Department of Medical and Surgical Sciences, St. Orsola-Malpighi Hospital, University of Bologna, Bologna, 40138, Italy.
Department of Molecular Genetics, Erasmus MC, Rotterdam, 3000 CA, The Netherlands.
Brain. 2021 Jun 22;144(5):1451-1466. doi: 10.1093/brain/awab056.
Abnormal gut motility is a feature of several mitochondrial encephalomyopathies, and mutations in genes such as TYMP and POLG, have been linked to these rare diseases. The human genome encodes three DNA ligases, of which only one, ligase III (LIG3), has a mitochondrial splice variant and is crucial for mitochondrial health. We investigated the effect of reduced LIG3 activity and resulting mitochondrial dysfunction in seven patients from three independent families, who showed the common occurrence of gut dysmotility and neurological manifestations reminiscent of mitochondrial neurogastrointestinal encephalomyopathy. DNA from these patients was subjected to whole exome sequencing. In all patients, compound heterozygous variants in a new disease gene, LIG3, were identified. All variants were predicted to have a damaging effect on the protein. The LIG3 gene encodes the only mitochondrial DNA (mtDNA) ligase and therefore plays a pivotal role in mtDNA repair and replication. In vitro assays in patient-derived cells showed a decrease in LIG3 protein levels and ligase activity. We demonstrated that the LIG3 gene defects affect mtDNA maintenance, leading to mtDNA depletion without the accumulation of multiple deletions as observed in other mitochondrial disorders. This mitochondrial dysfunction is likely to cause the phenotypes observed in these patients. The most prominent and consistent clinical signs were severe gut dysmotility and neurological abnormalities, including leukoencephalopathy, epilepsy, migraine, stroke-like episodes, and neurogenic bladder. A decrease in the number of myenteric neurons, and increased fibrosis and elastin levels were the most prominent changes in the gut. Cytochrome c oxidase (COX) deficient fibres in skeletal muscle were also observed. Disruption of lig3 in zebrafish reproduced the brain alterations and impaired gut transit in vivo. In conclusion, we identified variants in the LIG3 gene that result in a mitochondrial disease characterized by predominant gut dysmotility, encephalopathy, and neuromuscular abnormalities.
肠道运动异常是几种线粒体脑肌病的特征，TYMP 和 POLG 等基因的突变与这些罕见疾病有关。人类基因组编码三种 DNA 连接酶，其中只有一种，即连接酶 III（LIG3），具有线粒体剪接变体，对线粒体健康至关重要。我们研究了七个来自三个独立家族的患者中 LIG3 活性降低和由此导致的线粒体功能障碍的影响，这些患者表现出共同的肠道运动障碍和类似于线粒体神经胃肠脑肌病的神经表现。这些患者的 DNA 进行了全外显子组测序。在所有患者中，均发现了一个新疾病基因 LIG3 的复合杂合变异。所有变异均预测对蛋白质具有破坏性影响。LIG3 基因编码唯一的线粒体 DNA（mtDNA）连接酶，因此在 mtDNA 修复和复制中发挥关键作用。在患者来源的细胞中的体外测定表明 LIG3 蛋白水平和连接酶活性降低。我们证明，LIG3 基因缺陷会影响 mtDNA 的维持，导致 mtDNA 耗竭，而不会像其他线粒体疾病那样积累多个缺失。这种线粒体功能障碍可能导致这些患者出现表型。最突出和一致的临床体征是严重的肠道运动障碍和神经异常，包括脑白质病、癫痫、偏头痛、中风样发作和神经性膀胱炎。肠道中最突出的变化是肠神经元数量减少，纤维化和弹性蛋白水平增加。骨骼肌中也观察到细胞色素 c 氧化酶（COX）缺陷纤维。在斑马鱼中破坏 lig3 可重现体内的大脑改变和肠道转运受损。总之，我们鉴定了 LIG3 基因的变异，这些变异导致以肠道运动障碍、脑病和神经肌肉异常为主要特征的线粒体疾病。
