The detection of elevated blood levels of homocysteine in patients with acute intermittent porphyria (AIP), particularly in symptomatic individuals with high excretion of porphyrin precursors, suggest an impairment of one carbon metabolism (OCM). To investigate the impact of AIP and therapies on hepatic OCM, we conducted transcriptomic and metabolomic analyses in an AIP model developed by PBGD expression knockdown upon intrahepatic delivery (rAAV vector) of specific shRNA in non-human primates (NHP).

One month after AIP induction, hepatic PBGD inhibition led to 2-fold increase of porphyrin precursors despite normal ALAS1 gene expression. From month 1 to 7, NHPs (n = 8) were challenged with porphyrinogenic drugs to intensify precursor accumulation (up to 4-fold), replicating recurrent acute attacks. Multi-dose (MD) administration of hemin (n=1)(15 doses of 2 mg/kg, iv, with 3 doses every two weeks), givosiran (n=1)(5 doses of 2.5 mg/kg, sc, with one dose every 3 weeks), and hPBGD mRNA formulated in lipid nanoparticles (n=3)(7 doses of 0.5 mg/kg, iv, with one dose every 2 weeks) was implemented from month 3 to the end of the study (7 months post-rAAV injection). At sacrifice, hepatic levels of S-adenosyl-L-methionine (SAMe) decreased sharply, while those of polyamines, particularly putrescine, were increased. Given the critical role of SAMe in polyamine synthesis pathway, its reduced levels could be associated with polyamine accumulation. Accordingly, the hepatic levels of methylthioadenosine (MTA), a molecule generated from SAMe during polyamine synthesis, were also drastically reduced. Regarding the transsulfuration pathway, the hepatic enzymatic activity of cystathionine beta-synthase (CBS) was not altered in AIP NHPs. Consequently, metabolomic analyses showed no changes in cystathionine levels, a product of CBS, but low levels of reduced glutathione (GSH), the downstream product of this pathway. Transcriptomic studies showed no changes in the expression of MAT1A, coding for methionine adenosyltransferase (MAT I/III), the enzyme responsible for SAMe synthesis in the liver. However, MAT I/III is readily inactivated under oxidative stress conditions, which likely occur in the liver of AIP NHPs as indicated by the low GSH levels and the increased expression of SOD2 (superoxide dismutase 2). Interestingly, a significant induction of ODC1 (ornithine decarboxylase 1), together with decreased SAMe availability, may explain the accumulation of putrescine in AIP NHPs. Remarkably, hepatic levels of SAMe and GSH were restored only in hPBGD mRNA-treated animals.

In conclusion, our data demonstrate a significant impairment of OCM in the liver of AIP NHPs. Given the key role of SAMe in numerous hepatic and systemic metabolic pathways, its reduced availability may contribute to the progression of the disease and the alteration of central liver functions. OCM pathway tended to be normalized following MD administration of hPBGD mRNA.