Albumin Homodimers in Patients with Cirrhosis: Clinical and Prognostic Relevance of a Novel Identified Structural Alteration of the Molecule

Bibliographic info

• Authors: Maurizio Baldassarre, Marco Domenicali, Marina Naldi, Maristella Laggetta, Ferdinando A. Giannone, Maurizio Biselli, Daniela Patrono, Carlo Bertucci, Mauro Bernardi, Paolo Caraceni
• Journal: Scientific Reports, 2016, vol. 6, art. 35987; DOI: 10.1038/srep35987
• Institution: University of Bologna, S. Orsola-Malpighi Hospital, Italy

Key question

Do HSA homodimers form in vivo in patients with decompensated cirrhosis, and are they clinically and prognostically relevant?

Methods

• Sample type: Peripheral blood plasma
• Cohort: 123 patients with cirrhosis hospitalized for acute decompensation + 50 age/sex-comparable healthy controls; followed up to 1 year
• Technique: HPLC-ESI-MS (top-down); extended mass range 61,500–138,000 Da to detect dimers (compared to domenicali-2014 which only analyzed 65,800–67,100 Da for monomers)
• In vitro validation: tert-butyl hydroperoxide (t-BuOOH) dose-response experiment on healthy plasma
• Statistical: Spearman correlation with MELD/Child-Pugh; logistic regression for complications; Kaplan-Meier survival

Main findings

1. Three HSA homodimers identified: hdHA-DA (N-term truncated homodimer), hdHA-L (C-term truncated homodimer), hdHA-native (native homodimer)
2. All three homodimers elevated in cirrhosis patients vs healthy controls; further elevated in ACLF patients
3. Homodimerization mechanism: disulfide bond at Cys34 between two monomers — requires free (reduced) Cys34 → homodimerization is an alternative fate of native/non-cysteinylated forms
4. Explains the truncation paradox from domenicali-2014: HSA-DA and HSA-L monomers did not differ between patients and controls in the original study — because these truncated monomers are consumed by homodimerization, which was not detected in that study’s mass range
5. hdHA-DA independently associated with ascites (OR 1.321), renal impairment (OR 1.377), and bacterial infection (OR 1.537) — irrespective of MELD/Child-Pugh scores
6. Native monomeric HA reduced by the homodimerization process — further mechanism beyond direct Cys34 oxidation by which functional albumin is consumed
7. 1-year survival: homodimer levels stratify survival in patients with cirrhosis
8. In vitro: dose-dependent homodimer formation confirmed by t-BuOOH oxidative stress → confirms oxidative origin

PTMs reported

Protein	Species	Modification	Mass range (Da)	Clinical association
HSA	hdHA-DA	Homodimer of N-term truncated	~2× native − 2×115	Ascites, renal impairment, bacterial infection
HSA	hdHA-L	Homodimer of C-term truncated	~2× native − 2×(C-term)	Elevated in ACLF
HSA	hdHA-native	Homodimer of native	~2× 66,438 Da	Elevated in cirrhosis and ACLF

Clinical context

• Disease: Decompensated cirrhosis / Liver fibrosis (F4 / Child-Pugh B/C); ACLF
• Implication: Homodimerization is a clinically significant structural alteration in cirrhosis, independent of the oxidation landscape; adds to the pool of non-functional albumin; key driver of effective albumin deficit beyond cysteinylation/sulfinylation

Limitations

• Relative quantification only (%)
• Single-center (Bologna)
• Dimerization mechanism confirmed in vitro but not mapped to a specific site by BU in this paper
• Clinical relevance of individual dimer subtypes not fully disentangled

Connections

• HSA — adds homodimer layer to the isoform landscape
• Cysteinylation / Oxidation — Cys34 must be free (not oxidized) for homodimerization to occur
• Liver fibrosis / ACLF — disease context
• domenicali-2014 — predecessor; explains the truncation paradox from that paper
• baldassarre-2021-ealb — effective albumin concept; homodimers contribute to eAlb deficit
• oettl-2013 — parallel finding that irreversible oxidation (HNA2) correlates with survival; homodimers represent a third mechanism

My take home notes

• Conceptually important: the domenicali-2014 paper found no difference in HSA-DA and HSA-L between patients and controls. This paper resolves the paradox — those monomers are being consumed by homodimerization. The mass range extension (to 138 kDa) was the methodological key.
• Homodimerization is an additional loss of functional albumin beyond Cys34 oxidation. In advanced ACLF, multiple mechanisms simultaneously deplete the native, functional pool.
• The disulfide-mediated dimerization mechanism also implies that the Cys34 site in a dimer is completely unavailable for antioxidant function or drug binding — functional loss is total for these molecules.
• Not detected in standard top-down LC-MS analysis if mass range is restricted to monomer window — this is a methodological gap for any pipeline that does not extend to 130+ kDa.