Regulation of iron homeostasis by the sulfur assimilation pathway
Hale, Andrew Tully
Regulation of iron homeostasis is perturbed in numerous pathologic states. Thus, identifications of mechanisms responsible for iron metabolism have broad implication for disease modification. Sulfur assimilation is an evolutionarily conserved pathway that plays an essential role in cellular and metabolic processes including sulfation, nucleotide hydrolysis, amino-acid biosynthesis and organismal development. We report that loss of a key enzymatic component of the sulfur assimilation pathway, bisphosphate 3’-nucleotidase BPNT1, in mice, both whole animal and intestine specific, leads to iron-deficiency anemia (IDA). Analysis of mutant enterocytes demonstrate that modulation of BPNT1’s substrate 3’-phosphoadenosine 5’-phosphate (PAP) influences levels of key iron homeostasis factors involved in dietary iron reduction, import and transport, that in part mimic those reported for the loss of hypoxia inducible factor 2a, HIF-2a. In this dissertation we demonstrate three approaches that successfully reduce PAP metabolic toxicity and reverse IDA caused by loss of BPNT1: 1) genetic reduction of PAP synthesis through introduction of a hypomorphic mutation in PAP synthase, PAPSS2 2) dietary methionine restriction and 3) overproduction of a key transcriptional regulator Hif-2a. Genetic reduction of PAP synthesis and dietary methionine restriction reverses IDA in mice lacking BPNT1 through a mechanism of reduction of PAP metabolic toxicity. Gaining Hif-2a acts through a different mechanism by restoring iron homeostatic gene expression in BPNT1 deficient mouse intestinal organoids. Finally, as loss of BPNT1 impairs expression of known genetic modifiers of iron-overload, we demonstrate that intestinal-epithelium specific loss of BPNT1 attenuates hepatic iron accumulation in mice with homozygous C282Y mutations in homeostatic iron regulator (HFEC282Y), the most common cause of hemochromatosis in humans. Mechanistically, we provide preliminary biochemical evidence that histones may be targets of PAP metabolic toxicity. Overall, our studies 1) define a new genetic basis for iron-deficiency anemia, 2) reveal an unanticipated link between nucleotide hydrolysis in the sulfur assimilation pathway and iron homeostasis 3) demonstrate genetic, molecular and dietary approaches to overcome loss of BPNT1, 4) identify a genetic strategy to suppress hemochromatosis in mice, and 5) provide insights into potential targets of PAP metabolic toxicity.