Can mutations in FAM111A cause autosomal dominant hypocalcemia?
Introduction: Autosomal Dominant Hypocalcemia (ADH) is a childhood disorder resulting in low blood calcium and inappropriately low levels of parathyroid hormone (PTH), which functions to increase blood calcium. In 2014, a nine-year old female presented to the Stollery Emergency Department with seizures and was found to have low blood calcium levels (0.72 mM ionized), low PTH (1.2 nM) and increased urinary calcium excretion. She had no mutations in the genes (CASR and GNA11) known to cause ADH, although whole exome sequencing revealed a novel FAM111A gene mutation (c.1454G>A, p.C485Y). FAM111A mutations are known to cause Kenny Caffey syndrome and Gracile Bone disease, which are both characterized by low blood calcium levels, low PTH, short stature and bony abnormalities. The patient is normally grown, does not have bony abnormalities, but her other characteristics are consistent with a FAM111A gene mutation causing her disease. We therefore hypothesize that FAM111A mutations cause a range of diseases that all include low blood calcium and PTH levels. Methods: We generated with CRISPR/Cas9 a mutant mouse carrying the same mutation as the patient. FAM111A heterozygous (HET; Female n=22, Male n=16), homozygous (HOM; Female n=12, Male n=10), and wild-type (WT; Female n=10, Male n=18) mice receiving a low calcium (0.01%) diet were placed in metabolic cages for 3 days to collect urine and feces every 24 hours, tissues (i.e. kidney, intestine) and blood before euthanasia. Results: Blood calcium levels and PTH levels of all mice were within the normal physiological range and were not significantly different between WT and mutant mice. Additionally, there was no significant difference in the amount of intestinally absorbed calcium, fecal calcium excretion, or calcium balance. Urinary calcium excretion of females as assessed as a calcium/creatinine ratio (mM/mM) and 24-hour calcium excretion (mg/day) was significantly (p<0.05) lower in mutant HET (0.67±0.13, 0.12±0.03) and HOM (0.82±0.18, 0.12±0.03) than WT (1.15±0.35, 0.19±0.08). In females, the expression of intestinal genes involved in calcium absorption were not different among genotypes, however the expression of kidney genes (Cldn2, Cldn16, Cldn19, Pmca1b, Ncx1, Calb28k) involved in calcium reabsorption were significantly (p<0.05) decreased in HET compared to WT. Conclusions: The mutant mice do not show an ADH phenotype. They may have increased activity of compensatory systems to normalize blood calcium levels. In HET females, this could include excreting less calcium into the urine, however, unexpectedly there was decreased expression of the kidney genes involved in calcium reabsorption. Overall, mice may not be a good model for studying FAM111A mutations due to species differences.