Recombinant porphobilinogen deaminase targeted to the liver corrects enzymopenia in a mouse model of acute intermittent porphyria

Systemic and subcutaneous rApoAI-PBGD therapy protects against porphyrin precursor accumulation, pain, and motor neuropathy in AIP mice.

Perturbations in the Heme and Siroheme Biosynthesis Pathways Causing Accumulation of Fluorescent Free Base Porphyrins and Auxotrophy in Ogataea Yeasts

The biosynthesis of cyclic tetrapyrrol chromophores such as heme, siroheme, and chlorophyll involves the formation of fluorescent porphyrin precursors or compounds, which become fluorescent after oxidation. To identify Ogataea polymorpha mutations affecting the final steps of heme or siroheme biosynthesis, we performed a search for clones with fluorescence characteristic of free base porphyrins. One of the obtained mutants was defective in the gene encoding a homologue of Saccharomyces cerevisiae Met8 responsible for the last two steps of siroheme synthesis. Same as the originally obtained mutation, the targeted inactivation of this gene in O. polymorpha and O. parapolymorpha led to increased porphyrin fluorescence and methionine auxotrophy. These features allow the easy isolation of Met8-defective mutants and can potentially be used to construct auxotrophic strains in various yeast species. Besides MET8, this approach also identified the HEM3 gene encoding porphobilinogen deaminase, whose increased dosage led to free base porphyrin accumulation.

The crystal structures of the enzyme hydroxymethylbilane synthase, also known as porphobilinogen deaminase

The enzyme hydroxymethylbilane synthase (HMBS; EC 4.3.1.8), also known as porphobilinogen deaminase, catalyses the stepwise addition of four molecules of porphobilinogen to form the linear tetrapyrrole 1-hydroxymethylbilane. Thirty years of crystal structures are surveyed in this topical review. These crystal structures aim at the elucidation of the structural basis of the complex reaction mechanism involving the formation of tetrapyrrole from individual porphobilinogen units. The consistency between the various structures is assessed. This includes an evaluation of the precision of each molecular model and what was not modelled. A survey is also made of the crystallization conditions used in the context of the operational pH of the enzyme. The combination of 3D structural techniques, seeking accuracy, has also been a feature of this research effort. Thus, SAXS, NMR and computational molecular dynamics have also been applied. The general framework is also a considerable chemistry research effort to understand the function of the enzyme and its medical pathologies in acute intermittent porphyria (AIP). Mutational studies and their impact on the catalytic reaction provide insight into the basis of AIP and are also invaluable for guiding the understanding of the crystal structure results. Future directions for research on HMBS are described, including the need to determine the protonation states of key amino-acid residues identified as being catalytically important. The question remains – what is the molecular engine for this complex reaction? Thermal fluctuations are the only suggestion thus far.

Prophylactic Heme Arginate Infusion for Acute Intermittent Porphyria

Objectives: This study aimed to evaluate the efficacy of long-term weekly prophylactic heme arginate (HA) infusions in reducing attack frequency and severity in female AIP patients.Methods: We report the results of five female AIP patients with frequent recurrent attacks (>9/year) before and after institution of weekly prophylaxis with heme arginate (3 mg/kg body weight). All five cases had confirmed disease-associated mutations in the porphobilinogen deaminase gene, and all had received genetic and clinical counseling about AIP.Results: In the five included patients, average annual attack rate (AAR) in the year prior to HA prophylaxis was 11.82 (range 9.03–17.06), and average total HA usage was 32.60 doses (range: 13.71–53.13). After 2.58–14.64 years of HA prophylaxis, average AAR was reduced to 2.23 (range 0.00–5.58), and attack severity (i.e., doses required per attack) was reduced from 2.81 to 1.39 doses/attack. Liver and renal function remained stable during weekly administration of HA prophylaxis. The most common complications were port-A catheter-related events. No other complications or safety concerns occurred with long-term use of HA prophylaxis.Conclusion: Our study demonstrated women with AIP receiving weekly prophylactic HA infusions resulted in fewer episodes that required acute HA treatment while maintaining stable renal and liver function. Weekly prophylactic HA infusions effectively prevent frequent porphyric attacks and reduce attack severity.

Hyponatremia and Isolated Free T4 Elevation in a Patient With Acute Intermittent Porphyria

Background: Porphyrias represent a spectrum of diseases that stem from dysfunction within the heme biosynthetic pathway. Acute intermittent porphyria (AIP) is the most common type of porphyria due to a genetic deficiency of porphobilinogen deaminase which results in a wide range of neurovisceral symptoms. Hyponatremia and abnormalities with thyroid function have been found in AIP but the mechanisms behind these processes are unclear. Clinical Case: A 26-year-old male with a history of chronic, recurrent abdominal pain presented with 10 days of progressively worsening periumbilical abdominal pain and constipation. He describes that preceding the onset of symptoms he had been binging 5-10 standard alcoholic drinks each day for a few days. Initial laboratory workup demonstrated hyponatremia with Na 114 mmol/L (n: 134 – 137 mmol/L), hypochloremia with Cl 76 mmol/L (n: 95 – 108 mmol/L), and hyperbilirubinemia with total bilirubin 2.5 mg/dL (n: 0.2 – 1.3 mg/dL). CT abdomen/pelvis was negative for any concerning pathology. Further studies showed low serum osmolality at 243 mOsm/kg (n: 275 – 295 mOsm/kg), urine Na 62 mmol/L (n > 20mmol/L), and urine osmolality at 394 mOsm/kg (n: 300 – 900 mOsm/kg) consistent with SIADH. The patient was treated with 3% NaCl and free water restriction to 1.2L/day with improvement in Na levels. Further laboratory workup demonstrated normal TSH, but persistently elevated free T4 with maximum free T4 of 2.77 ng/dL (n: 0.80 – 1.70 ng/dL) which downtrended to 1.99 ng/dL by discharge. Thyroid ultrasound was unremarkable. Pituitary evaluation via hormonal workup and MRI brain was negative for any abnormalities. Given the symptomatology and laboratory findings, the patient was evaluated for porphyria. Laboratory evaluation demonstrated severe elevations in urine porphyrins, urine delta aminolevulinic acid (56.8 mg/24h, n < 4.5 mg/24h), and urine porphobilinogen (82.5 mg/g, n < 2.3 mg/g) consistent with AIP. The patient was treated with four days of hematin infusions which resolved his abdominal pain and was discharged in an improved state. Conclusion: AIP is a rare entity brought out by a deficiency in porphobilinogen deaminase, a key enzyme in the heme biosynthesis pathway. Various metabolic disturbances have been described in AIP including hyponatremia and alterations in thyroid function tests suggestive of thyrotoxicosis. Hyponatremia in our patient was likely due to SIADH from neurovisceral pain. Our patient displayed isolated free T4 elevation as well. We hypothesize this developed due to his acute illness causing a greater decrease in D2 deiodinase activity compared to any concomitant increase in D3 deiodinase activity. This was supported by his free T4 level downtrending following treatment of his AIP attack. More research is needed to further elucidate the mechanism behind these derangements in biochemical markers and their impact on patient prognosis.

Enigmatic Evolutionary History of Porphobilinogen Deaminase in Eukaryotic Phototrophs

In most eukaryotic phototrophs, the entire heme synthesis is localized to the plastid, and enzymes of cyanobacterial origin dominate the pathway. Despite that, porphobilinogen deaminase (PBGD), the enzyme responsible for the synthesis of hydroxymethybilane in the plastid, shows phylogenetic affiliation to α-proteobacteria, the supposed ancestor of mitochondria. Surprisingly, no PBGD of such origin is found in the heme pathway of the supposed partners of the primary plastid endosymbiosis, a primarily heterotrophic eukaryote, and a cyanobacterium. It appears that α-proteobacterial PBGD is absent from glaucophytes but is present in rhodophytes, chlorophytes, plants, and most algae with complex plastids. This may suggest that in eukaryotic phototrophs, except for glaucophytes, either the gene from the mitochondrial ancestor was retained while the cyanobacterial and eukaryotic pseudoparalogs were lost in evolution, or the gene was acquired by non-endosymbiotic gene transfer from an unspecified α-proteobacterium and functionally replaced its cyanobacterial and eukaryotic counterparts.

AbhemC encoding porphobilinogen deaminase plays an important role in chlorophyll biosynthesis and function in albino Ananas comosus var. bracteatus leaves

Background The chimeric leaves of Ananas comosus var. bracteatus are composed of normal green parts (Grs) and albino white parts (Whs). Although the underlying mechanism of albinism in A. comosus var. bracteatus leaves is not fully understood, it is likely associated with the chlorophyll (Chl) biosynthesis. In this biosynthetic process, porphobilinogen deaminase (PBGD) plays a crucial role by catalyzing the conversion of porphobilinogen (PBG) to uroporphyrinogen III (Urogen III). Therefore, its encoding gene AbhemC was investigated here in association with Chl biosynthesis and albinism in chimeric A. comosus var. bracteatus leaves. Methods The Chl content, main Chl biosynthesis precursor content, and main enzyme activity were determined and compared between the Whs and Grs of A. comosus var. bracteatus leaves. In addition, AbhemC was cloned and its transcriptional expression and prokaryotic protein expression were analyzed. Furthermore, RNAi-mediated silencing of AbhemC was produced and assessed in tobacco plants. Results The concentration of Chl a and Chl b in the Grs was significantly higher than that in the Whs, respectively. Additionally, the content of the Chl biosynthesis precursor Urogen III decreased significantly in the Whs compared with the Grs. Thus, the transition of PBG to Urogen III may be the first rate-limiting step leading to albinism in the chimeric leaves of A. comosus var. bracteatus. The gene AbhemC comprised 1,135 bp and was encoded into a protein with 371 amino acids; phylogenetically, AbhemC was most closely related to hemC of pineapple. Prokaryotic expression and in vitro enzyme activity analysis showed that the cloned mRNA sequence of AbhemC was successfully integrated and had PBGD activity. Compared with control plants, transgenic tobacco leaves with pFGC5941-AbhemC-RNAi vector were substantially less green with significantly reduced hemC expression and Chl content, as well as reduced PBGD enzyme activity and significantly decreased content of Chl biosynthesis precursors from Urogen III onwards. Our results suggest that the absence of hemC expression reduces the enzyme activity of PBGD, which blocks the transition of PBG to Urogen III, and in turn suppresses Chl synthesis leading to the pale-green leaf color. Therefore, we suggest that AbhemC plays an important role in Chl synthesis and may be an important factor in the albinism of A. comosus var. bracteatus leaves.

Reversible splenial lesion syndrome (RESLES) due to acute intermittent porphyria with a novel mutation in the hydroxymethylbilane synthase gene

Background: Reversible splenial lesion syndrome (RESLES) is a clinico-radiological syndrome characterized by the presence of reversible lesions specifically involving the splenium of the corpus callosum (SCC). The cause of RESLES is unknown. However, infectious-related mild encephalitis/encephalopathy (MERS) with a reversible splenial lesion remains the most common cause of reversible splenial lesions. Acute intermittent porphyria (AIP) is an autosomal dominant disorder caused by a partial deficiency of porphobilinogen deaminase (PBGD), the third enzyme in the heme biosynthetic pathway. It can affect the autonomic, peripheral, and central nervous system. Result: In this study, we report a 20-year-old woman with AIP who presented with MRI manifestations suggestive of RESLES, she had a novel HMBS nonsense mutation, a G to A mutation in base 594, which changed tryptophan to a stop codon (W198*). Conclusion: To the best of our knowledge, this is only one published case of RELES associated with AIP.