Biallelic loss-of-function mutations in JAM2 cause primary familial brain calcification
Biallelic loss-of-function mutations in JAM2 cause primary familial brain calcification
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JAM2双等位基因功能丧失突变导致原发性家族性脑钙化
DOI：
10.1093/brain/awz392
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发表时间：
2020-02-01
期刊：
影响因子：
14.5
通讯作者：
Luo, Wei
中科院分区：
文献类型：
--
作者：
Cen, Zhidong;Chen, You;Luo, Wei
原发性家族性脑钙化是一种单基因疾病，其特征是基底神经节和其他大脑区域的双侧钙化，通常表现为运动、精神和认知症状。目前，已鉴定出四种常染色体显性（SLC20A2、PDGFRB、PDGFB、XPR1）和一种常染色体隐性（MYORG）致病基因。与常染色体显性遗传性原发性家族性脑钙化患者相比，隐性遗传性脑钙化患者临床和影像学表型更为严重，值得更多的临床和研究关注。 MYORG 的双等位基因突变无法解释所有常染色体隐性遗传原发性家族性脑钙化病例，表明存在新的常染色体隐性基因。通过纯合性作图和全基因组测序，我们在一个近亲家庭的 JAM2 基因中检测到纯合移码突变（c.140delT，p.L48*），该家族中有两个受影响的兄弟姐妹被诊断患有原发性家族性脑钙化。对 398 名先证者的进一步基因筛查在两个不相关的家族中分别检测到纯合起始密码子突变 (c.1A>G, p.M1?) 和复合杂合突变 [c.504G> C, p.W168C 和 c.(67+1_68-1)_(394+1_395-1), p.Y23 V131delinsLj。这四名患者的临床表型包括帕金森病（3/4）、构音障碍（3/4）、癫痫发作（1/4）和可能无症状（1/4），且发病年龄各异。除了多个脑区域（豆状核、尾状核、丘脑、小脑半球、+/-脑干；总钙化评分：43-77）广泛钙化外，所有患者均出现皮质严重钙化。 JAM2 编码连接粘附分子 2，该分子在神经血管单位相关细胞类型（内皮细胞和星形胶质细胞）中高度表达，并且主要位于质膜上。它对于细胞间粘附和维持中枢神经系统的稳态可能很重要。在中国仓鼠卵巢细胞中，转染p.Y23_V131delinsL突变质粒后，Western blot检测到截短的His标签JAM2蛋白，而转染p.L48*或p.1M?后未检测到蛋白。突变质粒。在免疫荧光实验中，p.W168C突变型JAM2蛋白未能易位至质膜。我们推测突变的 JAM2 蛋白会导致细胞间粘附功能受损并降低神经血管单元的完整性。这与原发性家族性脑钙化或脑钙化综合征的其他致病基因（例如 PDGFRB、PDGFB、MYORG、JAM3 和 OCLN）的机制相似，所有这些基因在神经血管单元中都高度表达且具有重要的功能。我们的研究确定了原发性家族性脑钙化的一个新致病基因，其重要功能和在神经血管单元中的高表达进一步支持神经血管单元损伤是原发性家族性脑钙化发病机制的根源。
Primary familial brain calcification is a monogenic disease characterized by bilateral calcifications in the basal ganglia and other brain regions, and commonly presents motor, psychiatric, and cognitive symptoms. Currently, four autosomal dominant (SLC20A2, PDGFRB, PDGFB, XPR1) and one autosomal recessive (MYORG) causative genes have been identified. Compared with patients with autosomal dominant primary familial brain calcification, patients with the recessive form of the disease present with more severe clinical and imaging phenotypes, and deserve more clinical and research attention. Biallelic mutations in MYORG cannot explain all autosomal recessive primary familial brain calcification cases, indicating the existence of novel autosomal recessive genes. Using homozygosity mapping and whole genome sequencing, we detected a homozygous frameshift mutation (c.140delT, p.L48*) in the JAM2 gene in a consanguineous family with two affected siblings diagnosed with primary familial brain calcification. Further genetic screening in a cohort of 398 probands detected a homozygous start codon mutation (c.1A>G, p.M1?) and compound heterozygous mutations [c.504G> C, p.W168C and c.(67+1_68-1)_(394+1_395-1), p.Y23 V131delinsLj, respectively, in two unrelated families. The clinical phenotypes of the four patients included parkinsonism (3/4), dysarthria (3/4), seizures (1/4), and probable asymptomatic (1/4), with diverse onset ages. All patients presented with severe calcifications in the cortex in addition to extensive calcifications in multiple brain areas (lenticular nuclei, caudate nuclei, thalamus, cerebellar hemispheres, +/- brainstem; total calcification scores: 43-77). JAM2 encodes junctional adhesion molecule 2, which is highly expressed in neurovascular unitrelated cell types (endothelial cells and astrocytes) and is predominantly localized on the plasma membrane. It may be important in cell-cell adhesion and maintaining homeostasis in the CNS. In Chinese hamster ovary cells, truncated His-tagged JAM2 proteins were detected by western blot following transfection of p.Y23_V131delinsL mutant plasmid, while no protein was detected following transfection of p.L48* or p.1M? mutant plasmids. In immunofluorescence experiments, the p.W168C mutant JAM2 protein failed to translocate to the plasma membrane. We speculated that mutant JAM2 protein resulted in impaired cell-cell adhesion functions and reduced integrity of the neurovascular unit. This is similar to the mechanisms of other causative genes for primary familial brain calcification or brain calcification syndromes (e.g. PDGFRB, PDGFB, MYORG, JAM3, and OCLN), all of which are highly expressed and functionally important in the neurovascular unit. Our study identifies a novel causative gene for primary familial brain calcification, whose vital function and high expression in the neurovascular unit further supports impairment of the neurovascular unit as the root of primary familial brain calcification pathogenesis.