Parathyroid Hormone Resistance and Autoantibodies to the PTH1 Receptor

Abstract

We describe two cases of acquired parathyroid hormone (PTH) resistance consequent to the development of serum PTH type 1 receptor (PTH1R) autoantibodies, which block PTH binding and signaling. Both cases were associated with other autoimmune manifestations, and one case was associated with atypical membranous glomerulonephritis. In vitro binding and signaling assays identified the presence of PTH1R-blocking IgG autoantibodies, which were not present in serum samples from patients with other renal or autoimmune disorders. (Funded by the Intramural Research Programs of the National Institute of Diabetes and Digestive and Kidney Diseases and others.).

Figure 1 (facing page).. Mineral Metabolism and Renal Pathology in Patient 1.

Panel A shows the changes in the patient’s serum levels of parathyroid hormone (PTH) (in blue) and creatinine (in gray) and blood levels of ionized calcium (in purple) over 10 years during treatment. Black arrows denote representative time points of undertreatment and red arrows representative time points of more aggressive treatment. Panels B and C show representative glomeruli with atypical, membranous-like nephropathy manifested by diffuse thickening of the glomerular capillary loops (yellow arrows; hematoxylin and eosin staining in Panel B and periodic acid-Schiff staining in Panel C; magnification of 600 in both panels). In Panels D and E, images obtained on electron microscopy show the ultrastructure of a thickened glomerular capillary loop containing atypical, microspherule-shaped, intramembranous and subepithelial aggregates of unknown composition (white arrows; magnifications of 12,000 in Panel D and 20,000 in Panel E).

Figure 2.. Identification of PTH1R-Blocking Autoantibodies.

Panel A shows the results of a luciferase immunoprecipitation systems (LIPS) autoantibody analysis for human PTH in six healthy volunteers and in Patients 1 and 2, in which there is no evidence of autoantibodies to PTH. Panel B shows the results of LIPS autoantibody profiling for human, near full-length PTH type 1 receptors (PTH1Rs). Levels of PTH1R autoantibodies in the serum samples from both Patient 1 and Patient 2 were high as compared with the levels in samples from 14 healthy volunteers, 19 patients with various renal diseases (including membranoproliferative glomerulonephritis, C3 glomerulopathy, minimal change disease, thrombotic microangiopathy, vasculitis, light-chain deposition disease, and class III or IV lupus nephritis), 20 patients with autoimmune polyendocrinopathy candidiasis with ectodermal dystrophy (APECED), and 24 patients with membranous nephropathy (including 3 patients with uremia). Panel C shows LIPS autoantibody profiling against the PTH1R extracellular domain (PTH1R-ECD) and PTH1R transmembrane domain (PTH1R-TMD) in 15 healthy volunteers and in Patients 1 and 2, indicating that PTH1R autoantibodies bind predominantly to the extracellular domain in Patient 1 and exclusively to the extracellular domain in Patient 2. In Panels A, B, and C, raw light units reflect autoantibody levels determined by means of LIPS. The two data points for Patient 1 in Panels B and C represent repeated measurements of the same sample. Panel D shows stimulation of cyclic adenosine monophosphate (cAMP) in response to PTH 1-34 (182 pg per milliliter) in the presence of antibodies isolated from 4 healthy volunteers and from Patients 1 and 2, with all individual responses normalized to the mean response in healthy volunteers. In healthy volunteers, levels of PTH 1-34–stimulated light units were approximately five times as high as those of unstimulated light units. In Patient 1, results similar to those shown in Panel D were also observed at a higher PTH 1-34 dose (22,750 pg per milliliter), although twice the amount of IgG was included in that experiment (data not shown). Means and standard deviations are shown for each group.