Domínguez‑González et al. Orphanet J Rare Dis (2021) 16:407 https://doi.org/10.1186/s13023-021-02030-w REVIEW Open Access Collaborative model for diagnosis and treatment of very rare diseases: experience in Spain with thymidine kinase 2 deficiency Cristina Domínguez‑González1,2,3, Marcos Madruga‑Garrido4, Michio Hirano5, Itxaso Martí6, Miguel A. Martín3,7, Francina Munell8, Andrés Nascimento3,8,9, Montse Olivé10, Joanne Quan11, M. Dolores Sardina12, Ramon Martí3,13 and Carmen Paradas14,15* A portion of this work was presented September 28−October 2, 2020, at the Virtual World Muscle Society meeting, and November 30−December 2, 2020, at the Mitochondrial Medicine Virtual Conference. Abstract 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 dis‑ ease 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 Refer‑ ence 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. *Correspondence: cparadas@us.es 14 Neurology Department, Neuromuscular Disorders Unit, Instituto de Biomedicina de Sevilla, Hospital U. Virgen del Rocío, CSIC, Universidad de Sevilla, Avd. Manuel Siurot s/n, 41013 Sevilla, Spain Full list of author information is available at the end of the article © The Author(s) 2021. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/. The Creative Commons Public Domain Dedication waiver (http://​creat​iveco​ mmons.​org/​publi​cdoma​in/​zero/1.​0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Domínguez‑González et al. Orphanet J Rare Dis (2021) 16:407 Page 2 of 9 Conclusions: The Spanish experience in diagnosis and treatment of TK2d is a model for the diagnosis and develop‑ ment of new treatments for very rare diseases within an existing healthcare system. Keywords: Mitochondrial disease, Mitochondrial medicine, Thymidine kinase 2 deficiency (TK2d) Background patients with rare diseases are most efficiently organ- Patients with rare mitochondrial disorders predomi- ized in reference centers, which can provide specialized nantly manifesting with muscle symptoms experience assistance, ensure equity of access to care [1], and con- long diagnostic odysseys [1–4]. Most patients with mito- tribute to gathering and dissemination of knowledge [2]. chondrial myopathies will see an average of eight physi- In Spain, CSURs-NMD have simplified and streamlined cians over 8–10 years before receiving a diagnosis [4]. the referral and diagnostic process. Referrals of patients For patients with rapidly progressing, severe disease, this with TK2d and other mitochondrial myopathies among delay between onset of symptoms and initiation of treat- national Units of Neuromuscular Disorders within the ment could be fatal or could result in significant morbid- SNS facilitate rapid identification, diagnosis, and treat- ity [2]. ment of patients. Spain has been uniquely successful in identifying and treating patients with thymidine kinase 2 deficiency TK2 deficiency disease: clinical features (TK2d), an ultra-rare, autosomal recessive mitochon- TK2d is characterized by progressive muscle weakness, drial DNA (mtDNA) depletion and deletions syndrome with predominant distribution in the proximal, bul- (MDDS). A collaborative partnership, mainly between bar, and facial muscles together with severe respiratory research centers at the Vall d’Hebron Research Institute involvement [7–10]. Age of onset and rate of progression (VHIR) in Barcelona (Spain) and Columbia University are quite variable, but in all its clinical forms, TK2d clini- Irving Medical Center (CUIMC) in New York City (USA), cal manifestations can include proximal muscle weak- has facilitated understanding of disease pathology and ness, dysphagia, and respiratory insufficiency, leading the development of oral nucleosides as potential therapy to premature death in a majority of patients [7, 8, 11]. [5]. Spain continues to lead in identification, diagnosis, As is the case for most rare diseases, TK2d is probably and treatment of patients with TK2d due to collabora- underdiagnosed. Approximately 107 molecularly con- tion and networking through the Spanish National Ref- firmed cases were reported worldwide in the published erence Centers for Neuromuscular Disorders (CSURs). literature as of 2018 [11], but TK2d incidence is probably Here, we describe our experience in TK2d to exemplify underestimated due to early mortality and misdiagno- how CSURs can shorten the diagnostic odyssey and con- ses [3]. Given the progressive nature of the disease and solidate patient care for a rare mitochondrial disease that the availability of effective therapy with oral nucleosides, requires coordination of highly specialized, multidiscipli- earlier identification, diagnosis, and treatment are criti- nary care teams to improve patient outcomes and quality cal to patient prognosis, survival, and functionality in the of life. motor, respiratory, and feeding domains [7, 8, 10, 12, 13]. The objective of this work is to illustrate how CSURs in Spain (1) coordinate research, diagnosis, and treatment Collaborative care model in Spain: focus on TK2d efforts within the framework of the Spanish National The collaborative care model in Spain has identified and Health System (SNS), (2) optimize access to care for treated the largest cohort of patients worldwide: ~ 30 of patients with TK2d, and (3) could serve as a model for the ~ 107 total TK2d cases were diagnosed and treated in diagnosis and treatment of patients with rare neuromus- Spain (~ 30%). In Spain, most patients with genetic myo- cular diseases throughout the EU. pathies are referred to CSURs for neuromuscular disor- ders (CSURs-NMD) for diagnosis and treatment through Main body national neuromuscular units (Fig. 1) [1, 6]. The network Treatment of TK2d and rare neuromuscular disease of CSURs-NMD throughout Spain coordinate with the in Spain: the importance of reference centers Spanish collaboration among national Units of Neuro- In 2015, the Spanish Ministry of Health designated a net- muscular Disorders to facilitate identification and homo- work of National Reference Centers (CSURs) for Neu- geneous treatment (Fig. 2). CSURs-NMD at Hospital U romuscular Disorders by Real Decreto 1302/2006. The 12 Octubre (H12O) in Madrid and Hospital U Virgen TK2d experience in Spain is a uniquely successful exam- del Rocío (HUVR) in Sevilla were pioneers in coordinat- ple of a rare disease that has been diagnosed and treated ing the process to harmonize research, clinical care, and within the CSUR framework [6]. Diagnosis and care for management of TK2d cases. Domínguez‑González et al. Orphanet J Rare Dis (2021) 16:407 Page 3 of 9 Fig. 1 Collaboration among major reference centers across regions of Spain and research affiliates Fig. 2 Role of the CSURs in coordinating research, diagnosis, clinical care, and information dissemination Diagnosis and treatment paradigms within the CSUR Disorders [CIBERNED], Institute of Biomedicine in framework integrate ongoing research and develop- Seville [IBiS], Research Institute Hospital 12 de Octubre ment activities into clinical practice. Collaboration [imas12], VHIR). Neuromuscular disease specialists pro- occurs between local centers and CSURs within a vide expert opinion on obtaining differential diagnosis of regional network, which connects to the larger, national TK2d, including recommendations for additional test- CSUR network. CSURs-NMD further coordinate with ing. CSURs develop clinical protocols and provide treat- research affiliates in Spain (Center for Biomedical Net- ment for TK2d through clinical trials and compassionate work Research on Rare Diseases [CIBERER], Center for use programs to ensure timely access to care. CSURs- Biomedical Network Research on Neurodegenerative NMD, as part of the wider EURO-NMD, also disseminate Domínguez‑González et al. Orphanet J Rare Dis (2021) 16:407 Page 4 of 9 information and house registries for international access. 30 pathogenic variants in the TK2 gene have been iden- Although no archival registry of genetic mutations in tified, most of which are missense mutations (70%) [11] TK2d is available, patients with TK2d and their caregiv- and result in loss of function of the TK2 enzyme, which ers are invited to join the TK2d patient registry (https://​ phosphorylates deoxythymidine (dT) and deoxycy- www.​tk2d.​com/) to facilitate networking and data dis- tidine (dC) to deoxythymidine monophosphate (dTMP) semination. CSURs consolidate expertise to provide and deoxycytidine monophosphate (dCMP), respec- care of the highest standard for affected individuals. In tively (Fig. 4). These deoxynucleoside monophosphates the case of TK2d, the Spanish collaboration and CSURs- are consecutively phosphorylated to the deoxynucleo- NMD allowed rapid transition of targeted nucleoside side triphosphates (dNTPs) necessary for replication of therapy through the development pipeline by coordinat- mtDNA [11]. Ensuing mtDNA depletion/multiple dele- ing preclinical and clinical research activities, including tions lead to inadequate production of mitochondrial the recent Good Clinical Practice (GCP)-compliant ret- respiratory chain complexes needed for mitochondrial rospective protocol. Ongoing research initiatives seek to oxidative phosphorylation, the main source of cellular optimize patient treatment strategies (NCT03845712; energy [18]. Genotype–phenotype correlations were NCT03701568). Notably, of 38 total patients enrolled, 23 characterized in two mouse models of TK2d. Research- patients were from Spain (60%). Four of the eight (50%) ers at CUIMC developed a TK2 H126N knockin mouse participating sites were from the collaboration, and model of TK2d based on observed clinical pathogenic patients are continuing to receive nucleoside therapy. phenotypes [19, 20]. That same year, Swedish researchers independently characterized a TK2 knockout (TK2−/−) Key milestones in TK2d diagnosis and treatment: role mouse model [21]. Phenotype in the mouse models of CSURs‑NMD and Spanish research affiliates recapitulated clinical presentation of severe mtDNA Clinicians and scientists at VHIR (Spain) and CUIMC depletion in skeletal muscle, leading to mitochondrial (USA) have collaboratively contributed to TK2d research myopathy. Taken together, these data delineated the and development since the first TK2d patients were pathogenesis of TK2d and suggested targeted nucleoside described in 2001 [14, 15] (Fig. 3). The collaboration therapy as a plausible treatment modality to restore mito- represents a critical mass of individuals with multidis- chondrial function by restoring available pools of dNTPs. ciplinary expertise to interpret clinical findings and to Proof-of-concept using nucleotide therapy was first propose and test effective treatment modalities for TK2d published in mouse models in 2014 [22], paving the way and other mitochondrial disorders. Within 2 years of the for establishing dCMP/dTMP as the first effective phar- first reported TK2d cases [14, 15], mitochondrial disease macologic treatment strategy for TK2d in human clinical researchers at CUIMC characterized genetic and clinical studies [12]. The preclinical results with dTMP/dCMP features of TK2d and described its pathogenesis [16, 17]. in TK2-deficient mice encouraged treatment of the first TK2d is caused by mutations and deletions in the TK2 Spanish patient with dTMP/dCMP, and soon thereafter gene on chromosome 16q21 in nuclear DNA. More than a second patient was treated in the USA [5, 12]. Further Fig. 3 Key milestones in TK2d diagnostics and treatment: role of the Spanish research affiliates and collaborators [7, 10, 14] Domínguez‑González et al. Orphanet J Rare Dis (2021) 16:407 Page 5 of 9 Fig. 4 Pharmacological mechanism of nucleoside therapy in TK2d. MT1621 (dC/dT) deoxynucleosides drive production of dTMP and dCMP from the enzymes thymidine kinase 1 (TK1) and deoxycytidine kinase (dCK), respectively, which cross the mitochondrial membrane to provide substrate for dTTP and deoxycytidine triphosphate (dCTP) production in mtDNA synthesis and replication [23]. Both mechanisms help to restore the available pool of mtDNA for oxidative phosphorylation and energy production studies demonstrated that, when administered orally, the Therapy Designation was granted by the US-FDA. Clini- active compounds are not the monophosphates dTMP/ cal studies are under way to obtain regulatory approval dCMP, but the corresponding nucleosides dT/dC [5, 23]. for worldwide availability of MT1621 for patients with Pharmacologically, nucleoside therapy restores mito- TK2d. chondrial function through both a cytosolic nucleoside In parallel with drug development, three seminal salvage pathway and residual TK2 activity within the papers describing the natural history of TK2d were mitochondria (see Fig. 4). Approval for the use of nucleo- published in 2018 and 2019 (Fig. 3). These papers rep- sides in humans under compassionate use by the Spanish resented important milestones in characterizing the nat- Drug Agency (AEMPS), driven by CSURs-NMD at H12O ural history of TK2d. Case study and case series reports and HUVR, led to treatment of approximately 30 patients published over approximately 2 decades were consoli- in Spain between 2014 and 2018 [5, 12]. These clinical dated into the two 2018 publications describing the clini- data demonstrate the efficacy of nucleoside therapy in cal phenotype and spectrum of disease [8, 10]. A year slowing or reversing disease progression and prolong- later, as a consequence of Spanish success in identifying ing survival among patients with TK2d internationally, TK2d patients, Domínguez-González et al. published a with more than 40 individuals currently under treat- description of late-onset TK2d based on 18 patients, 16 ment in the USA, Central and South America, Eastern of whom were identified in Spain. The late-onset form, Europe, and Asia. Nucleoside therapy was licensed with which accounted for about 20% of cases, was first identi- industry stakeholders in 2016, based on patented intel- fied in 2010 as a slower progressing form of disease [11]. lectual property with Columbia University and the VHIR. Improved genetic testing enabled the characterization of PRIME (PRIorityMEdicines) designation was granted by late-onset TK2d in 2013 [24]. These papers led to pub- the EMA in 2018 for the product under development, lication of the first proposed diagnostic and treatment MT1621, which is a combination product of two nucle- algorithms [7, 8, 10, 11]. osides, deoxycytidine (dC) and deoxythymidine (dT), intended as a nucleoside therapy via a calculated dose of Diagnosis combination product (dC/dT) for the treatment of TK2d Detailed characterization of the natural history of TK2d (Fig. 4). MT1621 was granted Orphan Drug designa- has informed diagnostic algorithms and improved diag- tion by both the US-FDA and the EMA. Breakthrough nostic accuracy within the CSUR framework in Spain. Domínguez‑González et al. Orphanet J Rare Dis (2021) 16:407 Page 6 of 9 Typically, patients with TK2d in Spain would initially therapeutic landscapes by introducing disease-tailoring present to their primary care physician, but general prac- therapies that require identification of a genetic defect titioners cannot directly refer patients to CSURs. Patients [29]. presenting with muscle weakness are first referred to the Currently, access to genetic studies in the SNS is widely general neurology unit or to a neuromuscular neurolo- available; hence, we sometimes find patients with mito- gist at a local center. If patients do not receive a defini- chondrial disease who are referred from the local neu- tive diagnosis, or if the case shows high complexity, then romuscular unit to the CSUR-NMD unit after already patients with myopathy are referred to a CSUR-NMD. An receiving a genetic diagnosis. On the other hand, when appointment is scheduled within 15 days after a simple a referring physician sends a muscle sample to a labora- form is filled out to indicate referral to a CSUR. tory with expertise in mitochondrial diseases, the entire TK2d typically manifests as pure muscle weakness, diagnostic process, including thorough genetic testing a tissue-specific phenotype that is quite different from (i.e., both mtDNA and nuclear encoded gene analysis), is most infantile mitochondrial syndromes that exhibit mul- performed without the need for the referring doctor to tisystemic involvement and are usually referred to CSURs specify the tests to be performed. This makes it easier for for inborn metabolism errors. Therefore, in our experi- a definitive genetic diagnosis to be reached, even if the ence in Spain, patients with TK2d are identified for the doctor sending the sample is not an expert in mitochon- first time in neuromuscular units, where a muscle biopsy drial diseases. is frequently performed to guide the diagnosis of patients Clinical guidelines specific to TK2d are under develop- with muscle symptoms. In patients with TK2d, the biopsy ment, but an operational diagnostic algorithm is in place confirms the mitochondrial origin of the disorder, thus at reference centers in Spain based on consolidation of all directing the subsequent genetic study. If a muscle biopsy published case reports and case series, harmonized with is not performed, the use of next-generation sequencing guidelines recommended by the EURO-NMD Muscle (NGS) approaches using phenotype-driven gene panels Diseases and Mitochondrial Diseases Working Groups (many of which do not include the TK2 gene) may focus (Fig. 5, upper panel) [11, 30–32]. Identification of TK2 attention on analysis of genes involved in non-mitochon- as a causative gene has enabled genetic approaches to drial myopathies and muscle dystrophies, consequently molecular diagnosis, including whole exome sequenc- not reaching the diagnosis. However, as the use of whole ing, in addition to the mitochondrial diagnostic panel of exome sequencing (or, in the future, whole genome genes related to mtDNA maintenance after signs of mito- sequencing) becomes more widespread, diagnosis may chondrial dysfunction are detected on muscle biopsy [4, be reached via these techniques alone [11, 25]. Once the 15, 30, 32, 33]. TK2 gene has been implicated and a diagnosis of TK2d is in place, a muscle biopsy or assessment in other tissues Monitoring and treatment (e.g., urine epithelial cells, fibroblasts, white blood cells) Patients with TK2d require multidisciplinary care. Ref- is not mandatory because the associated metrics (e.g., erence centers play a crucial role in optimizing access to respiratory chain analysis, mtDNA quantification, TK2 care [1]. After confirming a diagnosis, CSURs consolidate activity from skeletal muscle biopsy specimens) are not resources to optimize patient access to care [2, 6] (Fig. 5, essential for management of the disease [7, 26]. Circulat- lower panel). CSURs consolidate care teams and link ing biomarkers such as GDF-15 and/or FGF-21 may also patients with appropriate specialists, including physical complement molecular and genetic diagnoses, as these therapists, speech therapists, pulmonologists, metabolic metrics provide additional information about monitoring specialists, and neuromuscular specialists [11, 30, 32]. disease progression and pharmacodynamic response to Consolidating care within the CSUR framework harmo- treatment [27]. nizes patient care, minimizing the number of appoint- Occasionally, a patient is diagnosed with mitochondrial ments and time lost from work for the primary caregiver myopathy due to muscle biopsy findings, with no deeper while avoiding contradictory recommendations. CSURs exploration into the genetic cause. This could be attrib- also play a pivotal role in patient education, including uted to the fact that reaching an accurate genetic diag- genetic counseling for parents, siblings, and prospective nosis is not easy in the field of mitochondrial disorders parents with late-onset TK2d. CSURs can recommend (including those manifesting with myopathy) because of patient associations for psychosocial and emotional sup- involvement of more than 300 genes encoded by nuclear port for patients and their caregivers and families. and mitochondrial genomes [28]. Furthermore, this approach has been favored due to lack of treatments Equity of access for these disorders, but the availability of new disease- The SNS is fully public, which represents a crucial modifying therapies has changed the diagnostic and aspect of care for patients with TK2d and other rare Domínguez‑González et al. Orphanet J Rare Dis (2021) 16:407 Page 7 of 9 Fig. 5 Proposed TK2d diagnostic algorithm [9, 28, 29]. COX, cytochrome C oxidase deficiency; CSUR, National Reference Centers; EMG, electromyography; ENT, ear, nose, and throat; MRI, magnetic resonance imaging; mtDNA, mitochondrial DNA; NIMV, non-invasive mechanical ventilation; TK2d, thymidine kinase 2 deficiency mitochondrial myopathies. Ensuring equity of access to Thus, disease frequency cannot be accurately estimated care removes any economic or social barriers that could or compared to diagnostic effectiveness in Spain. hinder access of certain patients to the healthcare system and to highly specialized CSURs [6]. The SNS facilitates universal and free access to specialists of any discipline, Conclusions along with complementary testing and therapeutic pro- The Spanish collaborative model supports the develop- cedures for any Spanish citizen and everyone holding ment of a new therapeutic approach for TK2d, a pro- the Spanish Health Card. The Spanish population is well gressive, ultra-rare disease with no approved therapies. accustomed to this universal characteristic of the Spanish Success in TK2d early identification, diagnosis, and Health System, established in the early 1970s. As such, treatment in Spain is attributable to (1) Spain’s fully Pub- there is little hesitation in obtaining medical consulta- lic National Healthcare System, which enables equity of tions on any health problem. access to care, and (2) the designation of CSURs-NMD, which homogenize a highly specialized diagnostic pro- cess, including muscle biopsy and molecular study. Uni- Limitations versal and free access to genetic studies ensures that In explaining the high rate of identification of TK2d a detailed understanding of the genetic causes of mito- patients in Spain without additional exploratory stud- chondrial disease is more available than in countries with ies, it is difficult to rule out completely the possibility of private healthcare systems. The Spanish experience in a genetic predisposition or founder effect. However, a diagnosis and treatment of TK2d is a model for the diag- founder effect is unlikely, given that patients carry dif- nosis and development of new treatments for very rare ferent mutations, both homozygous and compound diseases within an existing healthcare system. heterozygous [8, 10]. Further, TK2d does not appear to have an ethnic predisposition, although the p.Lys202del Abbreviations mutation, which is the most frequently reported muta- CIBERER: Center for Biomedical Network Research on Rare Diseases; CIBERNED: Center for Biomedical Network Research on Neurodegenerative Disorders; tion in late-onset cases so far, has been identified only in CSUR: Major national reference center; CSURs-NMD: CSURs for neuromuscular Spanish patients and patients of Hispanic ethnic back- disorders; CUIMC: Columbia University Irving Medical Center; dC/dT: Deoxy‑ ground [7]. Finally, carrier frequency by exome analysis cytidine/deoxythymidine; dCK: Deoxycytidine kinase; dCMP: Deoxycytidine monophosphate; dCTP: Deoxycytidine triphosphate; dNTP: Deoxynucleoside is currently unknown in Spain or elsewhere in the world. triphosphate; dTMP: Deoxythymidine monophosphate; dTTP: Deoxythymidine Domínguez‑González et al. Orphanet J Rare Dis (2021) 16:407 Page 8 of 9 triphosphate; ERN: EU reference network; EURO-NMD: Mitochondrial Disease Biodonostia Health Research Institute, University of the Basque Country, San Group within the Neuromuscular Disease ERN; GCP: Good clinical practice; Sebastián, Spain. 7 Mitochondrial Diseases Laboratory, Department of Bio‑ H12O: Hospital U 12 Octubre; HUVR: Hospital U Virgen del Rocío; IBiS: Institute chemistry, Research Institute Hospital 12 de Octubre (imas12), Madrid, Spain. 8 of Biomedicine in Seville; imas12: Research Institute Hospital 12 de Octubre; Pediatric Department, Vall d’Hebron Hospital, Barcelona, Spain. 9 Neuro‑ MDDS: Mitochondrial DNA depletion and deletions syndrome; mtDNA: muscular Unit, Neurology Department, Sant Joan de Déu Research Institute, Mitochondrial DNA; NGS: Next-generation sequencing; PRIME: PRIorityMEdi‑ Sant Joan de Déu Hospital, Barcelona, Spain. 10 Neuromuscular Disorders cines; SNS: Spanish National Health System; TK1: Thymidine kinase 1; TK2d: Unit, Department of Neurology, Hospital de la Santa Creu i Sant Pau/Center Thymidine kinase 2 deficiency; VHIR: Vall d’Hebron Research Institute. for Biomedical Network Research On Rare Diseases (CIBERER), Barcelona, Spain. 11 Zogenix, Inc., Emeryville, CA, USA. 12 Pediatric Neurology Department, Acknowledgements Badajoz Hospital Complex, Badajoz, Spain. 13 Research Group On Neuromuscu‑ Medical writing and editorial support, funded by Zogenix, Inc., was provided lar and Mitochondrial Diseases, Vall d’Hebron Research Institute, Autonomous by Danielle L. Ippolito, PhD, CMPP, MWC, and Dolores Matthews, MEd, ELS, of University of Barcelona, Barcelona, Spain. 14 Neurology Department, Neuro‑ PharmaWrite, LLC (Princeton, NJ, USA). muscular Disorders Unit, Instituto de Biomedicina de Sevilla, Hospital U. Virgen del Rocío, CSIC, Universidad de Sevilla, Avd. Manuel Siurot s/n, 41013 Sevilla, Authors’ contributions Spain. 15 Center for Biomedical Network Research On Neurodegenerative CP and CD-G drafted the manuscript. MM-G, MH, IM, MAM, FM, AN, MO, JQ, Disorders (CIBERNED), Instituto de Salud Carlos III, Madrid, Spain. MDS, and RM critically reviewed the manuscript. All authors approved the final submitted version of the manuscript. Received: 17 June 2021 Accepted: 18 September 2021 Funding This study was sponsored by Zogenix, Inc., and ERN EURO-NMD. The spon‑ sors reviewed the drafts and provided medical writing support for draft preparation. References 1. Miravitlles M, Nuñez A, Torres-Durán M, et al. 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Clinical application Springer Nature remains neutral with regard to jurisdictional claims in pub‑ of whole exome sequencing reveals a novel compound heterozygous lished maps and institutional affiliations. TK2-mutation in two brothers with rapidly progressive combined muscle-brain atrophy, axonal neuropathy, and status epilepticus. Mito‑ chondrion. 2015;20:1–6. Ready to submit your research ? Choose BMC and benefit from: • fast, convenient online submission • thorough peer review by experienced researchers in your field • rapid publication on acceptance • support for research data, including large and complex data types • gold Open Access which fosters wider collaboration and increased citations • maximum visibility for your research: over 100M website views per year At BMC, research is always in progress. Learn more biomedcentral.com/submissions