Department of Neurology, H. Houston Merritt Neuromuscular Research Center, Columbia University Medical Center, New York, NY 10032, USA.
Department of Biomedical Engineering, Ultrasound and Elasticity, Imaging Laboratory, Columbia University, New York, NY, USA.
EBioMedicine. 2019 Aug;46:356-367. doi: 10.1016/j.ebiom.2019.07.037. Epub 2019 Aug 2.
TK2 is a nuclear gene encoding the mitochondrial matrix protein thymidine kinase 2 (TK2), a critical enzyme in the mitochondrial nucleotide salvage pathway. Deficiency of TK2 activity causes mitochondrial DNA (mtDNA) depletion, which in humans manifests predominantly as a mitochondrial myopathy with onset typically in infancy and childhood. We previously showed that oral treatment of the Tk2 H126N knock-in mouse model (Tk2) with the TK2 substrates, deoxycytidine (dCtd) and thymidine (dThd), delayed disease onset and prolonged median survival by 3-fold. Nevertheless, dCtd + dThd treated Tk2 mice showed mtDNA depletion in brain as early as postnatal day 13 and in virtually all other tissues at age 29 days.
To enhance mechanistic understanding and efficacy of dCtd + dThd therapy, we studied the bioavailability of dCtd and dThd in various tissues as well as levels of the cytosolic enzymes, TK1 and dCK that convert the deoxynucleosides into dCMP and dTMP.
Parenteral treatment relative to oral treatment produced higher levels of dCtd and dThd and improved mtDNA levels in liver and heart, but did not ameliorate molecular defects in brain or prolong survival. Down-regulation of TK1 correlated with temporal- and tissue-specificity of response to dCtd + dThd. Finally, we observed in human infant and adult muscle expression of TK1 and dCK, which account for the long-term efficacy to dCtd + dThd therapy in TK2 deficient patients.
These data indicate that the cytosolic pyrimidine salvage pathway enzymes TK1 and dCK are critical for therapeutic efficacy of deoxynucleoside therapy for Tk2 deficiency. FUND: National Institutes of Health P01HD32062.
TK2 是一种核基因，编码线粒体基质蛋白胸苷激酶 2（TK2），这是线粒体核苷酸补救途径中的关键酶。TK2 活性缺乏会导致线粒体 DNA（mtDNA）耗竭，这在人类中主要表现为线粒体肌病，发病通常在婴儿期和儿童期。我们之前曾表明，用 TK2 底物脱氧胞苷（dCtd）和胸苷（dThd）对 Tk2 H126N 敲入小鼠模型（Tk2）进行口服治疗可使疾病发作延迟，中位生存期延长 3 倍。然而，dCtd+dThd 治疗的 Tk2 小鼠在出生后第 13 天以及在 29 天大时几乎所有其他组织中均表现出脑中线粒体 DNA 耗竭。
为了增强对 dCtd+dThd 治疗机制的理解和疗效，我们研究了各种组织中 dCtd 和 dThd 的生物利用度以及将脱氧核苷转化为 dCMP 和 dTMP 的细胞质酶 TK1 和 dCK 的水平。
与口服治疗相比，静脉内治疗可提高肝和心脏中 dCtd 和 dThd 的水平，并改善 mtDNA 水平，但不能改善脑内的分子缺陷或延长生存时间。TK1 的下调与 dCtd+dThd 反应的时间和组织特异性相关。最后，我们在人类婴儿和成人肌肉中观察到 TK1 和 dCK 的表达，这解释了 dCtd+dThd 治疗在 TK2 缺乏患者中的长期疗效。
这些数据表明，细胞质嘧啶补救途径酶 TK1 和 dCK 对 deoxynucleoside 治疗 Tk2 缺乏症的治疗效果至关重要。
美国国立卫生研究院 P01HD32062。
