Aberrant post-translational modifications and autoimmunity: the case of Primary Biliary Cirrhosis

Primary Biliary Cirrhosis (PBC) is a multifactorial autoimmune liver disease, characterized by the presence of highly specific serum anti-mitochondrial antibodies (AMAs) in 95% of patients. The immunodominant epitopes of AMAs were mapped to the inner lipoyl domain of dihydrolipoamide acetyltransferase, the E2 component of the pyruvate dehydrogenase complex (PDC-E2). In particular, the highly conserved amino acids surrounding the lipoylated K173 of the linear peptide AEIETDKATIGFEVQEEG corresponds to hPDC-E2(167-184) [1]. The importance of the lipoyl-Lysine seems to be confirmed by its relevant localization within the protein. Indeed, conformational studies indicate that K173 is exposed on the tip of a type I betaturn [2, 3]. Factors leading to PBC onset remain poorly understood. It was hypothesized that PBC tolerance breakdown could be elicited by aberrant post-translational modifications (PTMs) on the immunodominant epitope hPDC-E2(167-184) [4]. In this scenario, we decided to focus on 3 different aspects: 1) The importance of lipoic acid and its chiral center at C-6. Both the natural and non-natural diastereoisomers of lipoylated immunodominant epitope were synthesized: [(R)-LipoamideLys173]PDC-E2(167-186)-KKKK and [(S)-Lipoamide-Lys173]PDC-E2(167-186)-KKKK, respectively. The unlipoylated form of the peptide was also synthesized to elucidate the role of lipoic acid: [Lys173]PDC-E2(167-186)-KKKK. 2) The role of lipoyl-Lysine as minimal epitope, and 3) the relevance of K173 exposition on the tip of the beta-turn. We have previously identified CSF114, a 21-mer peptide able to expose optimally post-translationally modified residues on the tip of a type I’ beta-turn [5]. In order to verify the hypothesis of lipoyl-Lysine as minimal epitope and of K173 exposition in PDC-E2 we have synthesized: [(R)-Lipoamide-Lys7]CSF114, [(S)-lipoamide-Lys7]CSF114 and [Lys7]CSF114.