#234 Clinical spectrum and genetic variants of Alport syndrome and thin basement membrane nephropathy—single center experience

Abstract

Abstract Background and aims Alport syndrome (AS) and thin basement membrane nephropathy (TBMN) are genetically heterogeneous kidney diseases that manifest with kidney involvement, AS also with hearing and visual impairment. Electron microscopic examination shows characteristic ultrastructural changes in the glomerular basement membrane (GBM) in AS and TBMN, but cannot reliably distinguish between the two entities. Therefore, molecular genetic studies and the identification of individual pathogenic variants in the target genes are crucial to confirm the diagnosis. The aim of our study is to determine the significance of the genetic variants associated with the ultrastructural changes of GBM found in the renal biopsy. Method We retrospectively analyzed records of patients from the University Medical Centre Ljubljana in which ultrastructural GBM changes characteristic for AS and TBMN were found in the renal biopsy, including their clinical, laboratory and histological features. Molecular genetic investigations were made using the new generation sequencing method (NGS) to determine germline pathogenic variants in genes directly related to AS and TBMN (COL4A3, COL4A4 and COL4A5) and in genes that are important in syndromes with a related clinical picture (COL4A6, CD151, LMX1B, MYH9, FN1 and MYO1E). The pathogenicity of variants was determined according to the Clinical Molecular Genetic Society best practice guidelines. Results The study included 73 patients with ultrastructurally confirmed thin GBM. Indications for kidney biopsy were chronic nephritis syndrome in 33 patients (45%), acute worsening of renal function in 16 (22%), nephrotic syndrome in 9 (12%), nephrotic proteinuria and chronic kidney disease in 6 (8%), isolated proteinuria in 5 (7%) and suspected vasculitis in 4 patients (6%). At the time of kidney biopsy average age was 50 ± 16 years, average creatinine 112 ± 70 µmol/L and average estimated glomerular filtration rate (eGFR) 65 ± 24 ml/min/1.73 m2, 7 patients had eGFR below 30 ml/min/1.73 m2. Average daily proteinuria was 2.4 ± 3.3 gr and 88% of patients had hematuria. 38% of them have hearing impairment and 43% had positive familiar history of kidney disease. We recorded 33 patients (45%) with genetic variants associated with AS or TBMN: 22 patients (67%) with pathogenic variants, 2 patients (6%) with likely pathogenic and 9 patients (27%) with variants of uncertain significance. The presence of a positive genetic analysis was associated with a higher level of hematuria (p = 0.003), a lower level of proteinuria (p = 0.004) and a positive family history of kidney disease (p < 0.001). It was also associated with hearing loss, but the association was not statistically significant (p = 0.099). Patients with genetic variants had similar eGFR to those with negative genetics (63.9 ml/min/1.73 m2 (58.3-69.4) vs 66.2 ml/min/1.73 m2 (60.0-72.3)), p = 0.698. We also found no differences in GBM thickness in patients without compared to those with identified gene variants (213 nm (160-315) vs. 209 nm (140-410)), p = 0.761. 32 patients (44%) had only focal chronic changes and ultrastructural changes of thin GBM present on kidney biopsy and 41 patients (56%) had also been diagnosed with other renal pathology. When comparing these two groups, patients without associated pathology had lower daily proteinuria (p = 0.048), higher level of hematuria (p = 0.021), frequent hearing impairment (p = 0.008) and also positive genetics analysis (p = 0.016). Conclusion We have shown that in patients with proven ultrastructural alterations in GBM, the presence of so far recognized genetic variants associated with AS and TBMN is 45%, and corresponds to the frequency of other phenotypic characteristics of AS, including hearing impairment and positive family history. In the future, additional clinical tools will be required to accurately determine the role of coexistent thin GBM on the prognosis of patients with other relevant kidney pathology and other genetic tools beyond NGS should be employed to explore other pathologic genetic variants associated with the AS spectrum.