Clinical ApoA-IV amyloid is associated with fibrillogenic signal sequence

An unusual phenotype of hereditary AApoAI amyloidosis caused by a novel Asp20Tyr substitution is linked to pH-dependent aggregation of apolipoprotein A-I

Apolipoprotein A-I (apoA-I) plays beneficial roles as the major structural and functional protein on plasma high-density lipoproteins (HDL). However, APOA1 gene mutations can cause protein misfolding and pathologic amyloid deposition in various organs in human hereditary AApoAI amyloidosis, a potentially lethal systemic disease. We report esophageal and duodenal AApoAI amyloidosis in a 56-year-old patient with Barrett's esophagus, a condition involving chronic acid reflux. Amyloid deposits contained full-length apoA-I featuring a novel D20Y mutation identified by gene sequencing and protein mass spectrometry. Genetic analysis of asymptomatic family members revealed autosomal dominant inheritance. Fibril formation by the full-length variant apoA-I rather than its fragments and the location of the mutation in a conserved amyloid-prone N-terminal segment were highly unusual for hereditary AApoA-I amyloidosis. Structural and stability studies of the recombinant D20Y and wild-type apoA-I showed small but significant mutation-induced structural perturbations in the native lipid-free protein at pH 7.4. Major destabilization and aggregation of the variant protein were observed at pH 4.0. We propose that acidic conditions in Barrett's esophagus promoted protein misfolding and amyloid formation by the D20Y variant. These findings expand our understanding of the clinical features and molecular basis of AApoAI amyloidosis and suggest clinical strategies.

Small Bowel Perforation in Roux-en-Y Gastric Bypass (RYGB) Secondary to Apolipoprotein A-IV (AApoA-IV) Type Amyloidosis

BACKGROUND

ApoA-IV amyloidosis is a rare disease that involves the deposition of ApoA-IV protein aggregates in tissues. It commonly presents as cardiac or renal disease, but can, in rare cases, cause small bowel perforation.

METHODS

This study describes a case of ApoA-IV type amyloidosis causing small bowel perforation after conversion of a sleeve gastrectomy (SG) into a RYGB in a Crohn's disease (CD) and rheumatoid arthritis (RA) patient. It also considers the indications for bariatric and anti-reflux surgery in the setting of co-morbid inflammatory bowel disease (IBD), gastro-oesophageal reflux disease (GORD), and obesity.

RESULTS

Obesity can reduce the efficacy of IBD medications and drives a pro-inflammatory state that may worsen IBD, however IBD patients present an operative challenge due to risk of more intestinal adhesions, potential intolerance to intestinal bypass, and risk of affecting options for future bowel resections if required. SG is often chosen over RYGB for CD patients due to limited short-term complications. However, when considering co-morbid GORD, the long-term risk of medication resistant GORD, erosive oesophagitis, and Barrett's oesophagus with SG is significant, especially given that SG is an irreversible procedure.

CONCLUSION

There is growing evidence that bariatric surgery in IBD patients is both safe and effective, however the decision to perform bariatric surgery in an IBD patient involves consideration of the intricate interplay between obesity and IBD.

Age-related TFEB downregulation in proximal tubules causes systemic metabolic disorders and occasional apolipoprotein A4-related amyloidosis

With the aging of society, the incidence of chronic kidney disease (CKD), a common cause of death, has been increasing. Transcription factor EB (TFEB), the master transcriptional regulator of the autophagy/lysosomal pathway, is regarded as a promising candidate for preventing various age-related diseases. However, whether TFEB in the proximal tubules plays a significant role in elderly patients with CKD remains unknown. First, we found that nuclear TFEB localization in proximal tubular epithelial cells (PTECs) declined with age in both mice and humans. Next, we generated PTEC-specific Tfeb-deficient mice and bred them for up to 24 months. We found that TFEB deficiency in the proximal tubules caused metabolic disorders and occasionally led to apolipoprotein A4 (APOA4) amyloidosis. Supporting this result, we identified markedly decreased nuclear TFEB localization in the proximal tubules of elderly patients with APOA4 amyloidosis. The metabolic disturbances were accompanied by mitochondrial dysfunction due to transcriptional changes involved in fatty acid oxidation and oxidative phosphorylation pathways, as well as decreased mitochondrial clearance. This decreased clearance was reflected by the accumulation of mitochondria-lysosome-related organelles, which depended on lysosomal function. These results shed light on the presumptive mechanisms of APOA4 amyloidosis pathogenesis and provide a therapeutic strategy for CKD-related metabolic disorders and APOA4 amyloidosis.

Autosomal dominant chronic tubulointerstitial nephropathy: do not forget amyloidosis

Amyloidosis is a rare cause of inherited kidney disease, with most variants responsible for prominent glomerular involvement. In this issue, Kmochová et al. reported the first description of autosomal dominant medullary amyloidosis due to apolipoprotein A4 variants, resulting in slowly progressive chronic kidney disease with minimal proteinuria. Combining next-generation sequencing with histopathological studies incorporating Congo red staining and mass spectrometry should be considered in the diagnostic workup of hereditary tubulointerstitial disorders not identified after routine genetic testing.

Amyloid fibril cytotoxicity and associated disorders

Misfolded proteins assemble into fibril structures that are called amyloids. Unlike usually folded proteins, misfolded fibrils are insoluble and deposit extracellularly or intracellularly. Misfolded proteins interrupt the function and structure of cells and cause amyloid disease. There is increasing evidence that the most pernicious species are oligomers. Misfolded proteins disrupt cell function and cause cytotoxicity by calcium imbalance, mitochondrial dysfunction, and intracellular reactive oxygen species. Despite profound impacts on health, social, and economic factors, amyloid diseases remain untreatable. To develop new therapeutics and to understand the pathological manifestations of amyloidosis, research into the origin and pathology of amyloidosis is urgently needed. This chapter describes the basic concept of amyloid disease and the function of atypical amyloid deposits in them.