI. Patients with diabetes (n=90) also had lower BNP, BNP1-32 (both p&lt;0.01), NG-T71 and NT-proBNP concentrations (both p&lt;0.05), but not proBNP (p=0.46), and a trend towards a higher proBNP:BNP1-32 ratio (p=0.06).

Discussion and conclusion
The negative association between BMI and plasma NT-proBNP and BNP is not well understood. We recently reported that obese patients with HF have reduced circulating levels of proBNP unglycosylated at T71. In this expanded sample we show that whilst proBNP remains unaffected by BMI, both immunoreactive BNP and more specifically bioactive BNP1–32 levels, and NT-proBNP, are decreased with obesity in conjunction with increased glycosylation at T71. Increased glycosylation at proBNP-T71 reduces the amount of proBNP cleaved to form NT-proBNP and BNP resulting in decreased production and lowered circulating concentrations of these clinically used marker peptides. Our results provide a robust mechanism to explain the reduction in NT-proBNP and BNP levels observed in obese patients and confirm this is associated with reduced bioactive BNP1–32.

Funding Acknowledgement
Type of funding source: Public grant(s) – National budget only. Main funding source(s): National Heart Foundation of New Zealand, nHealth Research Council of New Zealand