Murat Bastepe, MD, PhD

Our broad goal is to understand the role of the complex GNAS locus in physiology and in disease.

GNAS encodes the α-subunit of the stimulatory G protein, a signaling protein that mediates the actions of numerous hormones, neurotransmitters, and autocrine/paracrine factors. A recently described variant of Gsα is the so-called extra-large α-subunit (XLαs), which uses a different first exon and an upstream promoter that is active exclusively on the paternal allele. Unlike Gsα, the cellular role of XLαs is unclear. Nonetheless, data from XLαs knockout mice and children with large deletions of the paternal GNAS allele indicate that XLαs is essential at least for postnatal adaptation to feeding and glucose and energy metabolism.

Mutations of GNAS are associated with several human diseases, including McCune-Albright syndrome, Albright’s Hereditary Osteodystrophy, progressive osseous heteroplasia (POH), and pseudohypoparathyroidism (PHP). With the exception of mutations in exon 1, all mutations of GNAS affect both Gsα and XLαs. Yet, the significance of XLαs deficiency or overactivity in the pathogenesis of these diseases is unknown. Our lab aims to elucidate the cellular roles of XLαs and, thereby, understand how alterations in XLαs activity modify GNAS-related disease pathogenesis. We have shown that XLαs can mimic Gsα in certain in vitro and in vivo settings. In addition, our findings have demonstrated that mutations found in patients with AHO and PHP impair cAMP generation through not only Gsα but also XLαs in vitro. We now extend these studies as follows:

Cumhur Omer Aydin

 

1. We aim to address the role of XLαs in disease pathogenesis in vivo. Using transgenic and knockout mouse models, we specifically ask whether XLαs can compensate for the lack of Gsα in the renal proximal tubule, an in vivo system where Gsα-mediated PTH actions are well defined and where Gsα levels are tightly controlled through genomic imprinting. Through similar in vivo approaches, we also ask whether a compensatory Gsα-like activity of XLαs explains the delay in the development of PTH-resistance observed in patients with PHP, a disorder of PTH resistance caused by maternally inherited GNAS mutations that affect activity and/or expression of Gsα. For these studies, we have generated transgenic mice with targeted expression of XLαs in the proximal tubule and mice with conditional knock-down of Gsα in the same tissue. Also included in our studies are mice with universal Gsα or XLαs knockout, which were obtained from collaborators.

2. XLαs mRNA gives rise to several different XLαs variants and alternative gene products. Using biochemical and molecular approaches, we aim to determine the structures and the functions of these variants and alternative proteins, particularly focusing on their potential roles in typical Gsα-mediated cell signaling. The XLαs variants under study include an N-terminally extended variant termed XXLαs, a C-terminally truncated variant termed XLN1, and two alternative translation products termed ALEX and ALEXX.