Folding Defects Leading to Primary Hyperoxaluria

Abstract

Protein misfolding is becoming one of the main mechanisms underlying inherited enzymatic deficits. This review is focused on primary hyperoxalurias, a group of disorders of glyoxylate detoxification associated with massive calcium oxalate deposition mainly in the kidneys. The most common and severe form, primary hyperoxaluria Type I, is due to the deficit of liver peroxisomal alanine/glyoxylate aminotransferase (AGT). Various studies performed in the last decade clearly evidence that many pathogenic missense mutations prevent the AGT correct folding, leading to various downstream effects including aggregation, increased degradation or mistargeting to mitochondria. Primary hyperoxaluria Type II and primary hyperoxaluria Type III are due to the deficit of glyoxylate reductase/hydroxypyruvate reductase (GRHPR) and 4-hydroxy-2-oxoglutarate aldolase (HOGA1), respectively. Although the molecular features of pathogenic variants of GRHPR and HOGA1 have not been investigated in detail, the data available suggest that some of them display folding defects. Thus, primary hyperoxalurias can be ranked among protein misfolding disorders, because in most cases the enzymatic deficit is due to the inability of each enzyme to reach its native and functional conformation. It follows that molecules able to improve the folding yield of the enzymes involved in each disease form could represent new therapeutic strategies.

Evolutionary Divergent Suppressor Mutations in Conformational Diseases

“…Structurally and functionally, each monomer can be divided in three regions: a short and extended N-terminal tail (NTT, residues 1–21) that grabs the monomers in the dimer, an N-terminal domain (NTD, residues 22–282) containing the active site and most of the dimerization interface, and a C-terminal domain (CTD, residues 283–392) containing the peroxisomal targeting sequence [ 59 ]. Human AGT activity is essential for detoxification of the metabolic intermediary glyoxylate in liver peroxisomes, preventing subsequent formation of oxalate and disease development [ 58 , 60 ]. AGT subcellular location varies among mammals, possibly as a reflection of evolutionary origins in metabolic partitioning between different subcellular organelles [ 61 , 62 , 63 , 64 ].…”

“…AGT subcellular location varies among mammals, possibly as a reflection of evolutionary origins in metabolic partitioning between different subcellular organelles [ 61 , 62 , 63 , 64 ]. There are over 200 mutations in the AGXT gene associated with a rare disease (primary hyperoxaluria type I, PH1) inherited in an autosomic recessive manner, in which patients accumulate oxalate that eventually causes renal failure and premature death [ 58 , 60 ]. The AGXT gene exists as two polymorphic variants, the most frequent, named as major allele (or wild type (WT)), and a less frequent minor allele (or LM), which carries two single amino acid changes (P11L and I340M) [ 58 ].…”

“…Although the mechanisms by which PH1 mutations cause AGT LOF are diverse, two of them have emerged as very common and particularly interesting for discussion here: aggregation and mitochondrial mistargeting [ 60 , 66 ]. Both are rooted in an enhanced tendency of the protein to populate NNS and altered interaction with the proteostasis machinery upon mutation [ 60 , 66 , 67 ], which can be thus framed into the mechanisms depicted in Figure 2 . However, in contrast to NQO1, in which the altered structure and dynamics of certain functional sites can be blamed to cause a given pathogenic effect ( Figure 2 ), such knowledge is not currently available for aggregation and mistargeting of PH1-causing mutants.…”

“…Nevertheless, the P11L polymorphism is known to strongly accelerate protein denaturation in the apo-state (without PLP) increasing the population of NNS, effects which are strengthened by PH1 mutations associated with aggregation (e.g., I244T) and mistargeting (the most common, G170R) [ 67 , 68 , 69 , 70 ]. This increased population of NNS could explain the enhanced interaction of missense variants with molecular chaperones (Hsp 40, 70, 90, and 60) and their increased aggregation propensity [ 60 , 66 , 67 , 68 , 69 , 71 ]. In this regard, the N-terminal region of AGT seems to protect towards misfolding [ 72 ], a role likely perturbed by the P11L substitution at structural and dynamic levels [ 67 ].…”

Dimerization Drives Proper Folding of Human Alanine:Glyoxylate Aminotransferase But Is Dispensable for Peroxisomal Targeting

“…Human AGT is present as two polymorphic forms, named major and minor allele, the latter characterized by two amino acid changes, Pro11Leu and Ile340Met [ 27 ]. Although the Pro11Leu mutation generates a putative N-terminal mitochondrial targeting sequence, protein mistargeting is prevented unless PH1-linked mutations are present that destabilize the dimeric structure and enhance interaction with molecular chaperones [ 28 , 29 , 30 , 31 ]. To assess the contribution of dimerization to AGT folding and peroxisomal targeting, here, we compared the two native dimeric forms of the protein encoded by the major (D Ma ) and the minor (D Mi ) allele, with the corresponding artificial monomeric forms (M Ma and M Mi ) obtained by the interfacial mutations R118A/F238S/F240S.…”