Eiken syndrome with parathyroid hormone resistance due to a novel parathyroid hormone receptor type 1 mutation: clinical features and functional analysis

We report two patients of east African ancestry with the same novel homozygous variant in the parathyroid hormone receptor type 1 (PTH1R). Both patients shared skeletal features including brachydactyly, extensive metacarpal pseudoepiphyses, elongated cone-shaped epiphyses, ischiopubic hypoplasia, deficient sacral ossification, suggestive of Eiken syndrome. Strikingly, both patients exhibited clinically manifest parathyroid hormone (PTH) resistance with hypocalcaemia and elevated serum phosphate levels. These laboratory and clinical abnormalities initially suggested pseudohypoparathyroidism, which is typically associated with GNAS abnormalities. In both patients, however, a homozygous novel PTH1R variant was identified (c.710 T > A; p.IIe237Asn, p.I237N) that is located in the second transmembrane helical domain. Previously, others have reported a patient with a nearby PTH1R mutation (D241E) who presented with similar clinical features, e.g. delayed bone mineralization as well as clinical PTH resistance. Functional analysis of the effects of both novel PTH1R variants (I237N- and D241E-PTH1R) in HEK293 reporter cells transfected with plasmid DNA encoding the wild-type or mutant PTH1Rs demonstrated increased basal cAMP signalling for both variants, with relative blunting of responses to both PTH and PTH-related peptide (PTHrP) ligands. The clinical presentation of PTH resistance and delayed bone mineralization combined with the functional properties of the mutant PTH1Rs suggest that this form of Eiken syndrome results from alterations in PTH1R-mediated signalling in response to both canonical ligands, PTH and PTHrP.