Exome-wide burden analysis of rare variants in individuals with thin basement membrane nephropathy

薄基底膜肾病（TBMN）是显微镜下血尿（MH）最常见的原因，在一般人群中发生率为1%。其特征是持续性MH和少量蛋白尿（如果有的话）。组织学上，电镜下肾活检标本肾小球基底膜（GBM）均匀变薄。成熟的GBM由COL4A3、COL4A4、COL4A5基因编码的IV型胶原α -链3、4、5异源三聚体组成。在大约40%的TBMN病例中，可以鉴定出COL4A3/COL4A4的单一杂合致病变异。TBMN具有常染色体显性遗传模式，外显率降低。尽管如此，在一项针对13个塞浦路斯TBMN家族的研究中，携带COL4A3/COL4A4杂合致病变异的个体中，约40%发展为慢性肾脏疾病，约20%发展为终末期肾脏疾病。此外，携带杂合致病性COL4A3/COL4A4变异的TBMN个体随着时间的推移可发展为局灶性节段性肾小球硬化。因此，TBMN不是一种良性的家族性血液病。TBMN的遗传数据来自COL4A3/COL4A4基因分析和特定单核苷酸多态性测序，而不是来自无偏倚的外显子组/全基因组罕见变异负担分析。与TBMN相比，Alport综合征（AS）是一种明确的高外显率单基因MH，可导致进行性肾衰竭，肾活检的病理改变，并经常出现双侧高频感音神经性听力损失和前晶状体等眼部变化等肾外特征。COL4A3和COL4A4的双等位致病变异与常染色体隐性AS（约占AS的15%）和COL4A5基因的半合子致病变异与x连锁AS相关。本研究的目的是对300名未怀疑AS但具有临床确定的TBMN的个体进行外显子组测序，以便与20,000个对照外显子组进行罕见变异的外显子组范围负担分析（已经可用；Munich exome Server）。本研究将解答罕见编码非同义变异体基因在TBMN中富集的问题，并计算罕见变异体负担的比值比。这一数据对于TBMN患者的遗传咨询和临床管理具有重要意义，并可能为TBMN和血尿肾小球疾病的遗传结构提供进一步的见解。这项研究将是TBMN研究中的第一个此类研究，TBMN是一种在普通人群中常见的疾病，具有很高的经济影响。

Thin basement membrane nephropathy (TBMN) is the most prevalent cause for microscopic hematuria (MH) and has a frequency of 1% in the general population. It is characterized by persistent MH and minimal proteinuria, if any. Histologically, a uniforme thinning of the glomerular basement membrane (GBM) on electron microscopy of a renal biopsy specimen can be identified. The mature GBM is composed of a heterotrimer of alpha-chains 3, 4 and 5 of type IV collagen encoded by genes COL4A3, COL4A4, COL4A5, respectively. In about 40% of TBMN cases, a single heterozygous pathogenic variant in COL4A3/COL4A4 can be identified. TBMN features an autosomal dominant inheritance pattern with reduced penetrance. Nonetheless, in one study featuring 13 Cypriot families with TBMN, about 40% of individuals with a heterozygous pathogenic variant in COL4A3/COL4A4 developed chronic kidney disease and about 20% progressed to end-stage renal disease. Moreover, individuals with TBMN carrying a heterozygous pathogenic COL4A3/COL4A4 variant can develop focal segemental glomerulosclerosis over time. Hence, TBMN is not a benign familial hematuric disease. Genetic data on TBMN comes from analysis of the genes COL4A3/COL4A4 and sequencing of specific single-nucleotide polymorphisms and not from unbiased exome-/genome-wide rare variant burden analyses. In comparison to TBMN, Alport syndrome (AS) is a clear-cut high-penetrance monogenic MH leading to progressive renal failure, pathognomonic changes on kidney biopsy and, frequently, extrarenal features like bilateral high-frequency sensorineural hearing loss and ocular changes such as anterior lenticonus. Biallelic pathogenic variants in COL4A3 and COL4A4 are associated with autosomal recessive AS (about 15% of AS) and hemizygous pathogenic variants in the gene COL4A5 with X-linked AS.The aim of this study is to perform exome sequencing in 300 individuals without suspicion of AS but with clinically-ascertained TBMN in order to conduct an exome-wide burden analysis of rare variants compared to 20,000 control exomes (already available; Munich Exome Server).This study will provide answers to the question in which genes rare coding non-synonymous variants are enriched in TBMN along with odds ratio calculations of rare variant burden. This data is important for genetic counselling and clinical management of individuals with TBMN and may give further insights in the genetic architecture of TBMN and hematuric glomerular disease in general. The study will be the first of its kind in TBMN research, a disorder common in the general population with a high economic impact.