Abstract
目的
纳入1例临床特征和遗传方式符合遗传性釉质发育不全的患者家系,在中国人群中发现RELT基因突变与遗传性釉质发育不全相关,分析其突变效应,探究基因型与表型的关系。
方法
收集患者及家系成员的临床资料,分析其临床表型;采集家系成员的外周静脉血等生物样本,提取基因组DNA,进行全外显子组测序(whole-exome sequencing, WES),分析其致病基因,采用Sanger测序验证。利用SIFT、PolyPhen-2等网站预测突变的致病性;利用Uniprot网站对比不同物种的蛋白序列,分析蛋白保守性; 利用Alphafold 2等生物信息学软件分析突变蛋白在三维结构等方面的改变。
结果
先证者表现为典型的钙化不全型遗传性釉质发育不全,磨耗重,釉质较软,表面粗糙着色,部分釉质丧失,其他家系成员不具有类似的口腔表现。WES和Sanger测序结果表明该先证者携带RELT基因的纯合移码突变,即NM_032871.3: c.1169_1170del,其父母均为携带者,该突变被预测为能致病。生物信息学分析结果显示,该突变位点在不同物种间高度保守。蛋白三维结构预测显示,与野生型RELT蛋白相比,突变蛋白p.Pro390fs35构象提前终止,影响该蛋白正常功能。
结论
通过对一个遗传性釉质发育不全家系进行表型分析、基因测序及功能预测等,发现RELT基因的纯合移码突变可造成蛋白结构异常,导致钙化不全型遗传性釉质发育不全。
Keywords: 遗传性釉质发育不全, RELT基因, 移码突变, 釉质矿化
Abstract
Objective
To analyze RELT gene mutation found in a pedigree with clinical features and inheritable pattern consistent with amelogenesis imperfecta (AI) in China, and to study the relationship between its genotype and phenotype.
Methods
Clinical and radiological features were recorded for the affected individuals. Peripheral venous blood samples of the patient and family members were collected for further study, and the genomic DNA was extracted to identify the pathogenic gene. Whole exome sequencing (WES) was performed to analyze the possible pathogenic genes, and Sanger sequencing was performed for validation. SIFT and PolyPhen-2 were used to predict and analyze the mutation effect. Comparison of RELT amino acids across different species were performed by using Uniprot website. In addition, the three-dimen-sional structures of the wild type and mutant proteins were predicted by Alphafold 2.
Results
The proband exhibited typical hypocalcified AI, with heavy wear, soft enamel, rough and discolored surface, and partial enamel loss, while his parents didn ' t have similar manifestations. WES and Sanger sequencing results indicated that the proband carries a homozygous frameshift mutation in RELT gene, NM_032871.3: c.1169_1170del, and both of his parents were carriers. This mutation was predicted to be pathogenic by SIFT and PolyPhen-2. Up to now, there were 11 mutation sites in RELT gene were reported to be associated with AI, and all of the patients exhibited with hypocalcified AI. Compared with the wild-type RELT protein, the mutant protein p. Pro390fs35 conformation terminated prematurely, affecting the normal function of the protein.
Conclusion
Through phenotype analysis, gene sequencing, and functional prediction of a Chinese family with typical amelogenesis imperfecta, this study found that RELT gene frameshift mutation can lead to protein dysfunction in AI patients. Further research will focus on the role and mechanism of RELT in enamel development at the molecular and animal levels, providing molecular biology evidence for the genetic counseling, prenatal diagnosis, and early prevention and treatment of AI.
Keywords: Amelogenesis imperfecta, RELT gene, Frameshift mutation, Enamel mineralization
牙发育是在外胚来源的上皮和间充质交互诱导下,经历牙板期、蕾状期、帽状期、钟状期、牙冠钙化形成、牙根形成并萌出的过程[1],遗传和环境因素的干扰会导致牙发育异常,可表现为牙数目、形态、结构和萌出缺陷[2-3]。遗传性釉质发育不全(amelogenesis imperfecta, AI)特指由基因突变导致的牙釉质发育缺陷,表现为釉质的颜色、厚度、矿化异常[4-5]。由于受累时间和程度不同,根据临床表现将AI分为发育不全型、成熟不全型、钙化不全型和混合型[2, 6]。
牙釉质是由高度矿化的羟基磷灰石晶体组成,牙发育过程中,成釉细胞分泌多种细胞外基质蛋白,引导羟基磷灰石晶核的形成与延伸,并最终经过基质蛋白酶的水解,实现釉柱最终成熟与矿化[7-8]。目前已报道超过27种基因突变与AI的发生有关[9],包括釉原蛋白(amelogenin, AMELX)基因、釉蛋白(enamelin, ENAM)基因、成釉蛋白(ameloblastin, AMBN)基因和基质金属蛋白酶20(matrix metallopeptidases 20, MMP20)基因、FAM83H基因、WDR72基因等[6, 9-13]。不同基因突变导致的临床表型存在差异,如ENAM的剪接突变可导致发育不全型AI,而MMP20突变可导致成熟不全型AI[10, 14]。
RELT是肿瘤坏死因子(tumor necrosis factor, TNF)受体家族成员之一,既往研究发现其主要表达在淋巴组织和血液系统[15],但目前发现有11例RELT基因突变可导致AI,表现为牙釉质矿化不良、釉柱结构排列紊乱,提示RELT基因可能与牙釉质成熟期发育缺陷相关[16]。在小鼠牙发育过程中,Relt基因在分泌前期和分泌期的小鼠成釉细胞中活跃表达[17]。RELT具有两个同源物RELL1和RELL2,具有相似氨基酸序列并共定位于细胞膜,奇数跳跃相关转录因子1(odd-skipped related transcription factor 1,OSR1)可以磷酸化RELT、RELL1和RELL2,过表达的RELL1和RELL2可以激活p38 MAPK信号通路, 诱导上皮细胞凋亡[18-19]。解离素和金属蛋白酶10(a disintegrin and metalloproteinase 10, ADAM10)可以水解RELT细胞外功能域,可能是RELT在釉柱形成中的重要调节因子[20],但RELT基因在釉质发育中的作用以及突变后的致病机制尚未有深入研究。
本研究在AI的患者中发现了RELT基因突变,对患者的临床表型进行了详细描述,对其致病基因进行了测序筛查和验证,探究RELT基因突变和AI的关系。
1. 资料与方法
本研究遵守赫尔辛基原则宣言,获得北京大学口腔医院生物医学伦理委员会审批(批准号:PKUSSIRB-202278104),研究对象均签署知情同意书。
1.1. 家系收集与临床检查
对1例2021年5月就诊于北京大学口腔医院牙体牙髓科的先证者(25岁男性)进行详细的临床及影像学检查,并采集其外周静脉血5 mL置于乙二胺四乙酸二钾抗凝采血管中。口腔检查包括牙齿数目、形态、颜色、牙周情况等,影像学检查包括根尖片、曲面体层和口腔锥形束CT(cone-beamed computerized tomography, CBCT)等,观察牙根、根尖周及颌骨情况;全身检查包括毛发、皮肤、骨骼、瞳孔等,询问先证者的居住情况、喂养史、用药史、既往史和家族史。
1.2. 基因组DNA提取和全外显子组测序
采用基因组DNA提取试剂盒(天根公司,中国)从先证者的外周静脉血中提取基因组DNA,送至北京优乐复生科技有限责任公司进行全外显子组测序(whole exome sequencing, WES),筛查致病基因,分析所得测序结果。分析流程与本课题组既往研究[10]类似,首先排除良性或可疑良性突变,排除同义突变,接着排除Exac数据库(Exome Aggregation Consortium, http://exac.broadinstitute.org)、1000G数据库(1000Gnomes Project, http://asia.ensembl.org/Homo_sapiens/Info/Index)、gnomAD数据库(Genome Aggregation Database, http://gnomad.broadinstitute.org)中的突变和最小等位基因频率(minor allele frequency, MAF)>0.01的突变。接下来,分析已报道过的与AI有关的基因,如AMELX、ENAM、AMBN、KLK4、MMP20等[6, 21-23]。使用SIFT(Sorting Intolerant from Tolerant, http://sift.jcvi.org/)、PolyPhen-2(Polymorphism Phenotyping v2, http://genetics.bwh.harvard.edu/pph2/)等网站预测剩余基因突变的致病性。
1.3. Sanger测序验证
通过多聚酶链反应(polymerase chain reaction, PCR)进行靶向扩增,使用Sanger测序对WES所得结果及家系成员进行验证,在PrimeSTAR@ HS DNA反应体系(Takara公司, 日本)中进行PCR反应,引物参照Kim等[17]的研究,产物送至北京睿博兴科生物技术有限公司进行纯化和PCR测序。
1.4. 突变位点保守性分析和突变蛋白三维结构预测
使用Uniprot(http://www.uniprot.org)网站查询不同物种的蛋白序列,勾选多个物种,包括人(Homo sapiens)、小鼠(Mus musculus)、家牛(Bos taurus)、野耗牛(Bos mutus)、食蟹猴(Macaca fasci-cularis)等,选择对比分析,即可得到不同物种间的保守性分析结果。在Protein Data Bank蛋白质结构数据库(https:www.rcsb.org)中,以目的基因为搜索关键词,未找到蛋白已知结构;接着在美国国家生物技术信息中心官方网站NCBI(National Center for Biotechnology Information, http://www.ncbi.nlm.nih.gov)上获取蛋白的氨基酸序列(NP_116260.2),使用AlphaFold2网站(https://colab.research.google.com/github/sokrypton/ColabFold/blob/main/Alpha-Fold2.ipynb)进行同源建模,选取最适合的模板作为野生型蛋白的同源三级结构。输入突变蛋白氨基酸序列,建模,选取合适模板作为突变型蛋白的三维结构。
2. 结果
2.1. 表型分析
先证者为25岁青年男性(图 1A),全口牙列呈现出轻中度磨耗,多数牙齿釉质缺损,以后牙为重,釉质表面粗糙,凹凸不平,呈现出棕黄色,质地较软(图 1B~D)。影像学检查可见受累牙齿釉质密度明显降低,接近牙本质透射密度,髓腔根管影像未见缩窄、增宽等异常表现(图 1E)。其他家系成员均不具有类似的口腔表现,先证者父亲牙釉质未观察到明显异常(图 1F、G)。先证者没有全身系统性疾病的表现,如骨发育不良、毛发稀疏、佝偻病、听力丧失等。结合先证者的婴幼儿期喂养史、居住史,排除环境因素导致的釉质发育不全,初步诊断为钙化不全型AI。
图 1.
先证者家系图谱和家系临床表现
Pedigree tree of the proband and clinical images of the pedigree
A, pedigree tree, the black arrow indicates the proband; B-D, intraoral photographs of the proband (Ⅱ-1); E, panoramic photograph of Ⅱ-1; F and G, intraoral photographs of Ⅰ-2.
2.2. 突变分析
对先证者进行WES,共获得17.439 G数据量,平均测序深度为148.944,与全基因组的比对率为99.944%,原始测序数据上传到Sequence Read Archive数据库(编号为PNJNA1184796)。先证者WES结果未发现常见的可导致AI的ENAM、AMELX、AMBN、LAMB3、MMP20、KLK4、WDR72、FAM83H、AMTN等基因的突变,进一步分析发现先证者携带RELT基因的纯合移码突变(NM_032871.3:c.1169_1170del),该突变位于RELT基因的第10外显子上,并经Sanger测序验证(图 2)。先证者父母均为该突变的携带者(图 2)。该突变(编号rs772929908)已被dbSNP数据库收录,并已被Kim等[17]报道与AI有关,经SIFT、PolyPhen-2、Mutation Taster(http://www.mutationtaser.org)等网站预测为能致病的突变。根据《美国医学遗传学与基因组学学会遗传变异分类标准与指南》(http://www.acmg.net),该突变被预测为致病的突变。
图 2.
先证者及家系成员Sanger测序峰图
Sanger sequencing of the pedigree
The proband (Ⅱ-1) was homozygous, while his parents (Ⅰ-1 and Ⅰ-2) were heterozygous. Red arrows indicated the mutation site.
2.3. 突变特征
RELT基因c.1169_1170del导致移码突变,第390位脯氨酸移码突变为精氨酸,并提前产生终止密码子,终止于第390号密码子下游35号密码子处,从而产生截短蛋白。保守性分析显示不同物种间p.390P进化高度保守(图 3A),利用AlphaFold 2软件预测野生型和突变型RELT蛋白三维结构,与野生型RELT蛋白相比,突变蛋白较野生型蛋白短,无规卷曲减少,模板建模评分(template modeling score, TM)得分(http://zhanggroup.org/TM-score,得分 < 0.5提示蛋白结构相似程度低)为0.34,二者结构相似性较差(图 3B、C)。
图 3.
RELT蛋白保守性分析及三维结构
Conservation analysis and 3D structure of the RELT protein
A, comparison of RELT amino acids across different species, the p.390 was highly conserved; B, 3D structure of wild-type RELT protein predicted by Alphafold 2 software; C, 3D structure of mutant RELT protein predicted by Alphafold 2 software, the red arrow indicates the random coil. WT, wild type; MT, mutant type.
2.4. AI患者中RELT基因突变分布和临床表现
目前已发现11个RELT基因突变位点与AI有关,内含子区域的突变均位于第3内含子,外显子区域的突变多数为错义突变,但其分布不集中,部分位于第4、第6外显子,部分位于第10、第11外显子,其临床表现见表 1。本研究中先证者牙釉质质地较软,釉质矿化降低,与目前已知的RELT基因突变患者的临床表现类似,如釉质矿化程度降低,大多数有磨耗等,提示这些突变位点对RELT蛋白功能的影响可能是类似的,干扰了釉质的矿化成熟。
表 1.
导致AI的RELT基因突变位点及临床表现
Description of the mutations in RELT causing AI according to location and clinical features
Location | cDNAa | Clinical features | References |
a, the locations of mutation sites refer to the sequence of RELT gene (NM_032871.3); b, the proband also carries the mutation of COL17A1 (c.4156+2dup). AI, amelogenesis imperfecta; dup, duplicate; del, delete. | |||
Intron 3 | c.120+1G>A | Hypomineralized and rough enamel, with attrition of occlusal surfaces | [16] |
Intron 3 | c.120+1G>T | Hypomineralized yellow-brown enamel, with attrition of occlusal surfaces and localized pitted hypoplastic enamel | [16] |
Intron 3 | c.121-2A>G | Rough hypoplastic enamel, attrition of occlusal surfaces with ring of thin enamel covering lateral surfaces of the crown | [17] |
Exon 4 | c.164C>T | Hypomineralized, normal enamel thickness, post-eruptive loss | [24] |
Exon 4 Exon11 | c.193T>C c.1260_1263dup |
Rough and hypomineralized enamel with pits localized in the middle of crown | [16] |
Exon 4 Exon 6 | c.260A>T c.521T>G |
Severe enamel hypoplasia affecting both primary molars and permanent teeth, loss of normal crown morphology | [25]b |
Exon 10 | c.1169_1170del | Rough and hypomineralized enamel, attrition of occlusal surfaces | This study and[17] |
Exon 11 | c.1264C>T | Rough enamel, normal enamel thickness | [24] |
Exon 11 | c.1265G>C | Severe generalized enamel attrition | [17] |
3. 讨论
本研究在中国人群中发现了RELT基因的移码突变可导致钙化不全型AI,先证者表现为广泛的釉质缺损,表面粗糙,凹凸不平,釉质厚度、硬度均不足,而在先证者父亲的全口牙列中未观察到釉质厚度、硬度和颜色的异常。基因测序发现先证者为RELT基因的纯合移码突变(NM_032871.3: c.1169_1170del), 预测会影响突变蛋白的三维空间结构,导致蛋白功能失常。RELT基因突变与AI的相关性提示了其在釉质形成与矿化过程中的重要作用。
AI可分为发育不全型、成熟不全型和钙化不全型[2, 6]。研究发现,ENAM、AMELX、AMBN、LAM亚基β3(LAMB3)等基因的突变可导致发育不全型AI,患者釉质厚度不足,硬度正常[21, 26-28]。成熟不全型AI的主要表现是釉质厚度基本正常,质地稍软易磨耗,常见的致病基因如MMP20基因、激肽释放酶4(kallikrein-4, KLK4)基因、WDR72基因[6, 9, 23];而FAM83H基因、釉成熟蛋白(amelotin, AMTN)基因的突变可导致钙化不全型AI,釉质质地较软,快速磨耗,牙面粗糙易着色[9, 29]。本研究中先证者釉质的质地较软,全口牙列广泛磨耗,表面粗糙着色,表现为典型的钙化不全型AI,且既往的报道[16-17, 24-25]中RELT基因突变后患者釉质均有矿化降低和明显磨耗,提示RELT基因突变患者更可能导致钙化不全型AI。
目前已发现的可导致AI的RELT基因内含子区域的突变均位于第3内含子区域[16-17]。体外研究发现RELT基因内含子区域的突变(NM_032871.3:c.121-2A>G)可导致第3内含子滞留、第3外显子跳读和第3、4外显子跳读[17]。考虑到目前发现的内含子的突变均位于第3内含子,且均位于内含子和外显子交界处,而该区域的突变通过可变剪接的方式影响蛋白功能[22, 30],推测RELT基因第3内含子的突变通过可变剪接的方式调控蛋白功能,第3外显子、第3内含子、第4外显子区域可能是RELT蛋白发挥功能的关键区域。
Relt-/-小鼠的临床表型和RELT基因突变患者的釉质表型类似,均表现为釉质表面粗糙,快速磨耗[17]。扫描电镜的结果显示Relt-/-小鼠的釉质矿化程度较低,但釉牙本质界处高度矿化,釉牙本质界机械应力降低,这可能是患牙易磨耗的原因之一[17]。此外,RELT基因突变患者离体恒牙和乳牙的扫描电镜结果显示,釉柱结构排列紊乱,矿物含量降低,表明釉柱结构的紊乱也可能是牙釉质难以抵抗咀嚼压力从而快速磨耗的原因[24]。本研究中先证者表现为釉质的广泛磨耗、缺损,进一步提示了RELT蛋白在釉质成熟过程中可能发挥着重要作用。
动物实验显示,RELT蛋白表达在分泌前期、分泌期成釉细胞和成牙本质细胞中,在成熟期成釉细胞中无表达,但其在牙发育过程中的具体作用尚未阐明[17]。本研究中先证者仅表现为釉质的发育缺陷,提示RELT蛋白在牙发育中可能主要影响成釉细胞。既往研究显示,RELT蛋白在血液系统、淋巴系统中主要调控细胞的凋亡[18-19]。有研究认为,RELT蛋白可能在成釉细胞p38激酶的激活中发挥作用,为正常的釉质形成提供必要的信号[31]。此外,ADAM10表达于成釉细胞的分泌期,可以破坏RELT蛋白的细胞外结构域,且Adam10条件性敲除小鼠的牙齿发育异常[20, 32]。上述研究提示,RELT基因突变可能会影响ADAM10的正常表达,影响成釉细胞凋亡,从而导致釉质发育异常,这一猜想仍需要进一步的研究。
本研究对RELT基因纯合移码突变导致的钙化不全型AI患者的临床表型进行了详细的描述,并对RELT基因的突变特点进行了总结分析。现有的证据表明,RELT基因突变患者主要表现为釉质的重度磨耗,属于钙化不全型AI,表明RELT蛋白在釉质的成熟矿化中发挥重要作用。进一步的研究将会针对RELT蛋白在釉质发育过程中的作用和致病机制进行分子和动物学水平的探索,以便为AI的发病机制和遗传咨询、早期防治等提供分子生物学证据。
Funding Statement
国家自然科学基金(81300839)、北大医学交叉研究种子基金(BMU2022MX019)和国家重大疾病多学科合作诊疗能力建设项目北京大学口腔医院牙体牙髓科子课题(PKUSSNMP-201906)
Supported by the National Natural Science Foundation of China (81300839), Peking University Medicine Seed Fund for Interdisciplinary Research (BMU2022MX019) and National Multidisciplinary Cooperative Diagnosis and Treatment Capacity Building Project for Major Diseases, Department of Cariology and Endodontology, Peking University School and Hospital of Stomatology (PKUSSNMP-201906)
Footnotes
利益冲突 所有作者均声明不存在利益冲突。
作者贡献声明 张真伟:收集、分析数据,撰写论文;徐欣然:分析数据;高学军、董艳梅:设计及指导研究;田华:设计及指导研究,分析数据,修改论文。
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