Introduction Characterisation of hydroxymethylbilane synthase (HMBS) variants contribute to genotype descriptions and may give an increased understanding of pathomechanisms in AIP. As part of the prospective clinical study Predictors of symptomatic disease and long-term complications in AIP (PredPor), we characterised and categorised identified HMBS variants based on their predicted and observed molecular effects and assessed markers of disease activity and kidney function across gene variant groups.

Method Norwegian patients with latent and current/previously active AIP were invited to participate in PredPor. Health data were recorded for 103 participants via standardised questionnaires, and urine and blood samples were collected. In vitro studies of 12 missense HMBS variants registered in PredPor were conducted using mass spectrometry, native-PAGE, and enzymatic activity. We extracted the highest ever recorded values for urinary porphobilinogen (uPBG) and estimated glomerular filtration rate (eGFR) at inclusion.

Results In the PredPor population, we identified 21 different HMBS variants (NM_000190.4), most common were c.593G>A (n=33) and c.644T>A (n=14). For all other variants, the number of occurrences was 6 or less. Increased uPBG was recorded in 64% of participants. Among women (n=67), 76% had increased uPBG, with 25% higher than 10 times the upper reference limit, as compared to 14% among the men (n=36). 45% of participants (n=102) had normal eGFR above 90ml/min at inclusion, whereas 45% had slightly reduced eGFR (60–89ml/min; G2) and 10% had eGFR of 30–59 ml/min (G3a/b).

Based on biophysical features of the expressed HMBS variants studied in vitro, we categorised them as i) unstable with folding defects; ii) stable apoenzyme; iii) catalytically inactive with accumulation of a single intermediate; and iv) with a distribution of intermediates and a degree of activity.

We established gene variant groups, taking observed features in expressed enzyme into account: 1) predicted to have no gene product through non-coding, null variants, and nonsense-mediated decay (8 variants, n=52); 2) gene product was expressed but unfunctional (categories i-iii above, 9 variants, n=30); and 3) gene product was expressed and relatively functional (category iv above, 4 variants, n=21). No clear differences in uPBG or eGFR classification were observed between the variant groups.

Conclusion There is a lack of genotype-phenotype correlations in AIP, but few studies have investigated relationships between clinical disease and variant groups established on observed and predicted molecular effects. In the PredPor study, we categorised HMBS variants into three groups. Although we were unable to demonstrate significant differences in metabolic activation or kidney function between the groups, further studies are warranted. Furhtermore, increased knowledge on HMBS variants on a molecular level can contribute to our understanding of disease mechanisms in AIP.