Hydroxymethylbilane synthase (HMBS), the third enzyme in the heme biosynthetic pathway, catalyzes the head-to-tail condensation of four molecules of porphobilinogen (PBG) to form the linear tetrapyrrole 1-hydroxymethylbilane (HMB). Mutations in human
Abstract Herein, we studied the expression of endocannabinoid receptor 2 (CB2R), a known inflammation mediator, in several lysosomal storage disorder (LSD) animal models and evaluated it as a potential biomarker and therapeutic target for these diseases. CB2R was highly elevated in the plasma of Farber disease and mucopolysaccharidosis (MPS) type IIIA mice, followed by Fabry disease and MPS type I mice. Mice with acid sphingomyelinase‐deficient Niemann‐Pick disease (ASMD) and rats with MPS type VI exhibited little or no plasma CB2R elevation. High‐level expression of CB2R was also observed in tissues of Farber and MPS IIIA mice. Treatment of MPS IIIIA patient cells with CB2R agonists led to a reduction of CB2R and monocyte chemoattractant protein‐1 (MCP‐1), a chemotactic factor that is elevated in this LSD. Treatment of MPS IIIA mice with one of these agonists (JWH133) led to a reduction of plasma and tissue CB2R and MCP‐1, a reduction of glial fibrillary acidic protein (GFAP) in the brain, and an improvement in hanging test performance. JWH133 treatment of Farber disease mice also led to a reduction of MCP‐1 in tissues and plasma, and treatment of these mice by enzyme replacement therapy (ERT) led to a reduction of plasma CB2R, indicating its potential to monitor treatment response. Overall, these findings suggest that CB2R should be further examined as a potential therapeutic target for the LSDs and may also be a useful biomarker to monitor the impact of therapies.
Abstract Acute intermittent porphyria (AIP) is an autosomal dominant inborn error of heme biosynthesis due to a pathogenic mutation in the Hmbs gene, resulting in half-normal activity of hydroxymethylbilane synthase. Factors that induce hepatic heme biosynthesis induce episodic attacks in heterozygous patients. The clinical presentation of acute attacks involves the signature neurovisceral pain and may include psychiatric symptoms. Here we used a knock-in mouse line that is biallelic for the Hmbs c.500G > A (p.R167Q) mutation with ~ 5% of normal hydroxymethylbilane synthase activity to unravel the consequences of severe HMBS deficiency on affective behavior and brain physiology. Hmbs knock-in mice (KI mice) model the rare homozygous dominant form of AIP and were used as tool to elucidate the hitherto unknown pathophysiology of the behavioral manifestations of the disease and its neural underpinnings. Extensive behavioral analyses revealed a selective depression-like phenotype in Hmbs KI mice; transcriptomic and immunohistochemical analyses demonstrated aberrant myelination. The uncovered compromised mitochondrial function in the hippocampus of knock-in mice and its ensuing neurogenic and neuroplastic deficits lead us to propose a mechanistic role for disrupted mitochondrial energy production in the pathogenesis of the behavioral consequences of severe HMBS deficiency and its neuropathological sequelae in the brain.
