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）的最常见原因，在一般人群中的发生率为1%。其特征是持续的MH和少量蛋白尿（如果有的话）。在组织学上，肾活检标本的电子显微镜下可见肾小球基底膜（GBM）均匀变薄。成熟GBM由分别由基因COL 4A 3、COL 4A 4、COL 4A 5编码的IV型胶原的α-链3、4和5的异源三聚体组成。在约40%的TBMN病例中，可以鉴定出COL 4A 3/COL 4A 4中的单个杂合致病性变体。TBMN的特征是常染色体显性遗传，遗传率降低。尽管如此，在一项针对13个塞浦路斯TBMN家族的研究中，约40%的COL 4A 3/COL 4A 4杂合致病性变异个体发展为慢性肾病，约20%进展为终末期肾病。此外，携带杂合致病性COL 4A 3/COL 4A 4变异体的TBMN个体可随时间发展成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尿病。TBMN的遗传数据来自基因COL 4A 3/COL 4A 4的分析和特定单核苷酸多态性的测序，而不是来自无偏倚的外显子组/全基因组罕见变异负荷分析。与TBMN相比，Alport综合征（AS）是一种明确的高转移率单基因MH，可导致进行性肾衰竭、肾活检病理改变以及常见的肾外特征，如双侧高频感音神经性听力损失和眼部改变，如前圆锥形晶状体。COL 4A 3和COL 4A 4的双等位基因致病性变异与常染色体隐性遗传AS相关本研究的目的是在300名没有疑似AS但具有临床确定的TBMN的个体中进行外显子组测序，以便进行罕见变体的外显子组范围的负荷分析，该研究将提供对以下问题的答案，其中编码罕见非同义变体的基因在TBMN中富集，沿着罕见变体负荷的比值比计算。这些数据对于TBMN患者的遗传咨询和临床管理非常重要，并可能进一步了解TBMN和血尿性肾小球疾病的遗传结构。这项研究将是TBMN研究中的第一项此类研究，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.