Unaltered ratio of circulating levels of growth hormone/GH isoforms in adults with Prader–Willi syndrome after GHRH plus arginine administration

Abstract

Human growth hormone (GH) is a heterogeneous protein hormone consisting of several isoforms, the most abundant being 22kDa- and 20kDa-GH. The availability of analytical methods to measure these GH isoforms might represent a valuable diagnostic tool to investigate GH hyposecretory states, including Prader–Willi syndrome (PWS), one of the most common causes of syndromic obesity. The aim of the present study was to measure circulating levels of 22kDa- and 20kDa-GH in PWS adults (n=14; M/F: 5/9; genotype DEL15/UPD15: 12/2; age: 19.0±3.7years; BMI: 29.9±8.7kg/m2) after combined GH releasing hormone (GHRH) plus arginine (ARG) administration. The results were analysed subdividing the study population in obese vs. nonobese (6/8) and GH deficient vs. nonGH deficient (GHD) (6/8) subjects, according to appropriate BMI-related diagnostic cut-off limits of GH peak response to the provocative test. Circulating levels of 22kDa-GH were measured by a chemiluminescent method based on a detection monoclonal antibody targeting an epitope in the loop connecting helix 1 and 2 of GH, which is missing in 20kDa-GH; the 20kDa-GH was measured using a time resolved fluorescence assay based on two monoclonal antibodies with no cross-reactivity to 22-kDa GH.GHRH plus ARG significantly stimulated the secretions of 22kDa- and 20kDa-GH in nonobese (at 30, 45, 60 and 90min and at 45, 60, 90 and 120min vs. 0min, p<0.05, with GH peaks of 15.8±10.3ng/ml and 2.7±1.2ng/ml, respectively) and in nonGHD PWS (at 30, 45 and 60min and at 45, 60 and 90min vs 0min, p<0.05, with GH peaks of 12.5±9.0ng/ml and 2.0±1.8ng/ml, respectively). No significant GHRH plus ARG-induced changes in 22kDa- and 20kDa-GH were observed in obese or GHD PWS patients, the only exception being the increase of 22kDa-GH (p<0.05) 60min after the stimulus administration in GHD group (with GH peaks of 6.9±4.7ng/ml and 0.8±0.6ng/ml in obese subjects and 8.5±6.0ng/ml and 1.2±1.0ng/ml in GHD subjects for 22kDa- and 20kDa-GH, respectively). The GH responses for both isoforms were significantly higher in nonobese than in obese PWS patients (at 45 and 60min for 22kDa-GH and at 45, 60, 90 and 120min for 20kDa-GH, p<0.05), while no differences were detected between GHD vs. nonGHD groups. As previously reported in healthy subjects, the ratios of circulating levels of 22kDa- to 20kDa-GH remained constant after GHRH plus ARG both in obese/non-obese and GHD/non-GHD groups, thus suggesting the preservation of a normal balance in GH isoforms in PWS.