#1941 About a case: ABBA disease in a patient with thymoma
Authors: De Spirt Alice , Guincestre Thomas , Francois Alexis , Promerat Aude , Duval Marion , Nicolazzi Paul , Portier Béatrice , Gibier Jean-Baptiste , Lemoine Cécile 
Published: May 2024

Abstract

Background and Aims
ABBA (antibrush border antibody) disease is a rare autoimmune tubulointerstitial kidney disease with few descriptions in the literature. Diagnosis is based on light microscopy, immunofluorescence, electron microscopy and immunostaining for LRP2 (LDL receptor related protein 2), also known as megalin.

Method
We report the case of a 47-year-old Caucasian man admitted in January 2023 for oedema and acute kidney injury (creatinine 3 mg/dl). Biology revealed nephrotic syndrome with hypoalbuminemia (12 g/L) and albuminuria (13 g/g). Haematuria was also noted (75/mm3). His medical history was notable for myasthenia gravis and epithelial Gougerot.
Serological tests were negative for HIV, hepatitis B and hepatitis C. Anti-GBM and anti-PLA2R were negative, antinuclear antibodies and antineutrophil cytoplasmic antibodies were both positive at 1/80 without specificity. Serum complement (C3 and C4) and protein electrophoresis were normal.
Renal biopsy showed membranous nephropathy, anti-PLA2R, anti-THSD7A, NELL1 and EXT1/2 negative. There was also IgG staining along Bowman's capsule and along part of the tubular basement membrane (TBM). The acute tubular injury was severe.
Morphological studies with thoracic tomodensitometry, thoracic MRI and PET scan revealed a thymoma.
The patient underwent surgery in May. It was a stage 3 thymoma with pulmonary and pericardial invasion.
In June 2023, the patient started haemodialysis due to progression of chronic renal failure.
We performed a second renal biopsy, which confirmed the atypical membranous nephropathy with glomerular and tubular deposits and 10% interstitial sclerosis.
The patient received two injections of RITUXIMAB (30 June and 14 July 2023).

Results
In October 2023, we performed a third renal biopsy due to dialysis dependency three months after RITUXIMAB.
The glomerular lesions were similar to the previous biopsy, with minimal extra-membranous deposits. However, tubulointerstitial lesions were more pronounced than before. There was 25% tubular atrophy and 25% interstitial sclerosis associated with interstitial inflammation. We also identified anti-LRP2 immune deposits along the TBM. All these histopathological features lead to the diagnosis of ABBA.

Conclusion
Anti-LRP2 nephropathy is an orphan disease. It's caused by autoantibodies directed against the proximal tubular brush border, leading to a progressive decline in kidney function. The largest recently published cohort includes 67 patients. Many patients may be misdiagnosed due to the insidious onset and atypical presentation with concomitant glomerular or tubulointerstitial disease. In the cohort, 30% of patients had cancer, including solid organ malignancies (including 1 thymoma). It seems that combined treatment with corticosteroids and another immunosuppressive therapy (cyclophosphamide or rituximab) has better results.
In our case, despite his thymoma surgery and rituximab perfusions, the patient is still on dialysis. The immunosuppressive treatment will be discussed with the oncologist soon.