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。ESKD的三个主要病因是2型糖尿病、高血压和肾小球硬化。非裔美国人患晚期ESKD的可能性是白人的3-4倍。局灶节段性肾小球硬化（FSGS）是成人原发性肾病综合征的主要原因，也是儿童ESKD的主要原因。FSGS包括一种综合征，包括特发性形式以及与肾细胞数量减少、高血压和HIV-1感染相关的形式。在此之前，我们使用混合映射来定位22号染色体上与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提供保护，防止感染布鲁氏锥虫；杂合子中的APOL1 G1和G2风险等位基因恢复了对APOL1抗性罗得西亚锥虫的保护，后者是昏睡病的一种原因。最近，在包含MYH9/APOL1的区域内，APOL1 G1和G2等位基因正选择导致单倍型纯合。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。一个尚未解决的重要问题是高风险基因型对活体肾供者或其肾受体的影响。尽管研究表明肾受体的基因型不影响移植物的存活，但尚不清楚供体的高风险基因型是否会增加活体供体未来肾脏疾病/衰竭的风险或受体的移植物存活。我们假设：1)具有高风险APOL1基因型的活体供体在其剩余肾脏中发生肾衰竭的风险更大；2)与移植肾衰竭相关的是供体的基因型，而不是受体的基因型。我们还在解决撒哈拉以南非洲地区这些变异对艾滋病毒感染者的影响问题。1)我们已经与芝加哥大学和韦恩州立大学的研究人员合作，研究APOL1变异在高血压、慢性肾脏疾病和活体供体肾脏存活中的作用。迄今为止，47名AA和71名（n=119）需要肾脏替代治疗的前活体供者已登记并进行APOL1基因分型。我们发现，超过一半的肾衰竭前献血者携带两个APOL1风险等位基因（57%），而在一般人群中这一比例为13%。这一结果与大多数活体供者是受者的一级亲属这一事实相混淆，因此更有可能携带APOL1风险变异。据报道，在患有ESRD和2个风险等位基因的非裔美国人中，23%的人与ESRD有一级亲属关系。2)有人假设，高度分化的单倍型上的两个新变体显示了可能与锥虫病抗性和肾脏疾病相关的近期选择信号，因为这些变体发生在编码锥虫裂解所需结构域的外显子内。此外，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，我们通过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。这些数据已被正式化以供出版。患有慢性肾脏疾病的非裔美国人患终末期肾脏疾病的风险增加。为了研究APOL1风险变异在非裔美国人高血压肾病进展到临床肾脏终点中的作用，我们研究了非裔美国人肾病和高血压研究（AASK）队列（n=693）和慢性肾功能不全队列（CRIC, n=2955）参与者中APOL1等位基因与肾脏终点的关系。我们发现，APOL1高危组中因高血压导致肾功能不全的非裔美国人进展到终末期肾病的速度更快（风险比1.9,P0.001），与治疗组或基线蛋白尿无相互作用。在CRIC研究中也观察到类似的结果，与糖尿病状态无关。4)在与南非威特沃特斯兰德大学的研究人员进行的一项国际研究中，我们表明APOL1变异与hiv感染人群中hiv相关的其他形式的肾脏疾病或FSGS密切相关，hiv是一种快速进展的肾脏疾病（OR 83）。这份手稿正在审查中。5)我们发现，在南非德班患有散发性类固醇抵抗性肾病综合征（SRNS）的黑人儿童中，超过30%的podocin基因（NPHS2）单一突变为纯合子，而非亚洲儿童。我们假设这种致命的突变在祖鲁人中出现的频率很高，是由一个创始人事件引起的，这种突变解释了该地区黑人儿童中SRNS的高频率。我们现在正在做实验来验证这一假设，并将数据正式化以发表。

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. The three leading causes of ESKD are type 2 diabetes, hypertension, and glomerulosclerosis. African Americans are 3-4 times more likely to develop end ESKD compared to their white counterparts. Focal segmental glomerulosclerosis (FSGS) is the leading cause of primary nephrotic syndrome in adults and the leading cause of ESKD in children. FSGS comprises a syndrome that includes idiopathic forms as well as forms associated with reduced nephron numbers, hypertension, and HIV-1 infection. Previously, we used admixture mapping to localize a region on chromosome 22 associated with 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 G1 and G2 risk alleles in heterozygotes restore protection against ApoL1 resistant T. b. rhodesiense, a cause of sleeping sickness. APOL1 G1 and G2 alleles have been under recent positive selection resulting in haplotype homozygosity across the region comprising MYH9/APOL1. 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. An important question, as yet unresolved, is the affect of high-risk genotypes on living kidney donors or their kidney recipients. Although studies suggest that the genotype of the kidney recipient does not impact graft survival, it is not known if high risk genotypes in the donor increases risk of future kidney disease/failure in the living donor or graft survival in the recipient. We hypothesize that: 1) living donors with high risk APOL1 genotypes are at greater risk for developing kidney failure in his or her remaining kidney and 2) it is the genotype of the donor and not the genotype of the recipient that is associated with transplant kidney failure. We also are addressing the question of the impact of these variants in sub-Saharan Africa on people with HIV infection. Accomplishments 1) We have entered into collaborations with researchers at the University of Chicago and Wayne State University to investigate the role of APOL1 variants on development of hypertension, chronic kidney disease, and kidney survival in the living donor. To date 47 AA and 71 (n=119) former living donors now in need of renal replacement therapy have been enrolled and genotyped for APOL1. We found that more than half of former donors with kidney failure carried two APOL1 risk alleles (57%) compared to 13% in the general population. This result is confounded by the fact that the majority of living donors were first-degree relatives of the recipients and therefore more likely to carry APOL1 risk variants. It was reported by others that amongst African Americans with ESRD and 2 risk alleles, 23% had a first degree relative with ESRD. 2) It has been hypothesized by others that two new variants are on a highly diverged haplotype showed signals of recent selection that might be associated with trypanosomiasis resistance and renal disease, as the variants occurred within exons encoding domains required for lysis of trypanosomes. Further, 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 has been formalized for publication. 3) African Americans with chronic renal disease are at increased risk for end-stage renal disease. To examine the role of APOL1 risk variants in African Americans with kidney disease attributed to hypertension on progression to clinical renal endpoints, we studied the association of APOL1 alleles on renal endpoints in in participants of the African American Study of Kidney Disease and Hypertension (AASK) cohort (n=693) and in the Chronic Renal Insufficiency Cohort (CRIC, n=2955). We showed that African Americans in the APOL1 high-risk group with renal insufficiency due to hypertension progressed faster to end-stage kidney disease (hazard ratio 1.9, P0.001), with no interaction with treatment arm or baseline proteinuria. Similar results were observed in the CRIC study, regardless of diabetes status. This study was published in the New England Journal of Medicine. 4) In a international study with researchers at the University of Witwatersrand in South Africa we have shown that APOL1 variants are strongly associated with HIVAN, a rapidly progressive kidney disease (OR 83) but not with other forms of HIV-related kidney disease or FSGS in the HIV-infected population. This manuscript is under review. 5) We have shown that in black children, but not Asian children, with sporadic steroid resistant nephrotic syndrome (SRNS) in Durban, South Africa, more than 30% are homozygous for a single mutation in the podocin gene (NPHS2). We hypothesize that this lethal mutation arose to high frequency among Zulu by a founder event and that this mutation accounts for the high frequency of SRNS among black children in this region. We are now doing experiments to test this hypothesis and are formalizing data for publications.