Acute intermittent porphyria: heterogeneity of mutations in the hydroxymethylbilane synthase gene in Italy

HMBS gene mutations and hydroxymethylbilane synthase activity in acute intermittent porphyria: A systematic review

BACKGROUND

Acute intermittent porphyria (AIP) is caused by a partial deficiency of hydroxymethylbilane synthase and affects heme biosynthesis. Mutations in the HMBS gene result in HMBS deficiency. AIP is a rare disease, and there been insufficient studies on it. This report describes the molecular epidemiology of HMBS gene defects and hydroxymethylbilane synthase activity levels in classical AIP.

METHODS

Databases of PubMed, CNKI, and Wang Fang Database were searched for eligible studies to investigate HMBS gene mutations in peripheral blood samples and HMBS activity in erythrocytes of patients with classical AIP. Relevant studies published up to July 15, 2023, from several databases were independently searched and selected by 2 reviewers. Accuracy data and relevant information were extracted from each eligible study by 2 independent researchers and analyzed using statistical software.

RESULTS

After pooling the accuracy data from 232 patients of the 15 eligible studies, 90.5% (210/232) of AIP patients had decreased erythrocyte hydroxymethylbilane synthase activity (<70%), and 96 different mutations were identified in 232 patients, including 33 missense (34.4%), 27 splice (28.1%), 19 deletion (19.8%), 8 nonsense (8.3%), 9 insertion (9.4%) mutations. Residual enzyme activities (%) for different groups of type were expressed using mean and 95% confidence interval (95% CI): missense (51.2, 48.5-53.9), splice (57.5, 52.0-59.1), deletion (54.9, 50.7-59.1), nonsense (52.2, 44.4-60.0), insertion (53.2, 47.4-59.0), group analysis P = .17. Subgroups of missense mutations, domain 1 (50.2, 46.0-54.4), domain 2 (52.8, 49.1-56.4), and domain 3 (49.2, 38.3-60.0), Subgroup analysis, P = .62.

CONCLUSION

Different mutation types and mutation positions are not associated with the level of hydroxymethylbilane synthase activity. Erythrocyte hydroxymethylbilane synthase activity is often reduced to half of normal in patients with AIP, and the enzyme activity assay has a high diagnostic value in AIP. AIP is highly molecularly heterogeneous, with missense mutations being the most common, followed by splice mutations. R173W and G111R are high-frequency mutations and have been found in multiple families from different countries.

Crystal structures of hydroxymethylbilane synthase complexed with a substrate analog: a single substrate-binding site for four consecutive condensation steps

Hydroxymethylbilane synthase (HMBS), which is involved in the heme biosynthesis pathway, has a dipyrromethane cofactor and combines four porphobilinogen (PBG) molecules to form a linear tetrapyrrole, hydroxymethylbilane. Enzyme kinetic study of human HMBS using a PBG-derivative, 2-iodoporphobilinogen (2-I-PBG), exhibited noncompetitive inhibition with the inhibition constant being 5.4 ± 0.3 µM. To elucidate the reaction mechanism of HMBS in detail, crystal structure analysis of 2-I-PBG-bound holo-HMBS and its reaction intermediate possessing two PBG molecules (ES2), and inhibitor-free ES2 was performed at 2.40, 2.31, and 1.79 Å resolution, respectively. Their overall structures are similar to that of inhibitor-free holo-HMBS, and the differences are limited near the active site. In both 2-I-PBG-bound structures, 2-I-PBG is located near the terminus of the cofactor or the tetrapyrrole chain. The propionate group of 2-I-PBG interacts with the side chain of Arg173, and its acetate group is associated with the side chains of Arg26 and Ser28. Furthermore, the aminomethyl group and pyrrole nitrogen of 2-I-PBG form hydrogen bonds with the side chains of Gln34 and Asp99, respectively. These amino acid residues form a single substrate-binding site, where each of the four PBG molecules covalently binds to the cofactor (or oligopyrrole chain) consecutively, ultimately forming a hexapyrrole chain. Molecular dynamics simulation of the ES2 intermediate suggested that the thermal fluctuation of the lid and cofactor-binding loops causes substrate recruitment and oligopyrrole chain shift needed for consecutive condensation. Finally, the hexapyrrole chain is hydrolyzed self-catalytically to produce hydroxymethylbilane.

CRISPR/Cas9-mediated β-globin gene knockout in rabbits recapitulates human β-thalassemia

β-thalassemia, an autosomal recessive blood disorder that reduces the production of hemoglobin, is majorly caused by the point mutation of the HBB gene resulting in reduced or absent β-globin chains of the hemoglobin tetramer. Animal models recapitulating both the phenotype and genotype of human disease are valuable in the exploration of pathophysiology and for in vivo evaluation of novel therapeutic treatments. The docile temperament, short vital cycles, and low cost of rabbits make them an attractive animal model. However, β-thalassemia rabbit models are currently unavailable. Here, using CRISPR/Cas9-mediated genome editing, we point mutated the rabbit β-globin gene HBB2 with high efficiency and generated a β-thalassemia rabbit model. Hematological and histological analyses demonstrated that the genotypic mosaic F0 displayed a mild phenotype of anemia, and the heterozygous F1 exhibited typical characteristics of β-thalassemia. Whole-blood transcriptome analysis revealed that the gene expression was altered in HBB2-targeted when compared with WT rabbits. And the highly expressed genes in HBB2-targeted rabbits were enriched in lipid and iron metabolism, innate immunity, and hematopoietic processes. In conclusion, using CRISPR-mediated HBB2 knockout, we have created a β-thalassemia rabbit model that accurately recapitulates the human disease phenotype. We believe this tool will be valuable in advancing the investigation of pathogenesis and novel therapeutic targets of β-thalassemia and associated complications.

Molecular Analysis of 55 Spanish Patients with Acute Intermittent Porphyria

Acute intermittent porphyria (AIP) results from a decreased activity of hepatic hydroxymethylbilane synthase (HMBS), the third enzyme in the heme biosynthetic pathway. AIP is an autosomal dominant disorder with incomplete penetrance, characterized by acute neurovisceral attacks precipitated by several factors that induce the hepatic 5-aminolevulinic acid synthase, the first enzyme in the heme biosynthesis. Thus, a deficiency in HMBS activity results in an overproduction of porphyrin precursors and the clinical manifestation of the disease. Early diagnosis and counselling are essential to prevent attacks, and mutation analysis is the most accurate method to identify asymptomatic carriers in AIP families. In the present study, we have investigated the molecular defects in 55 unrelated Spanish patients with AIP, identifying 32 HMBS gene mutations, of which six were novel and ten were found in more than one patient. The novel mutations included a missense, an insertion, two deletions, and two splice site variants. Prokaryotic expression studies demonstrated the detrimental effect for the missense mutation, whereas reverse transcription-PCR and sequencing showed aberrant splicing caused by each splice site mutation. These results will allow for an accurate diagnosis of carriers of the disease in these families. Furthermore, they increase the knowledge about the molecular heterogeneity of AIP in Spain.

Hydroxymethylbilane synthase gene mutations and polymorphisms in Brazilian families with acute intermittent porphyria

Acute intermittent porphyria (AIP), an autosomal dominant disorder, is caused by a deficiency of hydroxymethylbilane synthase (HMBS). In the present study, we sought to establish a correlation between HMBS activity with the presence of mutations and polymorphisms. Enzyme activity was measured in red blood cells of four Brazilian unrelated AIP families (n = 124) and in blood donors (n = 80). The HMBS mutations in AIP family members were studied by PCR-SSCP followed by direct sequencing. Six intragenic SNPs (1345 G>A, 1500 T>C, 2377 C>A, 2478 A>G, 3581 A>G, and 7064 C>A) were determined by PCR-RFLP. Abnormal SSCP patterns in exons 7, 9, 12, and 15 were observed. DNA sequencing analysis revealed one nonsense mutation, R149X, two missense mutations, G111R and L338P, and one deletion, CT 730-731. All mutation carriers had lower enzyme activity. All polymorphisms, except 2377 C>A and 7064 C>A, showed no significant differences compared with previous reports. Mutation screening allowed the detection of the missense mutation, L338P, and the 730_731delCT deletion, two as yet unreported mutations in Brazilian AIP patients. Our findings also showed a high frequency of 2478 A>G and 3581 A>G polymorphism combinations suggesting that these polymorphisms contributed to enzymatic activity reduction in our study population.

A challenging diagnosis for potential fatal diseases: recommendations for diagnosing acute porphyrias

Acute porphyrias are a heterogeneous group of metabolic disorders resulting from a variable catalytic defect of four enzymes out of the eight involved in the haem biosynthesis pathway; they are rare and mostly inherited diseases, but in some circumstances, the metabolic disturbance may be acquired. Many different environmental factors or pathological conditions (such as drugs, calorie restriction, hormones, infections, or alcohol abuse) often play a key role in triggering the clinical exacerbation (acute porphyric attack) of these diseases that may often mimic many other more common acute medical and neuropsychiatric conditions and whose delayed diagnosis and treatment may be fatal. In order to obtain an accurate diagnosis of acute porphyria, the knowledge and the use of appropriate diagnostic tools are mandatory, even in order to provide as soon as possible the more effective treatment and to prevent the use of potentially unsafe drugs, which can severely precipitate these diseases, especially in the presence of life-threatening symptoms. In this paper, we provide some recommendations for the diagnostic steps of acute porphyrias by reviewing literature and referring to clinical experience of the board members of the Gruppo Italiano Porfiria (GrIP).

Diagnosis of feline acute intermittent porphyria presenting with erythrodontia requires molecular analyses

Erythrodontia is the hallmark of human congenital erythropoietic porphyria (CEP), but is also a major phenotypic feature of acute intermittent porphyria (AIP) in cats. In this study, detailed biochemical and molecular analyses were performed on two unrelated cats with autosomal dominant AIP that presented with erythrodontia, yellow-brown urine and mild changes in erythrocytes. The cats had elevated concentrations of urinary 5-aminolevulinic acid and porphobilinogen, and half normal erythrocytic hydroxymethylbilane synthase (HMBS) activity. Two novel HMBS mutations were detected; one cat had a deletion (c.107_110delACAG) and one cat had a splicing alteration (c.826-1G>A), both leading to premature stop codons and truncated proteins (p.D36Vfs 6 and p.L276Efs 6, respectively). These studies highlight the importance of appropriate biochemical and molecular genetic analyses for the accurate diagnoses of porphyrias in cats and extend the molecular genetic heterogeneity of feline AIP. Thus, although erythrodontia is a classic sign of congenital erythropoietic porphyria in human beings, cats with erythrodontia may have acute intermittent porphyria, a hepatic porphyria.

Seven novel genetic mutations within the 5'UTR and the housekeeping promoter of HMBS gene responsible for the non-erythroid form of acute intermittent porphyria

Acute intermittent porphyria (AIP) is an autosomal dominant disorder caused by molecular abnormalities in the HMBS gene. This gene is transcribed from two promoters to produce ubiquitous and erythroid specific isoforms of porphobilinogen deaminase (PBGD). In the classical form of AIP, both isoforms are deficient, but about 5% of families have the non-erythroid variant in which only the ubiquitous isoform is affected. Only one mutation sited in the housekeeping promoter has been previously reported as causative for this form of AIP. In this study, we identified one small deletion and six nucleotide substitutions within the 5'UTR and the housekeeping promoter of HMBS gene: c.1-440_-427del14bp; c.1-421G>A; c.1-331C>T; c.1-270G>A; c.1-122T>A; c.1-103C>T; c.1-28A>C. Using luciferase reporter assays and quantitative PCR experiments, we characterized the functional role of these seven novel genetic variants demonstrating that all mutations cause a significant loss of transcriptional activity. Our investigations suggest that these nucleotide substitutions may alter critical binding sites for transcriptional factors, which confirms that these regions represent an important molecular target for pathogenesis of non-erythroid form of acute intermittent porphyria.

Porphyrias at a glance: diagnosis and treatment

Porphyrias are a group of eight rare inherited metabolic disorders of heme biosynthesis pathway. Porphyrias are still underdiagnosed, although examinations of urine and plasma are first-line tests for detecting excess of porphyrins or heme precursors in suspected patients. Diagnosis, particularly for the acute forms, is essential to avoid precipitating factors and the use of triggering drugs. Mutation screening of family members is recommended to identify presymptomatic carriers and to prevent acute attacks. The therapeutic approach should be appropriate regarding specific forms of porphyria and treatment should be started promptly.

The porphyrias: pathophysiology

Porphyrias are a group of inherited and acquired metabolic disorders due to a defect in haem biosynthesis. An enzymatic defect at different steps of haem synthesis leads to tissue accumulation and excessive excretion of porphyrins and/or their toxic precursors. The specific patterns of accumulation determine the variety of clinical manifestations, ranging from acute neurovisceral attacks to skin lesions and liver disease. Most enzyme defects represent partial deficiencies, while familial cases are linked to autosomal or recessive traits. The incomplete penetrance of the genetic defects often requires the triggering or aggravating effect of host-related or environmental factors. While genetics has a role in confirming clinical suspicion and in family screening, biochemical and clinical studies are still central in the diagnosis.

Pro-oxidant and antioxidant factors in acute intermittent porphyria: family studies

Given the crucial role of iron and porphyrins in oxidative cellular damage in the chronic porphyrias, we undertook an extensive study in families with acute porphyrias to evaluate the possible role of similar oxidative damage in these diseases, whose natural history is often also complicated by neoplastic evolution. Four unrelated patients with acute intermittent porphyria (AIP) were studied together with 37 members of four different families. Aminolevulinic acid and porphobilinogen were measured in urine, and porphyrins in urine, plasma and stools. The activity of the congenitally deficient enzyme, porphobilinogen deaminase, and the concentrations of plasma iron, transferrin, ferritin, and various antioxidants (ascorbic acid, retinol, tocopherol, alpha- and beta-carotene, by a personal HPLC method) and the urinary and plasma metabolites of nitrous oxide were also assayed. The results showed no relationship between the observed increase of porphyrin metabolites and the presence of markers of oxidative damage or the decrease of circulating antioxidants: however, when such a decrease was registered, it depended on spontaneous or iatrogenic iron accumulation. We conclude that family screening, recommended for the identification of AIP carriers, must also include evaluation of iron stores with a view to preventing the oxidative damage and in order to forestall the neoplastic evolution of the disease.