Genetics of Complex Diseases and Health Disparities

慢性肾脏疾病（CKD），影响超过2600万美国人，经常导致肾衰竭。每年有超过10万人患上终末期肾病（ESKD），近50万人接受肾脏移植或正在进行透析，每年花费300亿美元。在此之前，我们使用混合映射来定位22号染色体上与局灶节段性肾小球硬化（FSGS）和hiv相关肾病（HIVAN）相关的区域。随后，我们和其他人发现，该区域的APOL1编码变异体包括2个绝对连锁不平衡错义变异体（G1等位基因）和1个第6帧碱基对缺失（G2等位基因），与高血压ESKD、局灶节段性肾小球硬化（FSGS）和hiv相关肾病（HIVAN）的OR分别为7、19和27。ApoL1可保护人体免受布氏锥虫感染。APOL1风险等位基因最近在撒哈拉以南非洲出现，但由于非洲侨民，在世界其他地区也发现了apo1风险等位基因。非洲裔美国人G1和G2等位基因的总频率约为35%。这些等位基因几乎解释了非裔美国人患肾病的所有额外风险，从而为全球主要的健康差异提供了遗传基础。在与校内外研究者的合作研究中，我们继续对APOL1进行研究，以确定风险变异是否与其他显示种族差异的非肾脏或肾脏表型相关，如乳头状肾癌和心血管疾病。1)大约20%的原发性FSGS或hiv相关肾病患者携带1个或0个APOL1 G1或G2拷贝，这表明APOL1中的其他因素可能与疾病有关。在一项对全球2000多人进行的详尽研究中，包括1000多例FSGS和hiv相关肾病（HIVAN）的病例和对照，我们使用关联和负担试验表明，没有其他常见变异可以溶解锥虫，也没有其他常见或罕见变异与FSGS或HIVAN相关。这项研究的直接相关性是，在没有携带G1或G2肾脏风险变异的FSGS或HIVAN患者中，APOL1测序没有临床实用性。3)在与南非研究人员进行的一项国际研究中，我们表明APOL1变异与HIVAN（一种快速进展的肾脏疾病）密切相关（OR 89）。2)我们目前正在调查遗传在南非印度和黑人儿童儿童期肾病综合征中的作用。我们发现APOL1与儿童肾病综合征无关；然而，我们确实发现在南非德班的散发性类固醇抵抗性肾病综合征（SRNS）中，近三分之一是podocin基因（NPHS2）单一突变的纯合子。该研究表明，对于携带两个NPHS2变异拷贝的30%肾病综合征患儿，可以精确诊断为类固醇抵抗性局灶节段性肾小球硬化，无需肾活检或无效且可能有害的类固醇治疗。本研究已提交审查。3)在青壮年冠状动脉危险发展研究中，我们研究了APOL1在肾功能和蛋白尿中的作用，蛋白尿是慢性肾脏疾病和心血管疾病的预测因子。我们在3030名肾小球滤过率保持不变的年轻人中研究了APOL1与蛋白尿和肾功能下降的关系。蛋白尿是慢性肾脏疾病和心血管疾病的预测因子，在15年随访中，每1000人年的发病率为APOL1高危基因型为15.6，低风险黑人为7.8，白人为3.9。与白人相比，高风险黑人发生蛋白尿的比值比为5.71，低风险黑人为2.32。对危险因素的调整减弱了低风险黑人和白人之间的差异。与白人相比，APOL1低风险黑人的肾功能年下降速度也更快，但在调整了危险因素和社会经济地位后，这种差异减弱了。我们发现，携带两个APOL1风险等位基因的黑人在青年期发生蛋白尿和肾功能下降的风险最高，而低风险黑人和白人之间的差异与传统风险因素的差异有关。这项研究发表在《美国肾脏病学会杂志》上。我们评估了一组猝死患者的APOL1基因谱与肾脏组织学特征之间的相关性。肾小球数量和平均肾小球体积由我们的合作者在密西西比州杰克逊进行尸检的没有肾脏疾病的黑人和白人肾脏中测量。我们发现，随着年龄的增长，携带APOL1变异与肾小球数量的显著减少和肾小球体积的增加有关。回归分析预测，在具有两个风险等位基因的非裔美国人成年后的前38年，每个肾脏每年平均损失8834个肾小球。这些研究结果表明，APOL1风险等位基因与年龄相关的肾单位损失有关，可能在成年早期从较大的小肾小球池中衰减，同时剩余的肾小球也会增大。观察结果提示apol1阳性黑人对肾脏疾病易感性增加的机制。5)只有15%的两种APOL1高危基因型携带者发展为慢性肾脏疾病或进展为肾衰竭，提示疾病的发生和发展需要其他遗传或环境因素。在与NIDDK和弗吉尼亚大学的研究人员的一项合作研究中，我们发现GSTM1空等位基因，先前已被证明与CKD进展到肾衰竭或死亡相关，在APOL1高风险参与者中，它可能会促进进展到ESRD。GSTM1基因产物调节氧化应激。在参加AASK临床试验的黑人高血压慢性肾病患者中，GSTM1基因0拷贝或只有1拷贝放大了高风险APOL1基因型的影响。由于个体数量有限，我们无法确定携带2个GSTM1等位基因是否具有保护作用。需要更大的队列来进一步探索GSTM1无效和APOL1高危基因型之间的相互作用（发表在《美国肾脏学会杂志》上）。

Project Summary: Chronic kidney disease (CKD), affecting over 26 million Americans, frequently leads to kidney failure. More than 100,000 individuals develop end stage kidney disease (ESKD) annually and nearly 500,000 receive kidney transplants or are ongoing dialysis patients at an annual cost of $30 billion dollars. Previously, we used admixture mapping to localize a region on chromosome 22 associated with focal segmental glomerulosclerosis (FSGS) and HIV-associated nephropathy (HIVAN). Subsequently, we and others showed that APOL1 coding variants within this region comprising 2 missense variants in absolute linkage disequilibrium (G1 allele) and an in frame 6 base pair deletion (G2 allele) were responsible for the association, with OR of 7, 19, and 27 for hypertensive ESKD, focal segmental glomerulosclerosis (FSGS), and HIV-associated nephropathy (HIVAN), respectively. ApoL1 provides protection against infection with Trypanosoma brucei brucei. The APOL1 risk alleles emerged recently in sub-Saharan Africa, but are found in other regions of the world as a result of the African Diaspora. The combined frequencies of G1 and G2 alleles are approximately 35% in African Americans. These alleles explain nearly all the excess risk of kidney disease in African Americans, thus providing a genetic basis for a major global health disparity. We have continued our studies of APOL1 to determine if the risk variants are associated with other non-renal or renal phenotypes that show racial disparities, such as papillary renal cancer and cardiovascular disease, in collaborative studies with intramural and extramural investigators. Accomplishments 1) About 20% of patients with primary FSGS or HIV-associated nephropathy carry 1 or 0 copies of APOL1 G1 or G2, suggesting that additional factors in APOL1 might contribute to disease. In an exhaustive study of more than 2000 people world-wide, including over a 1000 case and controls for FSGS and HIV-associated nephropathy (HIVAN), we showed that no other common variants lysed trypanosomes and no additional common or rare variants were associated with FSGS or HIVAN using association and burden tests. The immediate relevance of this study is that there is no clinical utility in sequencing APOL1 in patients with FSGS or HIVAN who do not carrying G1 or G2 renal risk variants. This data was published in Kidney International. 3) In a international study with researchers in South Africa we have shown that APOL1 variants are strongly associated with HIVAN, a rapidly progressive kidney disease (OR 89).This manuscript was published in the Journal of the American Society of Nephrology. 2) We are now investigating the role of genetic role of childhood-onset nephrotic syndrome in Indian and Black children in in South Africa. We find that APOL1 does not contribute to childhood nephrotic syndrome; however, we did find that sporadic steroid resistant nephrotic syndrome (SRNS) in Durban, South Africa, nearly a third are homozygous for a single mutation in the podocin gene (NPHS2). This study indicates that for the 30% of children with nephrotic syndrome carrying two copies of the NPHS2 variant, a precision diagnosis of steroid resistant focal segmental glomerulosclerosis can be made, abrogating the need for a renal biopsy or ineffective and potentially harmful steroid therapy. This study as been submitted for review. 3) We investigated the role of APOL1 on kidney function and albuminuria, a predictor of chronic kidney disease and cardiovascular disease in the Coronary Artery Risk Development in Young Adults study. We examined associations of APOL1 with incident albuminuria and kidney function decline among 3030 young adults with preserved glomerular filtration rate. The incidence rate per 1000 person-years for albuminuria, a predictor of chronic kidney disease and cardiovascular disease over 15 years follow-up was 15.6 for APOL1 high-risk genotypes, 7.8 for low-risk blacks, and 3.9 for whites. Compared withwhites, the odds ratio for incident albuminuria was 5.71 ) for high-risk blacks and 2.32 for low-risk blacks. Adjustment for risk factors attenuated the difference between low-risk blacks and whites. APOL1 low-risk blacks also had a faster yearly decline in kidney function compared with whites, but this difference was attenuated after adjustment for risk factors and socioeconomic position. We found that blacks with two APOL1 risk alleles had the highest risk for albuminuria and kidney decline in young adulthood, whereas disparities between low-risk blacks and whites were related to differences in traditional risk factors. This study was published in Journal of the American Society of Nephrology. 4) 6) We assessed correlations between APOL1 profiles and renal histological features in a of individuals who experienced sudden death. Glomerular number and mean glomerular volume were measured by our collaborators in kidneys of blacks and whites without renal disease, undergoing autopsies in Jackson, Mississippi. We found that carriage of APOL1 variants was associated with significant reductions in glomerular number and increases in glomerular volume with increasing age. Regression analysis predicted an annual average loss of 8834 glomeruli per single kidney over the first 38 years of adult life in African Americans with two risk alleles. These findings indicate that APOL1 risk alleles are associated with exaggerated age-related nephron loss, probably decaying from a larger pool of smaller glomeruli in early adult life, along with enlargement of the remaining glomeruli. The observations suggest a mechanism of accentuated susceptibility to kidney disease in APOL1-positive blacks. This study was published in the Journal of the American Society of Nephrologists. 5) Only 15% of carriers of two APOL1 high risk genotypes develop chronic kidney disease or progress to kidney failure, suggesting that other genetic or environmental factor is required for disease initiation and progression. In a collaborative study with investigators at NIDDK and the University of Virginia, we found that the GSTM1 null allele, which previously has been shown to associate with CKD progression to kidney failure or death, potentiates progression to ESRD in APOL1 high-risk participants. The GSTM1 gene product modulates oxidative stress. Having 0 or only 1 copy of the GSTM1 gene amplified the effect of the high-risk APOL1 genotypes in black patients with chronic kidney disease attributed to hypertension enrolled in the AASK clinical trial. We were unable to determine if carriage of 2 GSTM1 alleles would be protective because of limited numbers of individuals. Larger cohorts are needed to further explore the interactions between GSTM1 null and APOL1 high-risk genotypes (published in Journal of the American Society of Nephrology).