title: Rare loss-of-function mutation in complement component C3 provides insight into molecular and pathophysiological determinants of complement activity.
abstract: The plasma protein C3 is a central element in the activation and effector functions of the complement system. A hereditary dysfunction of C3 that prevents complement activation via the alternative pathway (AP) was described previously in a Swedish family, but its genetic cause and molecular consequences have remained elusive. In this study, we provide these missing links by pinpointing the dysfunction to a point mutation in the β-chain of C3 (c.1180T > C; p.Met(373)Thr). In the patient's plasma, AP activity was completely abolished and could only be reconstituted with the addition of normal C3. The M373T mutation was localized to the macroglobulin domain 4 of C3, which contains a binding site for the complement inhibitor compstatin and is considered critical for the interaction of C3 with the AP C3 convertase. Structural analyses suggested that the mutation disturbs the integrity of macroglobulin domain 4 and induces conformational changes that propagate into adjacent regions. Indeed, C3 M373T showed an altered binding pattern for compstatin and surface-bound C3b, and the presence of Thr(373) in either the C3 substrate or convertase-affiliated C3b impaired C3 activation and opsonization. In contrast to known gain-of-function mutations in C3, patients affected by this loss-of-function mutation did not develop familial disease, but rather showed diverse and mostly episodic symptoms. Our study therefore reveals the molecular mechanism of a relevant loss-of-function mutation in C3 and provides insight into the function of the C3 convertase, the differential involvement of C3 activity in clinical conditions, and some potential implications of therapeutic complement inhibition.
authors.affiliation: Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, PA 19104;
authors.forename: Georgia
authors.initials: G
authors.lastname: Sfyroera
authors.affiliation: Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, PA 19104;
authors.forename: Daniel
authors.initials: D
authors.lastname: Ricklin
authors.affiliation: Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, PA 19104;
authors.forename: Edimara S
authors.initials: ES
authors.lastname: Reis
authors.affiliation: Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, PA 19104;
authors.forename: Hui
authors.initials: H
authors.lastname: Chen
authors.affiliation: Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455;
authors.forename: Emilia L
authors.initials: EL
authors.lastname: Wu
authors.affiliation: Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455;
authors.forename: Yiannis N
authors.initials: YN
authors.lastname: Kaznessis
authors.affiliation: Department of Immunology, Genetics and Pathology, Rudbeck Laboratory, Uppsala University, SE-751 85 Uppsala, Sweden; and Linnæus Center of Biomaterials Chemistry, Linnæus University, SE-391 82 Kalmar, Sweden.
authors.forename: Kristina N
authors.initials: KN
authors.lastname: Ekdahl
authors.affiliation: Linnæus Center of Biomaterials Chemistry, Linnæus University, SE-391 82 Kalmar, Sweden.
authors.forename: Bo
authors.initials: B
authors.lastname: Nilsson
authors.affiliation: Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, PA 19104; lambris@upenn.edu.
authors.forename: John D
authors.initials: JD
authors.lastname: Lambris
mesh_terms: D000328:Adult; D019943:Amino Acid Substitution; D003167:Complement Activation / Q000235:genetics / Q000276:immunology; D003176:Complement C3 / Q000737:chemistry / Q000235:genetics* / Q000276:immunology*; D050577:Complement C3-C5 Convertases / Q000378:metabolism; D003170:Complement Pathway, Alternative; D004252:DNA Mutational Analysis; D005091:Exons; D005260:Female; D006801:Humans; D008958:Models, Molecular; D009154:Mutation; D020125:Mutation, Missense; D010375:Pedigree; D011485:Protein Binding; D011487:Protein Conformation; D054730:Protein Interaction Domains and Motifs
chemical_list: D003176:Complement C3; D050577:Complement C3-C5 Convertases