Paediatric single mitochondrial DNA deletion disorders: an overlapping spectrum of disease

Hematologic Manifestations in Primary Mitochondrial Diseases

Primary mitochondrial disorders (PMDs) are known for their pleiotropic manifestations in humans, affecting almost any organ or system at any time. Hematologic manifestations, such as cytopenias and sideroblastic anemia, occur in 10% to 30% of patients with confirmed PMDs. These can be the initial presenting features or complications that develop over time. Surveillance for these manifestations allows for prompt identification and treatment. This article provides an overview of the pathophysiology underpinning the hematologic effects of mitochondrial dysfunction, discussing the 3 key roles of the mitochondria in hematopoiesis: providing energy for cell differentiation and function, synthesizing heme, and generating iron-sulfur clusters. Subsequently, the diagnosis and management of mitochondrial disorders are discussed, focusing on hematologic manifestations and the specific conditions commonly associated with them. Through this, we aimed to provide a concise point of reference for those considering a mitochondrial cause for a patient's hematologic abnormality, or for those considering a hematologic manifestation in a patient with known or suspected mitochondrial disease.

Mechanisms and pathologies of human mitochondrial DNA replication and deletion formation

Human mitochondria possess a multi-copy circular genome, mitochondrial DNA (mtDNA), that is essential for cellular energy metabolism. The number of copies of mtDNA per cell, and their integrity, are maintained by nuclear-encoded mtDNA replication and repair machineries. Aberrant mtDNA replication and mtDNA breakage are believed to cause deletions within mtDNA. The genomic location and breakpoint sequences of these deletions show similar patterns across various inherited and acquired diseases, and are also observed during normal ageing, suggesting a common mechanism of deletion formation. However, an ongoing debate over the mechanism by which mtDNA replicates has made it difficult to develop clear and testable models for how mtDNA rearrangements arise and propagate at a molecular and cellular level. These deletions may impair energy metabolism if present in a high proportion of the mtDNA copies within the cell, and can be seen in primary mitochondrial diseases, either in sporadic cases or caused by autosomal variants in nuclear-encoded mtDNA maintenance genes. These mitochondrial diseases have diverse genetic causes and multiple modes of inheritance, and show notoriously broad clinical heterogeneity with complex tissue specificities, which further makes establishing genotype-phenotype relationships challenging. In this review, we aim to cover our current understanding of how the human mitochondrial genome is replicated, the mechanisms by which mtDNA replication and repair can lead to mtDNA instability in the form of large-scale rearrangements, how rearranged mtDNAs subsequently accumulate within cells, and the pathological consequences when this occurs.

Expanding the spectrum of LAMB2: Pierson syndrome associated with neuromuscular junction disorder in two patients

LAMB2 gene disorders present with different phenotypes. Pierson syndrome (PS) is a common phenotype associated with LAMB2 variants. Neuromuscular phenotype has been reported including hypotonia and developmental delay. However, neuromuscular junction abnormalities represented as congenital myasthenic syndrome (CMS) was reported in one adult patient only. Here, in this paper, we present two pediatric cases with a severe presentation of PS and have CMS so expanding the knowledge of LAMB2 related phenotypes. The first patient had hypotonia and global developmental delay. Targeted genetic testing panel demonstrated homozygous pathogenic variant in the LAMB2 gene (c.5182C>T, pGln1728*) which was reported by Maselli et al. 2009. Repetitive nerve stimulation (RNS) showed a decremental response at low frequency of 3 Hz. On the other hand, the second patient had profound weakness since birth. Tri-Whole exome sequencing showed homozygous pathogenic variant in the LAMB2 gene c.2890C>T, pArg964*. A trial of salbutamol did not improve the symptoms. Both patients passed away from sequala of PS. The spectrum of phenotypic changes associated with LAMB2 mutations is still expanding, and further investigation into the various clinical and morphologic presentations associated with these mutations is important to better identify and manage affected individuals.

High number of mitochondrial DNA alterations in postmortem brain tissue of patients with schizophrenia compared to healthy controls

Previous studies have shown mitochondrial dysfunction in schizophrenia (SZ) patients, which may be caused by mitochondrial DNA (mtDNA) alterations. However, there are few studies in SZ that have analyzed mtDNA in brain samples by next-generation sequencing (NGS). To address this gap, we used mtDNA-targeted NGS and qPCR to characterize mtDNA alterations in brain samples from patients with SZ (n = 40) and healthy controls (HC) (n = 40). 35 % of SZ patients showed mtDNA alterations, a significantly higher prevalence compared to 10 % of HC. Specifically, SZ patients had a significantly higher frequency of deletions (35 vs. 5 in HC), with a mean number of deletions of 3.8 in SZ vs. 1.0 in HC. Likely pathogenic missense variants were also significantly more frequent in patients with SZ than in HC (10 vs. three HC), encompassing 14 variants in patients and three in HC. The pathogenic tRNA variant m.3243A>G was identified in one SZ patient with a high heteroplasmy level of 32.2 %. While no significant differences in mtDNA copy number (mtDNA-CN) were observed between SZ and HC, antipsychotic users had significantly higher mtDNA-CN than non-users. These findings suggest a potential role for mtDNA alterations in the pathophysiology of SZ that require further validation and functional studies.

Discerning clinicopathological features of congenital neutropenia syndromes: an approach to diagnostically challenging differential diagnoses

The congenital neutropenia syndromes are rare haematological conditions defined by impaired myeloid precursor differentiation or function. Patients are prone to severe infections with high mortality rates in early life. While some patients benefit from granulocyte colony-stimulating factor treatment, they may still face an increased risk of bone marrow failure, myelodysplastic syndrome and acute leukaemia. Accurate diagnosis is crucial for improved outcomes; however, diagnosis depends on familiarity with a heterogeneous group of rare disorders that remain incompletely characterised. The clinical and pathological overlap between reactive conditions, primary and congenital neutropenias, bone marrow failure, and myelodysplastic syndromes further clouds diagnostic clarity.We review the diagnostically useful clinicopathological and morphological features of reactive causes of neutropenia and the most common primary neutropenia disorders: constitutional/benign ethnic neutropenia, chronic idiopathic neutropenia, cyclic neutropenia, severe congenital neutropenia (due to mutations in ELANE, GFI1, HAX1, G6PC3, VPS45, JAGN1, CSF3R, SRP54, CLPB and WAS), GATA2 deficiency, Warts, hypogammaglobulinaemia, infections and myelokathexis syndrome, Shwachman-Diamond Syndrome, the lysosomal storage disorders with neutropenia: Chediak-Higashi, Hermansky-Pudlak, and Griscelli syndromes, Cohen, and Barth syndromes. We also detail characteristic cytogenetic and molecular factors at diagnosis and in progression to myelodysplastic syndrome/leukaemia.

Endocrine features of primary mitochondrial diseases

PURPOSE OF REVIEW

Primary mitochondrial diseases are one of the most prevalent groups of multisystem genetic disorders. Endocrinopathies associated with mitochondrial diseases may have clinical features that are distinct from the more common forms. We provide an overview of mitochondrial disorder genetics and phenotypes, focusing on recent studies regarding identification and treatment of associated endocrinopathies.

RECENT FINDINGS

Known endocrine phenotypes of mitochondrial disorders continue to expand, and now include growth hormone deficiency, hypogonadism, precocious puberty, hypoparathyroidism, hypo- and hyperthyroidism, diabetes, and adrenal insufficiency. Recent studies suggest several genotype-phenotype correlations, including those related to nuclear variants. Diagnosis is important, as special considerations should be made in the management of endocrinopathies in mitochondrial patients. Finally, new mitochondrial replacement strategies may soon be available for women interested in preventing mitochondrial disease transmission to offspring.

SUMMARY

Patients with multiple endocrinopathies or atypical endocrinopathies should be evaluated for primary mitochondrial disease, as a diagnosis may impact management of these individuals.

Mitochondrial Chronic Progressive External Ophthalmoplegia

BACKGROUND

Chronic progressive external ophthalmoplegia (CPEO) is a rare disorder that can be at the forefront of several mitochondrial diseases. This review overviews mitochondrial CPEO encephalomyopathies to enhance accurate recognition and diagnosis for proper management.

METHODS

This study is conducted based on publications and guidelines obtained by selective review in PubMed. Randomized, double-blind, placebo-controlled trials, Cochrane reviews, and literature meta-analyses were particularly sought.

DISCUSSION

CPEO is a common presentation of mitochondrial encephalomyopathies, which can result from alterations in mitochondrial or nuclear DNA. Genetic sequencing is the gold standard for diagnosing mitochondrial encephalomyopathies, preceded by non-invasive tests such as fibroblast growth factor-21 and growth differentiation factor-15. More invasive options include a muscle biopsy, which can be carried out after uncertain diagnostic testing. No definitive treatment option is available for mitochondrial diseases, and management is mainly focused on lifestyle risk modification and supplementation to reduce mitochondrial load and symptomatic relief, such as ptosis repair in the case of CPEO. Nevertheless, various clinical trials and endeavors are still at large for achieving beneficial therapeutic outcomes for mitochondrial encephalomyopathies.

KEY MESSAGES

Understanding the varying presentations and genetic aspects of mitochondrial CPEO is crucial for accurate diagnosis and management.

Red Flags in Primary Mitochondrial Diseases: What Should We Recognize?

Primary mitochondrial diseases (PMDs) are complex group of metabolic disorders caused by genetically determined impairment of the mitochondrial oxidative phosphorylation (OXPHOS). The unique features of mitochondrial genetics and the pivotal role of mitochondria in cell biology explain the phenotypical heterogeneity of primary mitochondrial diseases and the resulting diagnostic challenges that follow. Some peculiar features ("red flags") may indicate a primary mitochondrial disease, helping the physician to orient in this diagnostic maze. In this narrative review, we aimed to outline the features of the most common mitochondrial red flags offering a general overview on the topic that could help physicians to untangle mitochondrial medicine complexity.

Adrenocortical function in patients with Single Large Scale Mitochondrial DNA Deletions: a retrospective single centre cohort study

OBJECTIVE

Single Large Scale Mitochondrial DNA Deletions (SLSMDs), Pearson Syndrome (PS) and Kearns-Sayre Syndrome (KSS), are systemic diseases with multiple endocrine abnormalities. The adrenocortical function has not been systematically investigated with a few anecdotal reports of overt adrenal insufficiency (AI). The study aimed to assess the adrenocortical function in a large cohort of SLSMDs.

DESIGN AND METHODS

A retrospective monocentric longitudinal study involved a cohort of 18 SLSMDs patients. Adrenocortical function was evaluated by baseline adrenocorticotrophic hormone (ACTH) and cortisol measurements and by high- (HDT) and low-dose (LDT) ACTH stimulation tests and compared with 92 healthy controls (HC).

RESULTS

Baseline adrenocortical function was impaired in 39% of patients and by the end of the study, 66% of PS and 25% of KSS showed an insufficient increase after ACTH stimulation, with cortisol deficiency due to primary AI in most PS and subclinical AI in KSS. Symptomatic AI was recorded in 44% of patients. Peak cortisol levels after ACTH stimulation tests were significantly lower in patients than in HC (P < .0001), with a more reduced response to LDT vs HDT (P < .05).

CONCLUSIONS

Our study highlights that cortisol deficiency due to primary AI represents a relevant part of the clinical spectrum in SLSMDs, with more severe impairment in PS than in KSS. Basal and after-stimulus assessment of adrenocortical axis should be early and regularly investigated to identify any degree of adrenocortical dysfunction. The study allowed the elaboration of a diagnostic process designed for the diagnosis, treatment, and follow-up of adrenocortical abnormalities in SLSMDs.

Clinical and biochemical footprints of inherited metabolic diseases. XIV. Metabolic kidney diseases

Kidney disease is a global health burden with high morbidity and mortality. Causes of kidney disease are numerous, extending from common disease groups like diabetes and arterial hypertension to rare conditions including inherited metabolic diseases (IMDs). Given its unique anatomy and function, the kidney is a target organ in about 10% of known IMDs, emphasizing the relevant contribution of IMDs to kidney disease. The pattern of injury affects all segments of the nephron including glomerular disease, proximal and distal tubular damage, kidney cyst formation, built-up of nephrocalcinosis and stones as well as severe malformations. We revised and updated the list of known metabolic etiologies associated with kidney involvement and found 190 relevant IMDs. This represents the 14th of a series of educational articles providing a comprehensive and revised list of metabolic differential diagnoses according to system involvement.

Role of Mutations of Mitochondrial Aminoacyl-tRNA Synthetases Genes on Epileptogenesis

Mitochondria are the structures in cells that are responsible for producing energy. They contain a specific translation unit for synthesizing mitochondria-encoded respiratory chain components: the mitochondrial DNA (mt DNA). Recently, a growing number of syndromes associated with the dysfunction of mt DNA translation have been reported. However, the functions of these diseases still need to be precise and thus attract much attention. Mitochondrial tRNAs (mt tRNAs) are encoded by mt DNA; they are the primary cause of mitochondrial dysfunction and are associated with a wide range of pathologies. Previous research has shown the role of mt tRNAs in the epileptic mechanism. This review will focus on the function of mt tRNA and the role of mitochondrial aminoacyl-tRNA synthetase (mt aaRS) in order to summarize some common relevant mutant genes of mt aaRS that cause epilepsy and the specific symptoms of the disease they cause.

Modeling mitochondrial DNA diseases: from base editing to pluripotent stem-cell-derived organoids

Mitochondrial DNA (mtDNA) diseases are multi-systemic disorders caused by mutations affecting a fraction or the entirety of mtDNA copies. Currently, there are no approved therapies for the majority of mtDNA diseases. Challenges associated with engineering mtDNA have in fact hindered the study of mtDNA defects. Despite these difficulties, it has been possible to develop valuable cellular and animal models of mtDNA diseases. Here, we describe recent advances in base editing of mtDNA and the generation of three-dimensional organoids from patient-derived human-induced pluripotent stem cells (iPSCs). Together with already available modeling tools, the combination of these novel technologies could allow determining the impact of specific mtDNA mutations in distinct human cell types and might help uncover how mtDNA mutation load segregates during tissue organization. iPSC-derived organoids could also represent a platform for the identification of treatment strategies and for probing the in vitro effectiveness of mtDNA gene therapies. These studies have the potential to increase our mechanistic understanding of mtDNA diseases and may open the way to highly needed and personalized therapeutic interventions.

Genetics of mitochondrial diseases: Current approaches for the molecular diagnosis

Mitochondrial diseases are a genetically and phenotypically variable set of monogenic disorders. The main characteristic of mitochondrial diseases is a defective oxidative phosphorylation. Both nuclear and mitochondrial DNA encode the approximately 1500 mitochondrial proteins. Since identification of the first mitochondrial disease gene in 1988 a total of 425 genes have been associated with mitochondrial diseases. Mitochondrial dysfunctions can be caused both by pathogenic variants in the mitochondrial DNA or the nuclear DNA. Hence, besides maternal inheritance, mitochondrial diseases can follow all modes of Mendelian inheritance. The maternal inheritance and tissue specificity distinguish molecular diagnostics of mitochondrial disorders from other rare disorders. With the advances made in the next-generation sequencing technology, whole exome sequencing and even whole-genome sequencing are now the established methods of choice for molecular diagnostics of mitochondrial diseases. They reach a diagnostic rate of more than 50% in clinically suspected mitochondrial disease patients. Moreover, next-generation sequencing is delivering a constantly growing number of novel mitochondrial disease genes. This chapter reviews mitochondrial and nuclear causes of mitochondrial diseases, molecular diagnostic methodologies, and their current challenges and perspectives.

Phenotypic spectrum and clinical course of single large-scale mitochondrial DNA deletion disease in the paediatric population: a multicentre study

BACKGROUND

Large-scale mitochondrial DNA deletions (LMD) are a common genetic cause of mitochondrial disease and give rise to a wide range of clinical features. Lack of longitudinal data means the natural history remains unclear. This study was undertaken to describe the clinical spectrum in a large cohort of patients with paediatric disease onset.

METHODS

A retrospective multicentre study was performed in patients with clinical onset <16 years of age, diagnosed and followed in seven European mitochondrial disease centres.

RESULTS

A total of 80 patients were included. The average age at disease onset and at last examination was 10 and 31 years, respectively. The median time from disease onset to death was 11.5 years. Pearson syndrome was present in 21%, Kearns-Sayre syndrome spectrum disorder in 50% and progressive external ophthalmoplegia in 29% of patients. Haematological abnormalities were the hallmark of the disease in preschool children, while the most common presentations in older patients were ptosis and external ophthalmoplegia. Skeletal muscle involvement was found in 65% and exercise intolerance in 25% of the patients. Central nervous system involvement was frequent, with variable presence of ataxia (40%), cognitive involvement (36%) and stroke-like episodes (9%). Other common features were pigmentary retinopathy (46%), short stature (42%), hearing impairment (39%), cardiac disease (39%), diabetes mellitus (25%) and renal disease (19%).

CONCLUSION

Our study provides new insights into the phenotypic spectrum of childhood-onset, LMD-associated syndromes. We found a wider spectrum of more prevalent multisystem involvement compared with previous studies, most likely related to a longer time of follow-up.

Mitochondrial encephalomyopathy

Mitochondrial dysfunction, especially perturbation of oxidative phosphorylation and adenosine triphosphate (ATP) generation, disrupts cellular homeostasis and is a surprisingly frequent cause of central and peripheral nervous system pathology. Mitochondrial disease is an umbrella term that encompasses a host of clinical syndromes and features caused by in excess of 300 different genetic defects affecting the mitochondrial and nuclear genomes. Patients with mitochondrial disease can present at any age, ranging from neonatal onset to late adult life, with variable organ involvement and neurological manifestations including neurodevelopmental delay, seizures, stroke-like episodes, movement disorders, optic neuropathy, myopathy, and neuropathy. Until relatively recently, analysis of skeletal muscle biopsy was the focus of diagnostic algorithms, but step-changes in the scope and availability of next-generation sequencing technology and multiomics analysis have revolutionized mitochondrial disease diagnosis. Currently, there is no specific therapy for most types of mitochondrial disease, although clinical trials research in the field is gathering momentum. In that context, active management of epilepsy, stroke-like episodes, dystonia, brainstem dysfunction, and Parkinsonism are all the more important in improving patient quality of life and reducing mortality.

Mitochondrial augmentation of hematopoietic stem cells in children with single large-scale mitochondrial DNA deletion syndromes

Patients with single large-scale mitochondrial DNA (mtDNA) deletion syndromes (SLSMDs) usually present with multisystemic disease, either as Pearson syndrome in early childhood or as Kearns-Sayre syndrome later in life. No disease-modifying therapies exist for SLSMDs. We have developed a method to enrich hematopoietic cells with exogenous mitochondria, and we treated six patients with SLSMDs through a compassionate use program. Autologous CD34⁺ hematopoietic cells were augmented with maternally derived healthy mitochondria, a technology termed mitochondrial augmentation therapy (MAT). All patients had substantial multisystemic disease involvement at baseline, including neurologic, endocrine, or renal impairment. We first assessed safety, finding that the procedure was well tolerated and that all study-related severe adverse events were either leukapheresis-related or related to the baseline disorder. After MAT, heteroplasmy decreased in the peripheral blood in four of the six patients. An increase in mtDNA content of peripheral blood cells was measured in all six patients 6 to 12 months after MAT as compared baseline. We noted some clinical improvement in aerobic function, measured in patients 2 and 3 by sit-to-stand or 6-min walk testing, and an increase in the body weight of five of the six patients suffering from very low body weight before treatment. Quality-of-life measurements as per caregiver assessment and physical examination showed improvement in some parameters. Together, this work lays the ground for clinical trials of MAT for the treatment of patients with mtDNA disorders.

Primary Mitochondrial Disorders in the Neonate

Primary mitochondrial disorders (PMDs) are a heterogeneous group of disorders characterized by functional or structural abnormalities in the mitochondria that lead to a disturbance of cellular energy, reactive oxygen species, and free radical production, as well as impairment of other intracellular metabolic functions, causing single- or multiorgan dysfunction. PMDs are caused by pathogenic variants in nuclear and mitochondrial genes, resulting in distinct modes of inheritance. Onset of disease is variable and can occur in the neonatal period, with a high morbidity and mortality. In this article, we review the most common methods used for the diagnosis of PMDs, as well as their prenatal and neonatal presentations. We highlight the shift in the diagnostic approach for PMDs since the introduction of nontargeted molecular tests into clinical practice, which has significantly reduced the use of invasive studies. We discuss common PMDs that can present in the neonate, including general, nonsyndromic presentations as well as specific syndromic disorders. We also review current treatment advances, including the use of mitochondrial "cocktails" based on limited scientific evidence and theoretical reasoning, as well as the impending arrival of personalized mitochondrial-specific treatments.

Pearson syndrome: a multisystem mitochondrial disease with bone marrow failure

Pearson syndrome (PS) is a rare fatal mitochondrial disorder caused by single large-scale mitochondrial DNA deletions (SLSMDs). Most patients present with anemia in infancy. Bone marrow cytology with vacuolization in erythroid and myeloid precursors and ring-sideroblasts guides to the correct diagnosis, which is established by detection of SLSMDs. Non hematological symptoms suggesting a mitochondrial disease are often lacking at initial presentation, thus PS is an important differential diagnosis in isolated hypogenerative anemia in infancy. Spontaneous resolution of anemia occurs in two-third of patients at the age of 1-3 years, while multisystem non-hematological complications such as failure to thrive, muscle hypotonia, exocrine pancreas insufficiency, renal tubulopathy and cardiac dysfunction develop during the clinical course. Some patients with PS experience a phenotypical change to Kearns-Sayre syndrome. In the absence of curative therapy, the prognosis of patients with PS is dismal. Most patients die of acute lactic acidosis and multi-organ failure in early childhood. There is a great need for the development of novel therapies to alter the natural history of patients with PS.

Diabetes Out-of-the-Box: Diabetes Mellitus and Impairment in Hearing and Vision

PURPOSEOF REVIEW

This review aims to provide an update on the etiologies of diabetes that are due to genetic disorders and that co-occur with impaired hearing or vision and to compare them. The potential mechanisms, including novel treatments, will be detailed.

RECENT FINDINGS

Wolfram syndrome, Kearns-Sayre syndrome, thiamine-responsive megaloblastic anemia, and maternally inherited diabetes and deafness are genetic disorders characterized by diabetes, impaired hearing, and vision. They differ in mode of inheritance, age at presentation, and the involvement of other organs; they are often misdiagnosed as type 1 or type 2 diabetes. Suspicion of a genetic diabetes syndrome should be raised when pancreatic autoantibodies are negative, other organs are involved, and family history includes diabetes. Correct diagnosis of the various syndromes is important for tailoring the most advanced treatment, preventing disease progression, and enabling proper genetic counseling.

A child with mitochondrial DNA deletion presenting diabetes mellitus as an initial symptom

Children with mitochondrial disease may present with diabetes mellitus (DM) without autoimmune antibodies as an initial manifestation, however, it is difficult to make a precise diagnosis in early stages. We present a 2-year-old male patient with mitochondrial disease who showed insulin-dependent DM without autoimmune antibodies as an initial symptom. He later presented with progressive motor deterioration, hearing disability, ptosis, external ophthalmoplegia, and retinitis pigmentosa at 6 years and 6 months. T2- and diffusion-weighted imaging revealed high signal lesions in the subcortical white matter, anterior thalamus, globus pallidus, and brainstem. MR spectroscopy showed elevated lactate and low N-acetylaspartate in the affected white matter. Genetic analysis revealed a single large-scale mitochondrial DNA deletion at 7117-13994, leading to a diagnosis of mitochondrial DNA deletion syndrome associated with insulin-dependent DM. Although the frequency of DM in pediatric mitochondrial disease is low, mitochondrial disease, especially due to mitochondrial DNA deletion, should be considered as a differential diagnosis in those with insulin-dependent DM without autoimmune antibodies, and MRI and MR spectroscopy are recommended for an early diagnosis.

Mitochondrial Hepatopathy

Mitochondrial hepatopathies are a subset of mitochondrial diseases defined by primary dysfunction of hepatocyte mitochondria leading to a phenotype of hepatocyte cell injury, steatosis, or liver failure. Increasingly, the diagnosis is established by new sequencing approaches that combine analysis of both nuclear DNA and mitochondrial DNA and allow for timely diagnosis in most patients. Despite advances in diagnostics, for most affected children their disorders are relentlessly progressive, and result in substantial morbidity and mortality. Treatment remains mainly supportive; however, novel therapeutics and a more definitive role for liver transplantation hold promise for affected children.

Research priorities for mitochondrial disorders: Current landscape and patient and professional views

Primary mitochondrial disorders encompass a wide range of clinical presentations and a spectrum of severity. They currently lack effective disease-modifying therapies and have a high mortality and morbidity rate. It is therefore essential to know that competitively funded research designed by academics meets the core needs of people with mitochondrial disorders and their clinicians. Priority setting partnerships are an established collaborative methodology that brings patients, carers and families, charity representatives and clinicians together to try to establish the most pressing and unanswered research priorities for a particular disease. We developed a web-based questionnaire, requesting all patients affected by primary mitochondrial disease, their carers and clinicians to pose their research questions. This yielded 709 questions from 147 participants. These were grouped into overarching themes including basic biology, causation, health services, clinical management, social impacts, prognosis, prevention, symptoms, treatment and psychological impact. Following the removal of "answered questions", the process resulted in a list of 42 discrete, answerable questions. This was further refined by web-based ranking by the community to 24 questions. These were debated at a face-to-face workshop attended by a diverse range of patients, carers, charity representatives and clinicians to create a definitive "Top 10 of unanswered research questions for primary mitochondrial disorders". These Top 10 questions related to understanding biological processes, including triggers of disease onset, mechanisms underlying progression and reasons for differential symptoms between individuals with identical genetic mutations; new treatments; biomarker discovery; psychological support and optimal management of stroke-like episodes and fatigue.

Use of Next-Generation Sequencing for Identifying Mitochondrial Disorders

Mitochondria are major contributors to ATP synthesis, generating more than 90% of the total cellular energy production through oxidative phosphorylation (OXPHOS): metabolite oxidation, such as the β-oxidation of fatty acids, and the Krebs’s cycle. OXPHOS inadequacy due to large genetic lesions in mitochondrial as well as nuclear genes and homo- or heteroplasmic point mutations in mitochondrially encoded genes is a characteristic of heterogeneous, maternally inherited genetic disorders known as mitochondrial disorders that affect multisystemic tissues and organs with high energy requirements, resulting in various signs and symptoms. Several traditional diagnostic approaches, including magnetic resonance imaging of the brain, cardiac testing, biochemical screening, variable heteroplasmy genetic testing, identifying clinical features, and skeletal muscle biopsies, are associated with increased risks, high costs, a high degree of false-positive or false-negative results, or a lack of precision, which limits their diagnostic abilities for mitochondrial disorders. Variable heteroplasmy levels, mtDNA depletion, and the identification of pathogenic variants can be detected through genetic sequencing, including the gold standard Sanger sequencing. However, sequencing can be time consuming, and Sanger sequencing can result in the missed recognition of larger structural variations such as CNVs or copy-number variations. Although each sequencing method has its own limitations, genetic sequencing can be an alternative to traditional diagnostic methods. The ever-growing roster of possible mutations has led to the development of next-generation sequencing (NGS). The enhancement of NGS methods can offer a precise diagnosis of the mitochondrial disorder within a short period at a reasonable expense for both research and clinical applications.

Clinical and genetic features of four patients with Pearson syndrome: An observational study

Pearson syndrome (PS) is a multisystem mitochondrial cytopathy arising from deletions in mitochondrial DNA. Pearson syndrome is a sporadic disease that affects the hematopoietic system, pancreas, eyes, liver, and heart and the prognosis is poor. Causes of morbidity include metabolic crisis, bone marrow dysfunction, sepsis, and liver failure in early infancy or childhood. Early diagnosis may minimize complications, but suspicion of the disease is difficult and only mitochondrial DNA gene testing can identify mutations. There is no specific treatment for PS, which remains supportive care according to symptoms; however, hematopoietic stem cell transplantation may be considered in cases of bone marrow failure.We herein describe the clinical and genetic characteristics of four patients with PS. One patient presented with hypoglycemia, two developed pancytopenia, and the final patient had hypoglycemia and acute hepatitis as the primary manifestation. All patients had lactic acidosis. Additionally, all patients showed a variety of clinical features including coagulation disorder, pancreatic, adrenal, and renal tubular insufficiencies. Two patients with pancytopenia died in their early childhood. Our experience expands the phenotypic spectrum associated with PS and its clinical understanding.

Preterm twins with antenatal presentation of Pearson syndrome

Objectives

Pearson syndrome is a mitochondrial cytopathy with multisystemic involvement that typically presents in infancy and has poor prognosis. We aim to present a case that is distinct due to the timing of presentation and associated anomalies.

Case presentation

We report the case of preterm monochorionic twins with transfusion dependent fetal anemia that had post-natal multisystem dysfunction which led to the diagnosis of Pearson syndrome.

Conclusions

This case highlights the possibility of antenatal presentation of Pearson syndrome, which should be considered in cases of severe fetal anemia without an apparent cause.

Development of the human pancreas and its exocrine function

The pancreas has both endocrine and exocrine function and plays an important role in digestion and glucose control. Understanding the development of the pancreas, grossly and microscopically, and the genetic factors regulating it provides further insight into clinical problems that arise when these processes fail. Animal models of development are known to have inherent issues when understanding human development. Therefore, in this review, we focus on human studies that have reported gross and microscopic development including acinar-, ductal-, and endocrine cells and the neural network. We review the genes and transcription factors involved in organ formation using data from animal models to bridge current understanding where necessary. We describe the development of exocrine function in the fetus and postnatally. A deeper review of the genes involved in pancreatic formation allows us to describe the development of the different groups (proteases, lipids, and amylase) of enzymes during fetal life and postnatally and describe the genetic defects. We discuss the constellation of gross anatomical, as well as microscopic defects that with genetic mutations lead to pancreatic insufficiency and disease states.

Pediatric single large-scale mtDNA deletion syndromes: The power of patient reported outcomes

There is a limited understanding of system-level clinical outcomes and interventions associated with single large-scale mitochondrial DNA deletion syndromes (SLSMDS). Additionally, no research exists that describes patient reported outcomes (PROs) of children with SLSMDS. A global and observational registry was established to understand the multi-systemic course of SLSMDS and track clinical outcomes. The development and design of the registry is described. Demographic characteristics, history and diagnoses, and system level prevalence of problems and interventions are reported for 42 children. System level problems and interventions include information on the following body systems: audiology, cardiac, endocrine, gastrointestinal (including pancreatic and hepatobiliary system), hematological, metabolic, neurological (including autonomic, mobility, & learning), ophthalmic, psychiatric, renal, and respiratory. Results emphasize the need of patient registries and suggest that the diagnostic odyssey and burden of disease for children with SLSMDS is significant. System-level findings may help families and clinical providers with diagnosis, prognostication, and treatment. A multidisciplinary team of clinical experts with a central coordinating specialist for children with SLSMDS is recommended.

Mitochondrial hepatopathy: Respiratory chain disorders- ‘breathing in and out of the liver’

Mitochondria, the powerhouse of a cell, are closely linked to the pathophysiology of various common as well as not so uncommon disorders of the liver and beyond. Evolution supports a prokaryotic descent, and, unsurprisingly, the organelle is worthy of being labeled an organism in itself. Since highly metabolically active organs require a continuous feed of energy, any dysfunction in the structure and function of mitochondria can have variable impact, with the worse end of the spectrum producing catastrophic consequences with a multisystem predisposition. Though categorized a hepatopathy, mitochondrial respiratory chain defects are not limited to the liver in time and space. The liver involvement is also variable in clinical presentation as well as in age of onset, from acute liver failure, cholestasis, or chronic liver disease. Other organs like eye, muscle, central and peripheral nervous system, gastrointestinal tract, hematological, endocrine, and renal systems are also variably involved. Diagnosis hinges on recognition of subtle clinical clues, screening metabolic investigations, evaluation of the extra-hepatic involvement, and role of genetics and tissue diagnosis. Treatment is aimed at both circumventing the acute metabolic crisis and long-term management including nutritional rehabilitation. This review lists and discusses the burden of mitochondrial respiratory chain defects, including various settings when to suspect, their evolution with time, including certain specific disorders, their tiered evaluation with diagnostic algorithms, management dilemmas, role of liver transplantation, and the future research tools.

Blood biomarkers for assessment of mitochondrial dysfunction: An expert review

Although mitochondrial dysfunction is the known cause of primary mitochondrial disease, mitochondrial dysfunction is often difficult to measure and prove, especially when biopsies of affected tissue are not available. In order to identify blood biomarkers of mitochondrial dysfunction, we reviewed studies that measured blood biomarkers in genetically, clinically or biochemically confirmed primary mitochondrial disease patients. In this way, we were certain that there was an underlying mitochondrial dysfunction which could validate the biomarker. We found biomarkers of three classes: 1) functional markers measured in blood cells, 2) biochemical markers of serum/plasma and 3) DNA markers. While none of the reviewed single biomarkers may perfectly reveal all underlying mitochondrial dysfunction, combining biomarkers that cover different aspects of mitochondrial impairment probably is a good strategy. This biomarker panel may assist in the diagnosis of primary mitochondrial disease patients. As mitochondrial dysfunction may also play a significant role in the pathophysiology of multifactorial disorders such as Alzheimer's disease and glaucoma, the panel may serve to assess mitochondrial dysfunction in complex multifactorial diseases as well and enable selection of patients who could benefit from therapies targeting mitochondria.

Chronic neutropenia: how best to assess severity and approach management?

INTRODUCTION

Neutropenia is a relatively common finding in medical practice and the medical approach requires a gradual and pertinent diagnostic procedure as well as adapted management.

AREAS COVERED

The area of chronic neutropenia remains fragmented between diverse diseases or situations. Here physicians involved in different aspects of chronic neutropenia gather both the data from medical literature till the end of May 2021 and their experience to offer a global approach for the diagnosis of chronic neutropenia as well as their medical care.

EXPERT OPINION

In most cases, the neutropenia is transient, frequently related to a viral infection, and not harmful. However, neutropenia can be chronic (i.e. >3 months) and related to a number of etiologies, some clinically benign, such as so-called 'ethnic' neutropenia. Autoimmune neutropenia is the common form in young children, whereas idiopathic/immune neutropenia is a frequent etiology in young females. Inherited neutropenia (or congenital neutropenia) is exceptional, with approximately 30 new cases per 10⁶ births and 30 known subtypes. Such patients have a high risk of invasive bacterial infections, and oral infections. Supportive therapy, which is primarily based on daily administration of an antibiotic prophylaxis and/or treatment with granulocyte-colony stimulating factor (G-CSF), contributes to avoiding recurrent infections.

Two cases of sideroblastic anemia with B-cell immunodeficiency, periodic fevers, and developmental delay (SIFD) syndrome in Chinese Han children caused by novel compound heterozygous variants of the TRNT1 gene

Sideroblastic anemia with B-cell immunodeficiency, periodic fevers, and developmental delay (SIFD) syndrome is a serious autosomal recessive genetic disease. So far, <40 cases have been reported worldwide, and only one case has been reported in China. The main clinical features of SIFD are sideroblastic or microcytic anemia, immune deficiency, and recurrent episodes of inflammation. Here, we describe two unrelated cases of SIFD from China with different clinical manifestations and mild symptoms. Patient 1 was hospitalized at the age of 3.5 years due to persistent joint swelling with imaging of multiple joint effusions. Patient 2 was hospitalized at the age of 12 years due to repeated rashes on both lower limbs and oral ulcers. SIFD was detected using gene testing, which revealed the following compound heterozygous variants in TRNT1 in cases 1 and 2, respectively: c.88A > G/c.363G > T and c.302 T > C/c.1234cC > T. Searches of the HGMD databases revealed that these variants were all novel. Molecular dynamics simulations revealed that the missense variants c.363G > T and c.302 T > C would cause changes in protein structure and thus affect protein function. Finally, through literature reviewing, we found that the mortality in cases of SIFD was approximately 44% (14/32), and about 79% of individuals who died carried the hot-spot mutation c.668 T > C. Moreover, variants in the non-coding region were significantly more common among patients who died than among survivors. Our cases further expand the existing knowledge of the phenotype and variation spectrums of SIFD and suggest that genomic diagnosis is valuable for the hierarchical clinical management of this disease.

Application of Genome Sequencing from Blood to Diagnose Mitochondrial Diseases

Mitochondrial diseases can be caused by pathogenic variants in nuclear or mitochondrial DNA-encoded genes that often lead to multisystemic symptoms and can have any mode of inheritance. Using a single test, Genome Sequencing (GS) can effectively identify variants in both genomes, but it has not yet been universally used as a first-line approach to diagnosing mitochondrial diseases due to related costs and challenges in data analysis. In this article, we report three patients with mitochondrial disease molecularly diagnosed through GS performed on DNA extracted from blood to demonstrate different diagnostic advantages of this technology, including the detection of a low-level heteroplasmic pathogenic variant, an intragenic nuclear DNA deletion, and a large mtDNA deletion. Current technical improvements and cost reductions are likely to lead to an expanded routine diagnostic usage of GS and of the complementary “Omic” technologies in mitochondrial diseases.

Mitochondrial Syndromes Revisited

In the last ten years, the knowledge of the genetic basis of mitochondrial diseases has significantly advanced. However, the vast phenotypic variability linked to mitochondrial disorders and the peculiar characteristics of their genetics make mitochondrial disorders a complex group of disorders. Although specific genetic alterations have been associated with some syndromic presentations, the genotype–phenotype relationship in mitochondrial disorders is complex (a single mutation can cause several clinical syndromes, while different genetic alterations can cause similar phenotypes). This review will revisit the most common syndromic pictures of mitochondrial disorders, from a clinical rather than a molecular perspective. We believe that the new phenotype definitions implemented by recent large multicenter studies, and revised here, may contribute to a more homogeneous patient categorization, which will be useful in future studies on natural history and clinical trials.

Seeking impact: Global perspectives on outcome measure selection for translational and clinical research for primary mitochondrial disorders

Primary mitochondrial disorders (PMDs) are challenging due to overall poor outcomes, no proven treatments, and a history of failed clinical trials, leading to a critical need to design future trials that can prove efficacy of an intervention. Selection of outcome measures for PMDs is complicated by extreme clinical, biochemical and genetic heterogeneity; PMDs are effectively a collection of nearly 400 individually ultrarare diseases. In clinical trials, outcome measures aim to evaluate, and ideally quantitate, the efficacy of an intervention in ameliorating clinical phenotype(s). The heterogeneity and multisystemic nature of PMDs makes it unlikely that a universal outcome measure will be applicable to all PMDs. Instead, a composite score of the individual's most worrisome symptoms may be a preferable endpoint. A further challenge arises from the tension between finding outcomes suitable for use in clinical trials (able to produce a measurable change in a relatively short period of time, namely the duration of a clinical trial) vs measures that are clinically meaningful to individual patients. A number of clinical rating scales and proposed biomarkers have emerged to capture the features of PMDs for natural history and interventional trials. Here we review our collective experiences with clinical rating scales, patient-reported outcome measures, and physiological, imaging, biochemical and muscle phenotypes as outcome measures in paediatric and adult PMDs in natural history studies and recent clinical trials. There is a pressing need to agree on a set of validated, robust, clinically meaningful outcome measures internationally, to facilitate the multicentre international clinical trials needed for optimal evaluation of novel therapies for these ultrarare diseases.

Pulmonary Manifestations of Endocrine and Metabolic Diseases in Children

Advances in technology, methodology, and deep phenotyping are increasingly driving the understanding of the pathologic basis of disease. Improvements in patient identification and treatment are impacting survival. This is true in endocrinology and inborn errors of metabolism, where disease-modifying therapies are developing. Inherent to this evolution is the increasing awareness of the respiratory manifestations of these rare diseases. This review updates clinicians, stratifying diseases spirometerically; pulmonary hypertension and diseases with a predisposition to recurrent pulmonary infection are discussed. This division is artificial; many diseases have multiple pathologic effects on respiration. This review does not cover the impact of obesity.

Generation of two human iPSC lines, FINCBi002-A and FINCBi003-A, carrying heteroplasmic macrodeletion of mitochondrial DNA causing Pearson's syndrome

Pearson marrow pancreas syndrome (PMPS) is a sporadic mitochondrial disease, resulting from the clonal expansion of a mutated mitochondrial DNA (mtDNA) molecule bearing a macro-deletion, and therefore missing essential genetic information. PMPS is characterized by the presence of deleted (Δ) mtDNA that co-exist with the presence of a variable amount of wild-type mtDNA, a condition termed heteroplasmy. All tissues of the affected individual, including the haemopoietic system and the post-mitotic, highly specialized tissues (brain, skeletal muscle, and heart) contain the large-scale mtDNA deletion in variable amount. We generated human induced pluripotent stem cells (hiPSCs) from two PMPS patients, carrying different type of large-scale deletion.

Occurrence of Inborn Errors of Metabolism in Newborns, Diagnosis and Prophylaxis

Inborn errors of metabolism (IEM) are a heterogeneous group of rare genetic disorders that are generally transmitted as autosomal or X-linked recessive disorders. These defects arise due to mutations associated with specific gene(s), especially the ones associated with key metabolic enzymes. These enzymes or their product(s) are involved in various metabolic pathways, leading to the accumulation of intermediary metabolite(s), reflecting their toxic effects upon mutations. The diagnosis of these metabolic disorders is based on the biochemical analysis of the clinical manifestations produced and their molecular mechanism. Therefore, it is imperative to devise diagnostic tests with high sensitivity and specificity for early detection of IEM. Recent advances in biochemical and polymerase chain reaction-based genetic analysis along with pedigree and prenatal diagnosis can be life-saving in nature. The latest development in exome sequencing for rapid diagnosis and enzyme replacement therapy would facilitate the successful treatment of these metabolic disorders in the future. However, the longterm clinical implications of these genetic manipulations is still a matter of debate among intellectuals and requires further research.

The Phenotypic Spectrum of 47 Czech Patients with Single, Large-Scale Mitochondrial DNA Deletions

Background: In this retrospective study, we analysed clinical, biochemical and molecular genetic data of 47 Czech patients with Single, Large-Scale Mitochondrial DNA Deletions (SLSMD). Methods: The diagnosis was based on the long-range PCR (LX-PCR) screening of mtDNA isolated from muscle biopsy in 15 patients, and from the buccal swab, urinary epithelial cells and blood in 32 patients. Results: A total of 57% patients manifested before the age of 16. We did not find any significant difference between paediatric and adult manifestation in either the proportion of patients that would develop extraocular symptoms, or the timespan of its progression. The survival rate in patients with Pearson Syndrome reached 60%. Altogether, five patients manifested with atypical phenotype not fulfilling the latest criteria for SLSMD. No correlation was found between the disease severity and all heteroplasmy levels, lengths of the deletion and respiratory chain activities in muscle. Conclusions: Paediatric manifestation of Progressive External Ophthalmoplegia (PEO) is not associated with a higher risk of multisystemic involvement. Contrary to PEO and Kearns-Sayre Syndrome Spectrum, Pearson Syndrome still contributes to a significant childhood mortality. SLSMD should be considered even in cases with atypical presentation. To successfully identify carriers of SLSMD, a repeated combined analysis of buccal swab and urinary epithelial cells is needed.

Clinical Profile and Outcome of Pediatric Mitochondrial Myopathy in China

Introduction: Mitochondrial myopathy in children has notable clinical and genetic heterogeneity, but detailed data is lacking.

Patients and Methods: In this study, we retrospectively reviewed the clinical presentation, laboratory investigation, genetic and histopathological characteristics, and follow-ups of 21 pediatric mitochondrial myopathy cases from China.

Results: Twenty-four patients suspected with mitochondrial myopathy were enrolled initially and 21 were genetically identified. Fourteen patients were found to harbor mitochondrial DNA point mutations (14/21, 66.7%), including m.3243A>G (9/15, 60%), m.3303C>T (2/15, 13.3%), m.3302A>G (1/15, 6.7%), m.3250T>C (1/15, 6.7%), m.3251A>G (1/15, 6.7%), of whom 12 patients presented with progressive proximal mitochondrial myopathy (12/14, 85.7%). Three patients revealed large-scale deletion in blood or muscle tissue (3/21, 14.3%), presenting with Kearns-Sayer syndrome (1/3, 33.3%) or chronic progressive external ophthalmoplegia (2/3, 66.7%). Four patients were found to harbor pathogenic nuclear gene variants (4/21, 19.0%), including five variants in TK2 gene and two variants in SURF1 gene. During the follow-ups up to 7 years, 10 patients developed cardiomyopathy (10/21, 47.6%), 13 patients occurred at least once hypercapnic respiratory failure (13/21, 61.9%), six experienced recurrent respiratory failure and intubation (6/21, 28.6%), eight patients failed to survive (8/21, 38.1%). With nocturnal non-invasive ventilation of BiPAP, three patients recovered from respiratory failure, and led a relative stable and functional life (3/21, 14.3%).

Conclusion: Mitochondrial myopathy in children has great clinical, pathological, and genetical heterogeneity. Progressive proximal myopathy is most prevalent. Mitochondrial DNA point mutations are most common. And respiratory failure is a critical risk factor of poor prognosis.

MITO-FIND: A study in 390 patients to determine a diagnostic strategy for mitochondrial disease

Mitochondrial diseases, due to nuclear or mitochondrial genome mutations causing mitochondrial dysfunction, have a wide range of clinical features involving neurologic, muscular, cardiac, hepatic, visual, and auditory symptoms. Making a diagnosis of a mitochondrial disease is often challenging since there is no gold standard and traditional testing methods have required tissue biopsy which presents technical challenges and most patients prefer a non-invasive approach. Since a diagnosis invariably involves finding a disease-causing DNA variant, new approaches such as next generation sequencing (NGS) have the potential to make it easier to make a diagnosis. We evaluated the ability of our traditional diagnostic pathway (metabolite analysis, tissue neuropathology and respiratory chain enzyme activity) in 390 patients. The traditional diagnostic pathway provided a diagnosis of mitochondrial disease in 115 patients (29.50%). Analysis of mtDNA, tissue neuropathology, skin electron microscopy, respiratory chain enzyme analysis using inhibitor assays, blue native polyacrylamide gel electrophoresis were all statistically significant in distinguishing patients between a mitochondrial and non-mitochondrial diagnosis. From these 390 patients who underwent traditional analysis, we recruited 116 patients for the NGS part of the study (36 patients who had a mitochondrial diagnosis (MITO) and 80 patients who had no diagnosis (No-Dx)). In the group of 36 MITO patients, nuclear whole exome sequencing (nWES) provided a second diagnosis in 2 cases who already had a pathogenic variant in mtDNA, and a revised diagnosis (GLUL) in one case that had abnormal pathology but no pathogenic mtDNA variant. In the 80 NO-Dx patients, nWES found non-mitochondrial diagnosis in 26 patients and a mitochondrial diagnosis in 1 patient. A genetic diagnosis was obtained in 53/116 (45.70%) cases that were recruited for NGS, but not in 11/116 (9.48%) of cases with abnormal mitochondrial neuropathology. Our results show that a non-invasive, bigenomic sequencing (BGS) approach (using both a nWES and optimized mtDNA analysis to include large deletions) should be the first step in investigating for mitochondrial diseases. There may still be a role for tissue biopsy in unsolved cases or when the diagnosis is still not clear after NGS studies.

Using urine to diagnose large-scale mtDNA deletions in adult patients

OBJECTIVE

The aim of this study was to evaluate if urinary sediment cells offered a robust alternative to muscle biopsy for the diagnosis of single mtDNA deletions.

METHODS

Eleven adult patients with progressive external ophthalmoplegia and a known single mtDNA deletion were investigated. Urinary sediment cells were used to isolate DNA, which was then subjected to long-range polymerase chain reaction. Where available, the patient`s muscle DNA was studied in parallel. Breakpoint and thus deletion size were identified using both Sanger sequencing and next generation sequencing. The level of heteroplasmy was determined using quantitative polymerase chain reaction.

RESULTS

We identified the deletion in urine in 9 of 11 cases giving a sensitivity of 80%. Breakpoints and deletion size were readily detectable in DNA extracted from urine. Mean heteroplasmy level in urine was 38% ± 26 (range 8 - 84%), and 57% ± 28 (range 12 - 94%) in muscle. While the heteroplasmy level in urinary sediment cells differed from that in muscle, we did find a statistically significant correlation between these two levels (R = 0.714, P = 0.031(Pearson correlation)).

INTERPRETATION

Our findings suggest that urine can be used to screen patients suspected clinically of having a single mtDNA deletion. Based on our data, the use of urine could considerably reduce the need for muscle biopsy in this patient group.

Mitochondrial disease in children

Mitochondrial disease presenting in childhood is characterized by clinical, biochemical and genetic complexity. Some children are affected by canonical syndromes, but the majority have nonclassical multisystemic disease presentations involving virtually any organ in the body. Each child has a unique constellation of clinical features and disease trajectory, leading to enormous challenges in diagnosis and management of these heterogeneous disorders. This review discusses the classical mitochondrial syndromes presenting most frequently in childhood and then presents an organ-based perspective including systems less frequently linked to mitochondrial disease, such as skin and hair abnormalities and immune dysfunction. An approach to diagnosis is then presented, encompassing clinical evaluation and biochemical, neuroimaging and genetic investigations, and emphasizing the problem of phenocopies. The impact of next-generation sequencing is discussed, together with the importance of functional validation of novel genetic variants never previously linked to mitochondrial disease. The review concludes with a brief discussion of currently available and emerging therapies. The field of mitochondrial medicine has made enormous strides in the last 30 years, with approaching 400 different genes across two genomes now linked to primary mitochondrial disease. However, many important questions remain unanswered, including the reasons for tissue specificity and variability of clinical presentation of individuals sharing identical gene defects, and a lack of disease-modifying therapies and biomarkers to monitor disease progression and/or response to treatment.

Genetics of mitochondrial diseases: Identifying mutations to help diagnosis

Mitochondrial diseases are amongst the most genetically and phenotypically diverse groups of inherited diseases. The vast phenotypic overlap with other disease entities together with the absence of reliable biomarkers act as driving forces for the integration of unbiased methodologies early in the diagnostic algorithm, such as whole exome sequencing (WES) and whole genome sequencing (WGS). Such approaches are used in variant discovery and in combination with high-throughput functional assays such as transcriptomics in simultaneous variant discovery and validation. By capturing all genes, they not only increase the diagnostic rate in heterogenous mitochondrial disease patients, but accelerate novel disease gene discovery, and are valuable in side-stepping the risk of overlooking unexpected or even treatable genetic disease diagnoses.

A Systematic Review of the Health Economics of Pompe Disease

BACKGROUND

Pompe disease is a rare, severe neuromuscular disease with high mortality and substantial clinical and humanistic burden. However, the economic burden of Pompe disease and the health economic value of its treatments are not well understood. The objectives of this systematic review were to characterize the health economic evidence on Pompe disease, including healthcare resource use and costs (direct and indirect), health utilities, and the cost-effectiveness of current treatments used to manage patients with Pompe disease.

METHODS

A systematic search of MEDLINE^® and Embase^® was performed to retrieve publications on the health economics of Pompe disease. Publications were screened according to predefined criteria, extracted, and quality assessed using the Newcastle-Ottawa Scale. Data were narratively synthesized.

RESULTS

Eight publications evaluated patients with infantile-onset Pompe disease (IOPD) (two studies), late-onset Pompe disease (LOPD) (four studies), or both (two studies). In IOPD, total cost of supportive therapy (excluding treatment) was €32,871 (equivalent to US$41,667 when adjusted for currency and inflation to 2017 US dollars) over a life expectancy of 0.4 years. In adult LOPD, the average annual cost per patient of supportive therapy was €22,475 (adjusted $28,489). Resource use in LOPD was high, with nursing home admissions accounting for 19% of annual direct medical costs. Health economic evaluations estimating incremental costs per quality-adjusted life year (QALY) gained with enzyme-replacement therapy (ERT) versus supportive therapy ranged from £109,991 (adjusted, $186,851) per QALY gained in Columbia to €1,043,868 (adjusted, $1,323,207) in the Netherlands.

DISCUSSION

Despite a full systematic literature search, only eight relevant publications were identified, most of which were of relatively poor quality. However, a significant economic burden of Pompe disease on patients, families, healthcare systems, and society was found, with the majority of costs driven by the only currently approved treatment, ERT. Health economic evaluations of ERT versus supportive therapy vary significantly, with the majority well above willingness-to-pay thresholds. New therapies and approaches to care are needed to address the persistent and lifelong economic burden of Pompe disease and the large incremental cost-effectiveness ratios observed.