Department of Nephrology, Wuhan Children's Hospital (Wuhan Maternal and Child Healthcare Center), Tongji Medical College, Huazhong University of Science & Technology, Wuhan, China.
Clinical Research Center, Wuhan Children's Hospital (Wuhan Maternal and Child Healthcare Center), Tongji Medical College, Huazhong University of Science & Technology, Wuhan, China.
Mol Genet Genomic Med. 2024 Mar;12(3):e2353. doi: 10.1002/mgg3.2353.
Although proteinuria is long recognized as an independent risk factor for progressive chronic kidney diseases, not all forms of proteinuria are detrimental to kidney function, one of which is isolated proteinuria caused by cubilin (CUBN)-specific mutations. CUBN encodes an endocytic receptor, initially found to be responsible for the Imerslund-Gräsbeck syndrome (IGS; OMIM #261100) characterized by a combined phenotype of megaloblastic anemia and proteinuria.
After analyzing their clinical and pathological characterizations, next-generation sequencing for renal disease genes or whole-exome sequencing (WES) was performed on four patients with non-progressive isolated proteinuria. CUBN biallelic pathogenic variants were identified and further analyzed by cDNA-PCR sequencing, immunohistochemistry, minigene assay, and multiple in silico prediction tools, including 3D protein modeling.
Here, we present four patients with isolated proteinuria caused by CUBN C-terminal biallelic pathogenic variants, all of which showed no typical IGS symptoms, such as anemia and vitamin B12 deficiency. Their urine protein levels fluctuated between +~++ and estimated glomerular filtration rate (eGFR) were normal or slightly higher. Mild mesangial hypercellularity was found in three children's renal biopsies. A homozygous splice-site variant of CUBN (c.6821+3 (IVS44) A>G) was proven to result in the exon 44 skipping and premature translation termination by cDNA sequencing and immunohistochemistry. Compound heterozygous mutations were identified among the other three children, including another novel splice-site variant (c.10764+1 (IVS66) G>A) causing the retention of first 4 nucleotides in intron 66 by minigene assay, two unreported missense mutations (c.4907G>A (p.R1636Q); c. 9095 A>G (p.Y3032C)), and two reported missense mutations in China (c.8938G>A (p.D2980N); c. 9287T>C (p.L3096P)), locating behind the vitamin B12-binding domain, affecting CUB11, CUB16, CUB22, CUB23, and CUB27 domains, respectively.
These results demonstrate that above CUBN mutations may cause non-progressive and isolated proteinuria, expanding the variant spectrum of CUBN and benefiting our understanding of proteinuria and renal function.
尽管蛋白尿长期以来被认为是慢性肾脏病进展的独立危险因素，但并非所有形式的蛋白尿都对肾功能有害，其中一种是由巨核细胞（CUBN）特异性突变引起的孤立性蛋白尿。CUBN 编码一种内吞受体，最初发现其负责 Imerslund-Gräsbeck 综合征（IGS；OMIM #261100），其特征为巨幼细胞性贫血和蛋白尿的联合表型。
对四名非进行性孤立性蛋白尿患者进行肾脏疾病基因的下一代测序或外显子组测序（WES），分析其临床和病理特征。确定 CUBN 双等位基因致病性变异，并通过 cDNA-PCR 测序、免疫组织化学、小基因测定和多个计算机预测工具（包括 3D 蛋白质建模）进一步分析。
在此，我们介绍了四名由 CUBN C 端双等位基因致病性变异引起的孤立性蛋白尿患者，他们均无典型的 IGS 症状，如贫血和维生素 B12 缺乏。他们的尿蛋白水平在++~+++之间波动，估计肾小球滤过率（eGFR）正常或略高。三名儿童的肾活检显示轻度系膜细胞增生。通过 cDNA 测序和免疫组织化学证实，CUBN 的纯合剪接位点变异（c.6821+3（IVS44）A>G）导致外显子 44 跳过和提前翻译终止。另外三名儿童中发现了复合杂合突变，包括另一个新的剪接位点变异（c.10764+1（IVS66）G>A），通过小基因测定导致内含子 66 中前 4 个核苷酸的保留，两个未报道的错义突变（c.4907G>A（p.R1636Q）；c.9095 A>G（p.Y3032C））和两个在中国报道的错义突变（c.8938G>A（p.D2980N）；c.9287T>C（p.L3096P）），分别位于维生素 B12 结合域后面，影响 CUB11、CUB16、CUB22、CUB23 和 CUB27 结构域。
这些结果表明，上述 CUBN 突变可能导致非进行性和孤立性蛋白尿，扩展了 CUBN 的变异谱，有助于我们了解蛋白尿和肾功能。
