Genetics of Renal Disease in African Americans

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50万人接受透析或肾脏移植，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。足细胞中表达的结构蛋白被认为在影响水力流动和蛋白质从血浆空间进入肾脏尿空间的过程中起关键作用。FSGS是一种综合征，包括特发性形式和与肾细胞数量减少、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。最强的信号集中在MYH9上，编码肌球蛋白IIA，这是一个可能的候选基因。我们随后对MYH9进行了精细定位，但没有发现明显的功能变异。这些内含子变异在非裔美国人中很常见，在欧洲人中几乎没有；它们与高血压引起的FSGS、HIVAN和ESKD密切相关，这几乎解释了在非裔美国人中观察到的CKD和ESKD的4倍额外风险。随后，在编码载脂蛋白1 （APOL1）的APOL1基因中发现了包含2个绝对连锁不平衡错义变异（G1等位基因）和1个框内6碱基对缺失（G2等位基因）的编码变异。APOL1编码变体与FSGS的相关性比MYH9变体更强。ApoL1提供保护，防止感染布鲁氏锥虫；APOL1 G1和G2风险等位基因恢复了对罗得西亚锥虫（一种导致昏睡病的杂合子）的保护。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。G1和G2等位基因的频率在非洲裔美国人中约为35%，在尼日利亚约鲁巴人中约为60%。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。我们研究了活检证实为特发性局灶节段性肾小球硬化（FSGS）和HIVAN的患者，以更精确地定义遗传风险，并确定apol1相关的FSGS是否具有独特的临床表型。我们检测了271例非洲裔美国人FSGS和hiv病例、168例欧美人FSGS病例和939例对照者的APOL1基因型。在隐性模型中，与没有或只有一个APOL1风险等位基因的受试者相比，APOL1变异与FSGS的OR为17，与HIVAN的OR为29相关。与艾滋病毒的关联是基因-环境相互作用的有力例子。对于与两个APOL1风险等位基因相关的FSGS患者，与其他FSGS患者相比，发病年龄大约提前十年，进展为ESKD的速度明显更快。两个APOL1风险等位基因导致FSGS和HIVAN的归因风险为67%，FSGS和HIVAN的解释比例分别为18%和35%；这与吸烟导致小肺细胞癌的风险相似。具有两个APOL1风险等位基因的个体患FSGS的终生风险估计为4%，而未经治疗的HIV疾病患者患HIV的终生风险为50%。这些数据增加了确定APOL1基因检测在个性化医疗中的作用所需的证据基础。这些研究结果发表在《美国肾脏学会杂志》上。尽管在非裔美国人肾病和高血压研究（AASK）试验中，受试者使用血管紧张素转换酶抑制剂强化血压控制，但对肾脏进展终点（肌酐加倍或ESKD）没有显著影响。我们在675名患有CKD的AASK参与者和618名非裔美国人非肾病对照中评估了APOL1 G1和G2变异与高血压肾病相关的作用。在隐性模型中，与对照组相比，AASK病例的APOL1风险变异与临床终点显著相关。（OR 2.57; p=1.4 × 10-8）；在基线蛋白尿的参与者中，效果略高。我们还发现APOL1变异与较高的基线尿蛋白/肌酐比值（OR 6.29; p=2.6x10-14）和随访期间较高的血清肌酐（OR 4.61; p=5.6x10-15）相关。总之，APOL1基因的肾病风险变异与非糖尿病AASK参与者原发性高血压引起的CKD显著相关。在进行性肾功能下降和蛋白尿基线水平较高的个体中，结果最为明显。这些变异不太可能与原发性高血压本身相关，因为在将高血压asask病例与有或无高血压的对照组进行比较时，结果是一致的。这些结果强烈提示，以局灶性全局肾小球硬化、小动脉肾硬化和间质瘢痕为特征的进行性肾脏疾病，通常出现在AASK参与者的肾脏活检中，与特发性FSGS和HIVAN属于同一疾病谱系。这些结果发表在《肾脏国际》杂志上。APOL1变异不能解释高血压患者肾脏疾病的所有风险，也与高血压无关。使用仅限病例的设计，我们对AASK队列的1750个祖先信息标记进行了基因分型，通过连锁不平衡策略使用混合作图检测关联。简而言之，我们使用隐马尔科夫链分析来比较每个标记的祖先。肾脏病或高血压在非裔美国人中更为普遍，在利益区域中存在与之相关的因果变异，相对于基因组的其他部分，非洲血统应该有所增加。基因分型和质量控制步骤已完成，软件程序已适应数量性状。4. 我们对具有强烈种族差异的常见疾病的兴趣促使了对非裔美国人瘢痕疙瘩发展的研究。与肥厚性瘢痕不同，瘢痕瘤在伤口边缘过度生长，具有强烈的种族倾向；非洲裔美国人的患病率估计在3-15%之间。除外伤外，导致瘢痕疙瘩形成的确切病因尚未完全阐明。瘢痕疙瘩通常与负面伤口愈合因素有关，如感染、过度紧张、异物和重复性创伤；然而，瘢痕疙瘩可能在简单的清洁伤口中形成。它们通常首先表现在童年和成年早期；很少看到瘢痕疙瘩在生命的第三个十年之后发展。我们收集了多个三代家族分离的瘢痕疙瘩，其中遗传模式是常染色体显性与不完全外显。迄今为止，我们已经对60个家庭成员使用800个微卫星标记进行了连锁分析，以定位与瘢痕疙瘩表型相关的区域。我们获得了12个受影响和未受影响的家庭成员的外显子组序列，通过血统分析进行身份鉴定。我们已经确定了三个显示LOD分数的区域[gt；2]，但迄今为止的结果表明，产生瘢痕疙瘩的倾向是多基因的。

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 dialysis or kidney transplants 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 nephritic syndrome in adults and the leading cause of ESKD in children. Structural proteins expressed in podocytes are postulated to play a critical role in influencing hydraulic flow and protein exit from the plasma space into the urinary space in the kidney. FSGS represents a syndrome that includes idiopathic forms and 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). The strongest signal was centered on MYH9, encoding myosin IIA, a plausible candidate gene. We subsequently fine-mapped MYH9, but no obvious functional variants were identified. These intronic variants were common in African Americans and nearly absent from Europeans; they are strongly associated with FSGS, HIVAN, and ESKD attributable to hypertension, explaining nearly all the 4-fold excess risk for CKD and ESKD observed in African Americans. Subsequently, coding variants comprising 2 missense variants (G1 allele) in absolute linkage disequlibrium and an inframe 6 basepair deletion (G2 allele) were identified in the APOL1 gene encoding apolipoprotein 1 (APOL1). The APOL1 coding variants were more strongly associated with FSGS than were the MYH9 variants. ApoL1 provides protection against infection with Trypanosoma brucei brucei; the APOL1 G1 and G2 risk alleles restore protection against T. b. rhodesiense, a cause of sleeping sickness in heterozygotes. 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 frequencies of G1 and G2 alleles are approximately 35% in African Americans and 60% in Yoruba from Nigeria. 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.Accomplishments1. We studied patients with biopsy proven idiopathic focal segmental glomerulosclerosis (FSGS) and HIVAN to define more precisely the genetic risk and to determine whether APOL1-associated FSGS has a distinct clinical phenotype. We determined APOL1 genotypes for 271 African American FSGS and HIVAN cases, 168 European American FSGS cases and 939 control subjects. In the recessive model, APOL1 variants were associated with OR 17 for FSGS and 29 for HIVAN compared to subjects with no or one APOL1 risk allele. The association with HIV is a robust example of a gene-environment interaction. For FSGS associated with two APOL1 risk alleles, compared to other FSGS patients, onset age was approximately a decade earlier and progression to ESKD was significantly faster. Two APOL1 risk alleles confer an attributable risk of 67% for FSGS and HIVAN and an explained fraction of 18% for FSGS and 35% for HIVAN; this is similar to the risk conferred by smoking for small-lung-cell carcinoma. Individuals with two APOL1 risk alleles have an estimated 4% lifetime risk for FSGS and those with untreated HIV disease have a 50% lefetime risk for HIVAN. These data add to the evidence base required to define the role for APOL1 genetic testing in personalized medicine. These results were published in Journal of American Society of Nephrology.2. In spite of intensive blood pressure control with angiotensin converting enzyme inhibitors in subjects enrolled in the African American Study of Kidney Disease and Hypertension (AASK) trial, there was no significant effect on renal progression endpoints (creatinine doubling or ESKD). We evaluated the role of APOL1 G1 and G2 variants for association with hypertension-attributed nephropathy in 675 AASK participants with CKD and 618 African American non-nephrology controls. In a recessive model, APOL1 risk variants were significantly associated with clinical endpoints in AASK cases versus controls. (OR 2.57; p=1.4x10-8); the effects were slightly higher in participants with baseline proteinuria. We also found that APOL1 variants were associated with higher baseline urine protein/creatinine ratios (OR 6.29; p=2.6x10-14) and higher serum creatinine during follow-up (OR 4.61; p=5.6x10-15). In summary, nephropathy risk variants in the APOL1 gene are significantly associated with CKD attributed to essential hypertension in non-diabetic AASK participants. The results were most robust in individuals with progressive renal functional decline and higher baseline levels of proteinuria. It is unlikely that these variants are associated with essential hypertension per se, as results were consistent when comparing hypertensive AASK cases to controls with and without high blood pressure. These results strongly suggest that progressive kidney disease attributed to hypertensive nephrosclerosis and characterized by focal global glomerulosclerosis, arteriolar nephrosclerosis and interstitial scarring, commonly present in the renal biopsies of AASK participants, belongs to the same disease spectrum as idiopathic FSGS and HIVAN. These results were published in Kidney International.3. APOL1 variants do not explain all of the risk for kidney disease in persons with hypertension nor do they associate with hypertension. Using a case only design, we have genotyped the AASK cohort for 1750 ancestry informative markers to detect association using an admixture mapping by linkage disequilibrium strategy. In brief, we compare each marker for ancestry using a hidden Markoff chain analysis. Regions-of-interest harboring the causal variation associated with kidney disease or hypertension, both more prevalent in African Americans, should show an increase in African ancestry relative to the rest of the genome. The genotyping and quality control steps have been completed and the software programs adapted for quantitative traits. 4. Our interest in common diseases with strong racial disparities prompted a study of keloid development in African Americans. Keloids, unlike hypertrophic scarring, overgrow the wound border and have a strong racial propensity; estimates for prevalence in the African American population range from 3-15%. With the exception of trauma, precise etiologic factors responsible for keloid formation have not been fully elucidated. Keloids are frequently associated with negative wound healing factors, such as infection, excessive tension, foreign bodies, and repetitive trauma; however, keloids may form in simple clean wounds. They generally first manifest in childhood and early adulthood; it is rare to see keloids develop after the third decade of life. We have collected multiple three-generation families segregating for keloids, where the mode of inheritance is autosomal dominant with incomplete penetrance. To date we have performed a linkage analysis using 800 microsatellite markers for 60 family members to localize the region linked to the keloid phenotype. We have obtained exome sequences for 12 affected and unaffected family members for identity by descent analysis. We have identified three regions showing LOD scores >2, but results to date suggest that the propensity to develop keloids is polygenic.