Calcium-sensing receptor signaling is amplified in patients with a <i>FAM111A</i> gene mutation

Introduction: A six-year-old female presented with seizures, low blood calcium (Ca2+), low parathyroid hormone (PTH) and increased urinary Ca2+ excretion. She was diagnosed with Autosomal Dominant Hypocalcemia (ADH), a childhood disorder characterized by low blood Ca2+ and inappropriately low PTH. She had no mutations in genes known to cause ADH (Ca2+-sensing receptor ( CASR) and G-protein subunit alpha 11 ( GNA11)). Consequently, we performed whole exome sequencing and a trio analysis that identified a novel FAM111A gene mutation (c.1454G>A, p.C485Y). Although FAM111A is a protease, the specific protein targets are unknown. In addition, FAM111A regulates DNA replication and thus gene expression. Interestingly, FAM111A gene mutations cause Kenny Caffey syndrome (KCS) and Osteocraniostenosis (OCS), conditions characterized by low blood Ca2+, low PTH, and bony abnormalities. The molecular mechanism mediating these phenotypes are unknown. We hypothesize that mutations in FAM111A cause increased CASR signaling, resulting in low blood Ca2+ and high urine Ca2+ levels. The CASR is a G-protein coupled receptor, which upon activation by extracellular Ca2+, initiates signaling cascades that increase intracellular Ca2+ levels and decrease cyclic adenosine monophosphate (cAMP). CASR activation also increases the expression of the tight-junction protein claudin-14 (CLDN14). In the kidney, CLDN14 blocks Ca2+ reabsorption leading to increased urine Ca2+ excretion and lower blood Ca2+. Our objective was to determine if FAM111A wild-type (WT) suppresses CASR signaling and if the mutants fail to do so. Methods: Human embryonic cells (HEK293) were transfected with empty vector (EV) as a control, CASR and EV, FAM111A WT with EV or CASR and FAM111A WT or mutants (C485Y, ADH; Y511H, KCS; R569H, KCS; T338A, OCS; P527R, OCS; D528G, OCS; S541A, inactivated protease). We performed Fura2AM imaging of intracellular Ca2+ in transfected cells in the presence of increasing extracellular Ca2+ (0.5-11.3 mM) levels. Also, as an indication of CASR activity, we measured CLDN14 expression and cAMP levels, in transfected cells incubated in 0.5 mM or 5 mM extracellular Ca2+, via a dual luciferase assay and cAMP assay kit, respectively. Results: The change in peak intracellular Ca2+ concentration was two times higher (p<0.05) in cells with CASR plus EV or FAM111A mutants, compared to EV or FAM111A WT plus CASR. Similarly, luciferase activity as an indication of CLDN14 levels, was significantly higher (p<0.05) in CASR plus EV compared to EV and FAM111A WT plus CASR. Some mutants (C485Y, T338A, P527T, D528G, S542A) had similar CLDN14 levels to CASR plus EV, while others (Y511H, R569H) showed similar levels to FAM111A plus CASR. The cAMP levels were not different in cells with CASR and FAM111A WT compared to CASR with FAM111A mutants. Conclusions: FAM111A WT attenuates CASR activity. All FAM111A mutations assessed, enhanced CASR activity when measured by an increase in intracellular Ca2+. Some mutants increase CLDN14 expression, while others may affect a different CASR signaling pathway. Elucidating the mechanism of how FAM111A affects CASR activity and Ca2+ homeostasis will provide a better understanding of our patient’s condition as well as those with KCS or OCS. Canadian Institute of Health Research; Stollery Children’s Hospital Foundation through the Women and Children's Health Research Institute (WCHRI); Alberta Innovates Graduate Student Scholarship; WCHRI Graduate Studentship Award. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

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