The interplay between genotype, metabolic state and cofactor treatment governs phenylalanine hydroxylase function and drug response

The discovery of a pharmacological treatment for phenylketonuria (PKU) raised new questions about function and dysfunction of phenylalanine hydroxylase (PAH), the enzyme deficient in this disease. To investigate the interdependence of the genotype, the metabolic state (phenylalanine substrate) and treatment (BH4 cofactor) in the context of enzyme function in vitro and in vivo, we (i) used a fluorescence-based method for fast enzyme kinetic analyses at an expanded range of phenylalanine and BH4 concentrations, (ii) depicted PAH function as activity landscapes, (iii) retraced the analyses in eukaryotic cells, and (iv) translated this into the human system by analyzing the outcome of oral BH4 loading tests. PAH activity landscapes uncovered the optimal working range of recombinant wild-type PAH and provided new insights into PAH kinetics. They demonstrated how mutations might alter enzyme function in the space of varying substrate and cofactor concentrations. Experiments in eukaryotic cells revealed that the availability of the active PAH enzyme depends on the phenylalanine-to-BH4 ratio. Finally, evaluation of data from BH4 loading tests indicated that the patient's genotype influences the impact of the metabolic state on drug response. The results allowed for visualization and a better understanding of PAH function in the physiological and pathological state as well as in the therapeutic context of cofactor treatment. Moreover, our data underscore the need for more personalized procedures to safely identify and treat patients with BH4-responsive PAH deficienc

The Domain-Specific and Temperature-Dependent Protein Misfolding Phenotype of Variant Medium-Chain acyl-CoA Dehydrogenase

The implementation of expanded newborn screening programs reduced mortality and morbidity in medium-chain acyl-CoA dehydrogenase deficiency (MCADD) caused by mutations in the ACADM gene. However, the disease is still potentially fatal. Missense induced MCADD is a protein misfolding disease with a molecular loss-of-function phenotype. Here we established a comprehensive experimental setup to analyze the structural consequences of eight ACADM missense mutations (p. Ala52Val, p. Tyr67His, p. Tyr158His, p. Arg206Cys, p. Asp266Gly, p. Lys329Glu, p. Arg334Lys, p. Arg413Ser) identified after newborn screening and linked the corresponding protein misfolding phenotype to the site of side-chain replacement with respect to the domain. With fever being the crucial risk factor for metabolic decompensation of patients with MCADD, special emphasis was put on the analysis of structural and functional derangements related to thermal stress. Based on protein conformation, thermal stability and kinetic stability, the molecular phenotype in MCADD depends on the structural region that is affected by missense-induced conformational changes with the central beta-domain being particularly prone to structural derangement and destabilization. Since systematic classification of conformational derangements induced by ACADM mutations may be a helpful tool in assessing the clinical risk of patients, we scored the misfolding phenotype of the variants in comparison to p. Lys329Glu (K304E),the classical severe mutation, and p. Tyr67His (Y42H),discussed to be mild. Experiments assessing the impact of thermal stress revealed that mutations in the ACADM gene lower the temperature threshold at which MCAD loss-of-function occurs. Consequently, increased temperature as it occurs during intercurrent infections, significantly increases the risk of further conformational derangement and loss of function of the MCAD enzyme explaining the life-threatening clinical courses observed during fever episodes. Early and aggressive antipyretic treatment thus may be life-saving in patients suffering from MCADD

Pahenu1 is a mouse model for tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency and promotes analysis of the pharmacological chaperone mechanism in vivo

The recent approval of sapropterin dihydrochloride, the synthetic form of 6[R]-l-erythro-5,6,7,8-tetrahydrobiopterin (BH4), for the treatment of phenylketonuria (PKU) as the first pharmacological chaperone drug initiated a paradigm change in the treatment of monogenetic diseases. Symptomatic treatment is now replaced by a causal pharmacological therapy correcting misfolding of the defective phenylalanine hydroxylase (PAH) in numerous patients. Here, we disclose BH4 responsiveness in Pahenu1, a mouse model for PAH deficiency. Loss of function resulted from loss of PAH, a consequence of misfolding, aggregation, and accelerated degradation of the enzyme. BH4 attenuated this triad by conformational stabilization augmenting the effective PAH concentration. This led to the rescue of the biochemical phenotype and enzyme function in vivo. Combined in vitro and in vivo analyses revealed a selective pharmaceutical action of BH4 confined to the pathological metabolic state. Our data provide new molecular-level insights into the mechanisms underlying protein misfolding with loss of function and support a general model of pharmacological chaperone-induced stabilization of protein conformation to correct this intracellular phenotype. Pahenu1 will be essential for pharmaceutical drug optimization and to design individually tailored therapie

Protein misfolding is the molecular mechanism underlying MCADD identified in newborn screening

Newborn screening (NBS) for medium-chain acyl-CoA dehydrogenase deficiency (MCADD) revealed a higher birth prevalence and genotypic variability than previously estimated, including numerous novel missense mutations in the ACADM gene. On average, these mutations are associated with milder biochemical phenotypes raising the question about their pathogenic relevance. In this study, we analyzed the impact of 10 ACADM mutations identified in NBS (A27V, Y42H, Y133H, R181C, R223G, D241G, K304E, R309K, I331T and R388S) on conformation, stability and enzyme kinetics of the corresponding proteins. Partial to total rescue of aggregation by co-overexpression of GroESL indicated protein misfolding. This was confirmed by accelerated thermal unfolding in all variants, as well as decreased proteolytic stability and accelerated thermal inactivation in most variants. Catalytic function varied from high residual activity to markedly decreased activity or substrate affinity. Mutations mapping to the β-domain of the protein predisposed to severe destabilization. In silico structural analyses of the affected amino acid residues revealed involvement in functionally relevant networks. Taken together, our results substantiate the hypothesis of protein misfolding with loss-of-function being the common molecular basis in MCADD. Moreover, considerable structural alterations in all analyzed variants do not support the view that novel mutations found in NBS bear a lower risk of metabolic decompensation than that associated with mutations detected in clinically ascertained patients. Finally, the detailed insight into how ACADM missense mutations induce loss of MCAD function may provide guidance for risk assessment and counseling of patients, and in future may assist delineation of novel pharmacological strategies

Pah enu1 is a mouse model for tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency and promotes analysis of the pharmacological chaperone mechanism in vivo

The recent approval of sapropterin dihydrochloride, the synthetic form of 6[R]-l-erythro-5,6,7,8-tetrahydrobiopterin (BH4), for the treatment of phenylketonuria (PKU) as the first pharmacological chaperone drug initiated a paradigm change in the treatment of monogenetic diseases. Symptomatic treatment is now replaced by a causal pharmacological therapy correcting misfolding of the defective phenylalanine hydroxylase (PAH) in numerous patients. Here, we disclose BH4 responsiveness in Pahenu1, a mouse model for PAH deficiency. Loss of function resulted from loss of PAH, a consequence of misfolding, aggregation, and accelerated degradation of the enzyme. BH4 attenuated this triad by conformational stabilization augmenting the effective PAH concentration. This led to the rescue of the biochemical phenotype and enzyme function in vivo. Combined in vitro and in vivo analyses revealed a selective pharmaceutical action of BH4 confined to the pathological metabolic state. Our data provide new molecular-level insights into the mechanisms underlying protein misfolding with loss of function and support a general model of pharmacological chaperone-induced stabilization of protein conformation to correct this intracellular phenotype. Pahenu1 will be essential for pharmaceutical drug optimization and to design individually tailored therapie

Mutations.

<p>Mutations.</p

Mean transition midpoints of thermal denaturation (CD spectroscopy, FAD fluorescence, ANS fluorescence).

<p>Mean transition midpoints of thermal denaturation (CD spectroscopy, FAD fluorescence, ANS fluorescence).</p

Molecular phenotypes of thermal, kinetic, and conformational stability.

<p>Data derived from experiments depicted in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0093852#pone-0093852-g002" target="_blank">Figures 2</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0093852#pone-0093852-g003" target="_blank">3</a>, and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0093852#pone-0093852-g005" target="_blank">5</a> were combined and visualized as 3D plot. Thermal stability (y-axis) refers to midpoints of thermal denaturation determined by CD spectroscopy, FAD-DSF, and ANS-DSF. Kinetic stability (x-axis) refers to the E_A determined by thermal inactivation experiments. Conformational stability (z-axis) refers to secondary structure (CD spectra), hydrophobicity (ANS fluorescence), and FAD binding capacity (intrinsic FAD fluorescence) in the absence of heat-induced stress.</p

Missense mutations induce conformational alterations of variant MCAD proteins.

<p>(A) The composition of secondary structure elements of wild-type and variant MCAD in the absence of thermal stress was determined by circular dichroism applying the CD spectra deconvolution software CDNN <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0093852#pone.0093852-Bhm1" target="_blank">[23]</a>. The spectra showed typical secondary structure characteristics of proteins with predominating α-helical content. (B) Hydrophobicity of wild-type and variant MCAD proteins in the absence of thermal stress was determined using the fluorescent dye ANS. An increase in the fluorescence signal points to increased binding of the ANS dye to hydrophobic groups. Data represent means and SEM of three independent experiments. (C) Structural integrity of wild-type and variant MCAD proteins in the absence of thermal stress was determined by intrinsic FAD fluorescence. An increase in the fluorescence signal points to a release of FAD from the MCAD protein. Data represent means and SEM of three independent experiments. (B, C) Data are grouped according to the domain affected by mutations (αD_N, N-terminal α-helical domain; βD, β-sheet domain; αD_C, C-terminal α-helical domain).</p