线粒体tRNA^(ser(UCN))基因突变致线粒体脑肌病1例并文献复习

Clinical and mtDNA analysis in a mitochondrial cytopathy child due to the mutation of mitochondrial tRNA^(ser(UCN)) gene

目的分析1例线粒体细胞病患儿的临床表现及其基因突变特点。方法对1例临床诊断为线粒体脑肌病患儿归纳总结其临床表现及实验室检查结果,并运用PCR法扩增患儿外周血线粒体基因3243、8344和8993热点突变及已报道的62个常见突变位点所在片段,对扩增片段采用直接测序方法检测突变。在某医院年度体检中选择70名无血缘关系的健康成人作为正常对照,采用PCR-RFLP方法进行多态性分析。结果男性患儿,1岁9个月时出现持续高乳酸血症、反复严重代谢性酸中毒和高氨血症,头颅CT扫描显示双侧额顶叶对称性空泡样低密度灶,考虑线粒体性脑肌病;脑萎缩。2岁时死亡。患儿外周血线粒体基因3243、8344和8993热点突变及已报道的62个常见突变位点均未见突变,患儿线粒体tRNAser(UCN)基因存在7496 T→C突变。为证实在正常人群中线粒体tRNAser(UCN)基因是否存在7496 T→C突变,70名正常对照组皆未发现这一位点突变。结论线粒体脑肌病可以表现为代谢紊乱和神经损伤,应提高警惕。线粒体tRNAser(UCN)基因7496 T→C突变可能导致线粒体细胞病。该突变尚未见报道。

Objective Mitochondrial cytopathies represent a heterogeneous group of multisystem disorders which preferentially affect the muscle and nervous systems.The disease has emerged as a major entity among the neurometabolic diseases of childhood with an incidence of 1 in 11,000 children,and also has a high prevalence in adults.They can be caused either by mutations in mitochondrial genome,or by nuclear DNA mutations.The first pathogenic mutation of human mitochondrial DNA（mtDNA） was discovered in 1988.Since then more than 100 mutations of mtDNA have been reported,including point mutations of genes encoding transfer RNA,ribosomal RNA,and proteins,as well as large-scale deletions.However,mitochondrial cytopathy could be easily misdiagnosed because of its complicated clinical features.This study aimed to review the clinical features and analyze mitochondrial DNA mutations in a Chinese patient presenting mitochondrial encephalopathy.Methods Clinical manifestations and laboratory findings of the patient were summarized.The genomic DNA of the patient and 70 normal controls were extracted from peripheral blood with QIAamp DNA Mini Kit.All fragments including 3243、8344、8993 point mutations and 62 other common mutations reported frequently in mtDNA of the patient was amplified by polymerase chain reaction（PCR）.Direct-sequencing was performed for mutation analysis.If new mutations were detected,the fragment of the new mutation was amplified by PCR,then restriction fragment length polymorphism（RFLP） was used for polymorphism detection in 70 normal controls.These controls were healthy medical staffs of a hospital,meeting the following conditions： firstly,blood biochemistry,routine blood test and abdomen ultrasound in physical examination were normal;secondly,blood specimen after blood biochemistry remained 〉2 mL;thirdly,there was no kinship with each other;fourthly,they were under 40 years old;male or female were not limited;lastly,they were all orally informed consent.Results The patient was a boy,admitted into hospital when he was 21 months of age because of anepithymia and hyperdiuresis.He presented as elevated lactate,severe metabolic acidosis,hyperammonniemia and hearing impairment.Brain CT scan showed symmetric diffuse low-density lesions at frontal lobe and parietal lobe and the patient was considered to be with mitochondrial encephalopathy.While muscle biopsy was suggested,his parents refused because of his poor physical conditions.He showed gradually retrocession： firstly hypotonia,then myotonia and coma.The patient died at two years old.His true family history was unknown because he was adopted by his parents.MtDNA 3243、8344、8993 and 62 reported common mutations were not detected in the patient.mtDNA 7496 T→C mutation in tRNAser（UCN） gene was detected in the patient,which affected a highly conserved site.The mutation led to the loss of a restriction site for the endonuclease NcoⅠ,so the tRNAser gene of the patient could not be cleaved while that of 70 normal controls were completely cleaved,suggesting no same mutation was identified in 70 normal controls.Conclusions Mitochondrial cytopathy might be presented as metabolic disorder and neuropathy and should be alerted to.The analysis of mtDNA mutation was worthwhile in the diagnostic evaluation of patients with clinically probable mitochondrial encephalomyopathy.MtDNA 7496 T→C mutation in tRNAser（UCN） gene may cause mitochondrial encephalopathy which has never been reported.This study would be helpful for the future diagnosis and genetic counseling of mitochondrial encephalopathy.