Preferent Diaphragmatic Involvement in TK2 Deficiency: An Autopsy Case Study

Novel biallelic TK2 mutations cause mitochondrial DNA depletion syndrome with infantile early-onset lipid storage myopathy

BACKGROUND

Mutations in the TK2 gene are strongly associated with mitochondrial DNA depletion syndrome (MDS), a severe condition with high mortality and poor outcomes. Although many MDS cases are reported, those linked to TK2 mutations with lipid deposition are rare. Large deletions in the TK2 gene are even rarer.

METHODS

We conducted whole-exome sequencing to find the gene linked to MDS, followed by genomic and structural analyses, histopathological, and functional analyses to assess the mutations' pathogenicity. Additionally, a HEK293T cell model with TK2 mutations was created to investigate the impact of large deletions on mitochondrial function.

RESULTS

The patient was found to have a novel compound heterozygous mutation in the TK2 gene, consisting of a large deletion spanning exons 5-10 (E5-E10 del) and a previously reported missense mutation (c.311C > A, p.Arg104His). Analysis of the patient's muscle tissue demonstrated a marked reduction in mtDNA content and a significant impairment in overall mitochondrial function. In the HEK293T cell model, the group with the deletion mutation exhibited a notable reduction in TK2 protein expression and levels of mitochondrial complex subunits when compared to the control group. Furthermore, there was an observed increase in ROS levels, a decrease in ATP production, and compromised mitochondrial respiratory chain function. Moreover, we conducted a comprehensive review of the previously reported genotypic and phenotypic spectrum of TK2 mutations in the literature.

CONCLUSIONS

This case report underscores the detrimental impact of large fragment deletion mutations in the TK2 gene and elucidates their role in the pathogenesis of MDS. It broadens the spectrum of known TK2 mutations and enhances our understanding of the structural and functional consequences of these mutations.

Clinical and molecular spectrum of TK2-deficiency: a large Brazilian cohort

Biallelic pathogenic variants at TK2 lead to a severe and progressive myopathy (TK2d). For a disease with unspecific clinical findings, and the possibility of a supplementation therapy that changes the natural history of the disease, highlighting clinical features that increase suspicion and accelerate diagnosis is essential. Clinical and genetic findings of 36 Brazilian patients with TK2d were identified and presented in this work. Genotype-phenotype correlation was performed for recurrent and novel variants. Motor and respiratory assessments were systematically performed in 13 patients, three of them were receiving the nucleosides replacement therapy. Natural history data was gathered from the follow up of five adult patients. Eight patients with the infantile form, 19 with childhood-onset and five with late-onset form were described. Extramuscular features were present in 30% of the cohort. Neuropathy and encephalopathy were the clinically predominant features for some patients. Four variants were recurrent (p.Thr108M, p.His121Asn, p.Arg183Trp and c.536_538 + 8del) allowing genotype-phenotype correlations, and one was novel (G91D). P.Thr108Met patients presented a milder presentation when compared to the p.His121Asn group. P.Arg183Trp was associated with peripheral nerve involvement and c.536_538 + 8del with encephalomyopathy. Long-term follow-up of 5 patients harbouring p.Thr108Met showed decreased motor, bulbar, and respiratory function, compared to a dramatic improvement in the treated patients. TK2d is a very debilitating and progressive disease among all forms including the childhood-onset as we demonstrated. Early diagnosis is essential since a potential treatment can change the natural history of the disease. Extramuscular involvement plays an important role for diagnostic strategies.

Remarkable clinical improvement with oral nucleoside treatment in a patient with adult-onset TK2 deficiency: A case report

OBJECTIVES

Thymidine kinase 2 deficiency (TK2d) is a rare autosomal recessive mitochondrial disorder. It manifests as a continuous clinical spectrum, from fatal infantile mitochondrial DNA depletion syndromes to adult-onset mitochondrial myopathies characterized by ophthalmoplegia-plus phenotypes with early respiratory involvement. Treatment with pyrimidine nucleosides has recently shown striking effects on survival and motor outcomes in the more severe infantile-onset clinical forms. We present the response to treatment in a patient with adult-onset TK2d.

METHODS

An adult with ptosis, ophthalmoplegia, facial, neck, and proximal muscle weakness, non-invasive nocturnal mechanical ventilation, and dysphagia due to biallelic pathogenic variants in TK2 received treatment with 260 mg/kg/day of deoxycytidine (dC) and deoxythymidine (dT) under a Compassionate Use Program. Prospective motor and respiratory assessments are presented.

RESULTS

After 27 months of follow-up, the North Star Ambulatory Assessment improved by 11 points, he walked 195 m more in the 6 Minute-Walking-Test, ran 10 s faster in the 100-meter time velocity test, and the Forced Vital Capacity stabilized. Growth Differentiation Factor-15 (GDF15) levels, a biomarker of respiratory chain dysfunction, normalized. The only reported side effect was dose-dependent diarrhea.

DISCUSSION

Treatment with dC and dT can significantly improve motor performance and stabilize respiratory function safely in patients with adult-onset TK2d.

Mitochondrial DNA-targeted therapy: A novel approach to combat cancer

Mitochondrial DNA (mtDNA) encodes proteins and RNAs that are essential for mitochondrial function and cellular homeostasis, and participates in important processes of cellular bioenergetics and metabolism. Alterations in mtDNA are associated with various diseases, especially cancers, and are considered as biomarkers for some types of tumors. Moreover, mtDNA alterations have been found to affect the proliferation, progression and metastasis of cancer cells, as well as their interactions with the immune system and the tumor microenvironment (TME). The important role of mtDNA in cancer development makes it a significant target for cancer treatment. In recent years, many novel therapeutic methods targeting mtDNA have emerged. In this study, we first discussed how cancerogenesis is triggered by mtDNA mutations, including alterations in gene copy number, aberrant gene expression and epigenetic modifications. Then, we described in detail the mechanisms underlying the interactions between mtDNA and the extramitochondrial environment, which are crucial for understanding the efficacy and safety of mtDNA-targeted therapy. Next, we provided a comprehensive overview of the recent progress in cancer therapy strategies that target mtDNA. We classified them into two categories based on their mechanisms of action: indirect and direct targeting strategies. Indirect targeting strategies aimed to induce mtDNA damage and dysfunction by modulating pathways that are involved in mtDNA stability and integrity, while direct targeting strategies utilized molecules that can selectively bind to or cleave mtDNA to achieve the therapeutic efficacy. This study highlights the importance of mtDNA-targeted therapy in cancer treatment, and will provide insights for future research and development of targeted drugs and therapeutic strategies.

Metrics of progression and prognosis in untreated adults with thymidine kinase 2 deficiency: An observational study

This historical cohort study evaluated clinical characteristics of progression and prognosis in adults with thymidine kinase 2 deficiency (TK2d). Records were available for 17 untreated adults with TK2d (mean age of onset, 32 years), including longitudinal data from 6 patients (mean follow-up duration, 26.5 months). Pearson's correlation assessed associations between standard motor and respiratory assessments, clinical characteristics, and laboratory values. Longitudinal data were assessed by linear regression mixed models. Respiratory involvement progressed at an annual rate of 8.16% decrement in forced vital capacity (FVC). Most patients under noninvasive ventilation (NIV) remained ambulant (12/14, 86%), reduced FVC was not associated with concomitant decline in 6-minute walk test (6MWT), and 6MWT results were not correlated with FVC. Disease severity, assessed by age at NIV onset, correlated most strongly at diagnosis with: creatinine levels (r = 0.8036; P = 0.0009), followed by FVC (r = 0.7265; P = 0.0033), mtDNA levels in muscle (r = 0.7933; P = 0.0188), and age at disease onset (r = 0.7128; P = 0.0042). This population of adults with TK2d demonstrates rapid deterioration of respiratory muscles, which progresses independently of motor impairment. The results support FVC at diagnosis, mtDNA levels in muscle, and age at disease onset as prognostic indicators. Creatinine levels may also be potentially prognostic, as previously reported in other neuromuscular disorders.

Collaborative model for diagnosis and treatment of very rare diseases: experience in Spain with thymidine kinase 2 deficiency

Background

Mitochondrial diseases are difficult to diagnose and treat. Recent advances in genetic diagnostics and more effective treatment options can improve patient diagnosis and prognosis, but patients with mitochondrial disease typically experience delays in diagnosis and treatment. Here, we describe a unique collaborative practice model among physicians and scientists in Spain focused on identifying TK2 deficiency (TK2d), an ultra-rare mitochondrial DNA depletion and deletions syndrome.

Main Body

This collaboration spans research and clinical care, including laboratory scientists, adult and pediatric neuromuscular clinicians, geneticists, and pathologists, and has resulted in diagnosis and consolidation of care for patients with TK2d. The incidence of TK2d is not known; however, the first clinical cases of TK2d were reported in 2001, and only ~ 107 unique cases had been reported as of 2018. This unique collaboration in Spain has led to the diagnosis of more than 30 patients with genetically confirmed TK2d across different regions of the country. Research affiliate centers have led investigative treatment with nucleosides based on understanding of TK2d clinical manifestations and disease mechanisms, which resulted in successful treatment of a TK2d mouse model with nucleotide therapy in 2010. Only 1 year later, this collaboration enabled rapid adoption of treatment with pyrimidine nucleotides (and later, nucleosides) under compassionate use. Success in TK2d diagnosis and treatment in Spain is attributable to two important factors: Spain’s fully public national healthcare system, and the designation in 2015 of major National Reference Centers for Neuromuscular Disorders (CSURs). CSUR networking and dissemination facilitated development of a collaborative care network for TK2d disease, wherein participants share information and protocols to request approval from the Ministry of Health to initiate nucleoside therapy. Data have recently been collected in a retrospective study conducted under a Good Clinical Practice–compliant protocol to support development of a new therapeutic approach for TK2d, a progressive disease with no approved therapies.

Conclusions

The Spanish experience in diagnosis and treatment of TK2d is a model for the diagnosis and development of new treatments for very rare diseases within an existing healthcare system.