Time to harmonize mitochondrial syndrome nomenclature and classification: A consensus from the North American Mitochondrial Disease Consortium (NAMDC).

OBJECTIVE: To harmonize terminology in mitochondrial medicine, we propose revised clinical criteria for primary mitochondrial syndromes. METHODS: The North American Mitochondrial Disease Consortium (NAMDC) established a Diagnostic Criteria Committee comprised of members with diverse expertise. It included clinicians, researchers, diagnostic laboratory directors, statisticians, and data managers. The Committee conducted a comprehensive literature review, an evaluation of current clinical practices and diagnostic modalities, surveys, and teleconferences to reach consensus on syndrome definitions for mitochondrial diseases. The criteria were refined after manual application to patients enrolled in the NAMDC Registry. RESULTS: By building upon published diagnostic criteria and integrating recent advances, NAMDC has generated updated consensus criteria for the clinical definition of classical mitochondrial syndromes. CONCLUSIONS: Mitochondrial diseases are clinically, biochemically, and genetically heterogeneous and therefore challenging to classify and diagnose. To harmonize terminology, we propose revised criteria for the clinical definition of mitochondrial disorders. These criteria are expected to standardize the diagnosis and categorization of mitochondrial diseases, which will facilitate future natural history studies and clinical trials.

Biallelic disruption of PKDCC is associated with a skeletal disorder characterised by rhizomelic shortening of extremities and dysmorphic features.

BACKGROUND: During mouse embryonic development the protein kinase domain containing, cytoplasmic (Pkdcc) gene, also known as Vlk, is expressed in several tissues including the ventral midbrain, with particularly strong expression in branchial arches and limb buds. Homozygous Pkdcc knockout mice have dysmorphic features and shortened long bones as the most obvious morphological abnormalities. The human PKDCC gene has currently not been associated with any disorders. OBJECTIVE: To use clinical diagnostic exome sequencing (DES) for providing genetic diagnoses to two apparently unrelated patients with similar skeletal abnormalities comprising rhizomelic shortening of limbs and dysmorphic features. METHODS: Patient-parents trio DES was carried out and the identified candidate variants were confirmed by Sanger sequencing. RESULTS: Each patient had a homozygous gene disrupting variant in PKDCC considered to explain the skeletal phenotypes shared by both. The first patient was homozygous for the nonsense variant p.(Tyr217*) (NM_1 38 370 c.651C>A) expected to result in nonsense-mediated decay of the mutant transcripts, whereas the second patient was homozygous for the splice donor variant c.639+1G>T predicted to abolish the donor splice site by three in silico splice prediction algorithms. CONCLUSIONS: Biallelic gene disrupting variants in PKDCC in humans, just like in mice, cause dysmorphic features and rhizomelic shortening of limbs.

Recurrent De Novo Mutations Affecting Residue Arg138 of Pyrroline-5-Carboxylate Synthase Cause a Progeroid Form of Autosomal-Dominant Cutis Laxa.

Progeroid disorders overlapping with De Barsy syndrome (DBS) are collectively denoted as autosomal-recessive cutis laxa type 3 (ARCL3). They are caused by biallelic mutations in PYCR1 or ALDH18A1, encoding pyrroline-5-carboxylate reductase 1 and pyrroline-5-carboxylate synthase (P5CS), respectively, which both operate in the mitochondrial proline cycle. We report here on eight unrelated individuals born to non-consanguineous families clinically diagnosed with DBS or wrinkly skin syndrome. We found three heterozygous mutations in ALDH18A1 leading to amino acid substitutions of the same highly conserved residue, Arg138 in P5CS. A de novo origin was confirmed in all six probands for whom parental DNA was available. Using fibroblasts from affected individuals and heterologous overexpression, we found that the P5CS-p.Arg138Trp protein was stable and able to interact with wild-type P5CS but showed an altered sub-mitochondrial distribution. A reduced size upon native gel electrophoresis indicated an alteration of the structure or composition of P5CS mutant complex. Furthermore, we found that the mutant cells had a reduced P5CS enzymatic activity leading to a delayed proline accumulation. In summary, recurrent de novo mutations, affecting the highly conserved residue Arg138 of P5CS, cause an autosomal-dominant form of cutis laxa with progeroid features. Our data provide insights into the etiology of cutis laxa diseases and will have immediate impact on diagnostics and genetic counseling.

Nonreentrant atrial tachycardia occurs independently of hypertrophic cardiomyopathy in RASopathy patients.

Multifocal atrial tachycardia (MAT) has a well-known association with Costello syndrome, but is rarely described with related RAS/MAPK pathway disorders (RASopathies). We report 11 patients with RASopathies (Costello, Noonan, and Noonan syndrome with multiple lentigines [formerly LEOPARD syndrome]) and nonreentrant atrial tachycardias (MAT and ectopic atrial tachycardia) demonstrating overlap in cardiac arrhythmia phenotype. Similar overlap is seen in RASopathies with respect to skeletal, musculoskeletal and cutaneous abnormalities, dysmorphic facial features, and neurodevelopmental deficits. Nonreentrant atrial tachycardias may cause cardiac compromise if sinus rhythm is not restored expeditiously. Typical first-line supraventricular tachycardia anti-arrhythmics (propranolol and digoxin) were generally not effective in restoring or maintaining sinus rhythm in this cohort, while flecainide or amiodarone alone or in concert with propranolol were effective anti-arrhythmic agents for acute and chronic use. Atrial tachycardia resolved in all patients. However, a 4-month-old boy from the cohort was found asystolic (with concurrent cellulitis) and a second patient underwent cardiac transplant for heart failure complicated by recalcitrant atrial arrhythmia. While propranolol alone frequently failed to convert or maintain sinus rhythm, fleccainide or amiodarone, occasionally in combination with propranolol, was effective for RASopathy patient treatment for nonreentrant atrial arrhythmia. Our analysis shows that RASopathy patients may have nonreentrant atrial tachycardia with and without associated cardiac hypertrophy. While nonreentrant arrhythmia has been traditionally associated with Costello syndrome, this work provides an expanded view of RASopathy cardiac arrhythmia phenotype as we demonstrate mutant proteins throughout this signaling pathway can also give rise to ectopic and/or MAT.

Patient care standards for primary mitochondrial disease: a consensus statement from the Mitochondrial Medicine Society.

The purpose of this statement is to provide consensus-based recommendations for optimal management and care for patients with primary mitochondrial disease. This statement is intended for physicians who are engaged in the diagnosis and management of these patients. Working group members were appointed by the Mitochondrial Medicine Society. The panel included members with several different areas of expertise. The panel members utilized surveys and the Delphi method to reach consensus. We anticipate that this statement will need to be updated as the field continues to evolve. Consensus-based recommendations are provided for the routine care and management of patients with primary genetic mitochondrial disease.

De novo heterozygous mutations in SMC3 cause a range of Cornelia de Lange syndrome-overlapping phenotypes.

Cornelia de Lange syndrome (CdLS) is characterized by facial dysmorphism, growth failure, intellectual disability, limb malformations, and multiple organ involvement. Mutations in five genes, encoding subunits of the cohesin complex (SMC1A, SMC3, RAD21) and its regulators (NIPBL, HDAC8), account for at least 70% of patients with CdLS or CdLS-like phenotypes. To date, only the clinical features from a single CdLS patient with SMC3 mutation has been published. Here, we report the efforts of an international research and clinical collaboration to provide clinical comparison of 16 patients with CdLS-like features caused by mutations in SMC3. Modeling of the mutation effects on protein structure suggests a dominant-negative effect on the multimeric cohesin complex. When compared with typical CdLS, many SMC3-associated phenotypes are also characterized by postnatal microcephaly but with a less distinctive craniofacial appearance, a milder prenatal growth retardation that worsens in childhood, few congenital heart defects, and an absence of limb deficiencies. While most mutations are unique, two unrelated affected individuals shared the same mutation but presented with different phenotypes. This work confirms that de novo SMC3 mutations account for ∼ 1%-2% of CdLS-like phenotypes.

Fetal akinesia deformation sequence due to a congenital disorder of glycosylation.

Congenital disorders of Glycosylation (CDG) are increasingly emerging as a major underlying etiology for patients with complex neurogenetic malformations and dysmorphic features. We describe a newborn female with arthrogryposis multiplex due to fetal akinesia secondary to CDG-DPAGT1. Pregnancy was complicated by reduced fetal movements. At birth, the patient was evaluated for intrauterine growth restriction, bilateral cataracts, and multiple joint contractures. She had markedly reduced spontaneous movements, hypotonia, weak cry, and poor suck. She had ventilator-dependent central respiratory depression. Brain MRI showed delayed myelination and an incomplete cerebellar vermis. Transferrin isoelectric focusing was suggestive of a type I congenital disorder of glycosylation. Sequencing revealed a homozygous missense mutation in dolichyl-phosphate N-acetylglucosaminephosphotransferase (DPAGT1), exon 3, p.Leu118Val, consistent with DPAGT1-CDG. There have been seventeen previously reported cases of DPAGT1-CDG, including two similar cases with multiple contractures. This case highlights the importance of considering congenital disorders of glycosylation in the differential diagnosis for arthrogryposis.

A Case of DNAJC12-Deficient Hyperphenylalaninemia Detected on Newborn Screening: Clinical Outcomes from Early Detection.

DNAJC12-deficient hyperphenylalaninemia is a recently described inborn error of metabolism associated with hyperphenylalaninemia, neurotransmitter deficiency, and developmental delay caused by biallelic pathogenic variants of the DNAJC12 gene. The loss of the DNAJC12-encoded chaperone results in the destabilization of the biopterin-dependent aromatic amino acid hydroxylases, resulting in deficiencies in dopamine, norepinephrine, and serotonin. We present the case of a patient who screened positive for hyperphenylalaninemia on newborn screening and was discovered to be homozygous for a likely pathogenic variant of DNAJC12. Here, we review the management of DNAJC12-related hyperphenylalaninemia and compare our patient to other reported cases in the literature to investigate how early detection and management may impact clinical outcomes.

Shwachman-Diamond syndrome mimicking mitochondrial hepatopathy.

Shwachman-Diamond syndrome (SDS) is a genetic disorder caused by mutations in the Shwachman-Bodian-Diamond syndrome (SBDS) gene. The syndrome is characterized by multiorgan dysfunction primarily involving the bone marrow and exocrine pancreas. Frequently overlooked is the hepatic dysfunction seen in early childhood which tends to improve by adulthood. Here, we report a child who initially presented with failure to thrive and elevated transaminases, and was ultimately diagnosed with SDS. A liver biopsy electron micrograph revealed hepatocytes crowded with numerous small mitochondria, resembling the hepatic architecture from patients with inborn errors of metabolism, including mitochondrial diseases. To our knowledge, this is the first report of the mitochondrial phenotype in an SDS patient. These findings are compelling given the recent cellular and molecular research studies which have identified SBDS as an essential regulator of mitochondrial function and have also implicated SBDS in the maintenance of mitochondrial DNA.

ScreenPlus: A comprehensive, multi-disorder newborn screening program.

The increasing availability of novel therapies highlights the importance of screening newborns for rare genetic disorders so that they may benefit from early therapy, when it is most likely to be effective. Pilot newborn screening (NBS) studies are a way to gather objective evidence about the feasibility and utility of screening, the accuracy of screening assays, and the incidence of disease. They are also an optimal way to evaluate the complex ethical, legal and social implications (ELSI) that accompany NBS expansion for disorders. ScreenPlus is a consented pilot NBS program that aims to enroll over 100,000 infants across New York City. The initial ScreenPlus panel includes 14 disorders and uses an analyte-based, multi-tiered screening platform in an effort to enhance screening accuracy. Infants who receive an abnormal result are referred to a ScreenPlus provider for confirmatory testing, management, and therapy as needed, along with longitudinal capture of outcome data. Participation in ScreenPlus requires parental consent, which is obtained in active and passive manners. Patient-facing documents are translated into the ten most common languages spoken at our nine pilot hospitals, all of which serve diverse communities. At the time of consent, parents are invited to receive a series of online surveys to capture their opinions about specific ELSI-related topics, such as NBS policy, residual dried blood spot retention, and the types of disorders that should be on NBS panels. ScreenPlus has developed a stakeholder-based, collective funding model that includes federal support in addition to funding from 14 advocacy and industry sponsors, all of which have a particular interest in NBS for at least one of the ScreenPlus disorders. Taken together, ScreenPlus is a model, multi-sponsored pilot NBS program that will provide critical data about NBS for a broad panel of disorders, while gathering key stakeholder opinions to help guide ethically sensitive decision-making about NBS expansion.

Mitochondrial tRNA(Phe) mutation as a cause of end-stage renal disease in childhood.

BACKGROUND: We identified a mitochondrial tRNA mutation (m.586 G > A) in a patient with renal failure and symptoms consistent with a mitochondrial cytopathy. This mutation was of unclear significance due to the absence of consistent reports of linkage to specific disease phenotypes and any data pertaining to its effects on mitochondrial function. CASE-DIAGNOSIS/TREATMENT: A 16-month-old girl with failure-to-thrive, developmental regression, persistent lactic acidosis, hypotonia, gastrointestinal dysmotility, adrenal insufficiency, and hematologic abnormalities developed hypertension and renal impairment with chronic tubulointerstitial fibrosis, progressing to renal failure with the need for peritoneal dialysis. Evaluation of her muscle and blood led to the identification of a mutation of the mitochondrial tRNA for phenylalanine, m.586 G > A. CONCLUSIONS: The m.586 G > A mutation is pathogenic and a cause of end-stage renal disease in childhood. The mutation interferes with the stability of tRNA(Phe) and affects the translation of mitochondrial proteins and the stability of the electron transport chain.

PPA2 Deficiency in 2 Sisters: A Rare Cause of Sudden Cardiac Death.

Inorganic pyrophosphatase 2 (PPA2) deficiency is a genetic cause of sudden cardiac death, often triggered by viral infection or alcohol consumption. Literature on management is limited because most cases are diagnosed post mortem. We report lethal and nonlethal cardiac presentations of PPA2 deficiency in 2 adolescent sisters that resulted from a novel pathogenic PPA2 variant. (Level of Difficulty: Advanced.).

Recessive pathogenic variants in MCAT cause combined oxidative phosphorylation deficiency.

Malonyl-CoA-acyl carrier protein transacylase (MCAT) is an enzyme involved in mitochondrial fatty acid synthesis (mtFAS) and catalyzes the transfer of the malonyl moiety of malonyl-CoA to the mitochondrial acyl carrier protein (ACP). Previously, we showed that loss-of-function of mtFAS genes, including Mcat, is associated with severe loss of electron transport chain (ETC) complexes in mouse immortalized skeletal myoblasts (Nowinski et al., 2020). Here, we report a proband presenting with hypotonia, failure to thrive, nystagmus, and abnormal brain MRI findings. Using whole exome sequencing, we identified biallelic variants in MCAT. Protein levels for NDUFB8 and COXII, subunits of complex I and IV respectively, were markedly reduced in lymphoblasts and fibroblasts, as well as SDHB for complex II in fibroblasts. ETC enzyme activities were decreased in parallel. Re-expression of wild-type MCAT rescued the phenotype in patient fibroblasts. This is the first report of a patient with MCAT pathogenic variants and combined oxidative phosphorylation deficiency.

Novel inborn error of folate metabolism: identification by exome capture and sequencing of mutations in the MTHFD1 gene in a single proband.

OBJECTIVE: An infant was investigated because of megaloblastic anaemia, atypical hemolytic uraemic syndrome, severe combined immune deficiency, elevated blood levels of homocysteine and methylmalonic acid, and a selective decreased synthesis of methylcobalamin in cultured fibroblasts. METHODS: Exome sequencing was performed on patient genomic DNA. RESULTS: Two mutations were identified in the MTHFD1 gene, which encodes a protein that catalyses three reactions involved in cellular folate metabolism. This protein is essential for the generation of formyltetrahydrofolate and methylenetetrahydrofolate and important for nucleotide and homocysteine metabolism. One mutation (c.727+1G>A) affects the splice acceptor site of intron 8. The second mutation, c.517C>T (p.R173C), changes a critical arginine residue in the NADP-binding site of the protein. Mutations affecting this arginine have previously been shown to affect enzyme activity. Both parents carry a single mutation and an unaffected sibling carries neither mutation. The combination of two mutations in the MTHFRD1 gene, predicted to have severe consequences, in the patient and their absence in the unaffected sibling, supports causality. CONCLUSION: This patient represents the first case of an inborn error of folate metabolism affecting the trifunctional MTHFD1 protein. This report reinforces the power of exome capture and sequencing for the discovery of novel genes, even when only a single proband is available for study.

Phenotypic continuum of NFU1-related disorders.

Bi-allelic variants in Iron-Sulfur Cluster Scaffold (NFU1) have previously been associated with multiple mitochondrial dysfunctions syndrome 1 (MMDS1) characterized by early-onset rapidly fatal leukoencephalopathy. We report 19 affected individuals from 10 independent families with ultra-rare bi-allelic NFU1 missense variants associated with a spectrum of early-onset pure to complex hereditary spastic paraplegia (HSP) phenotype with a longer survival (16/19) on one end and neurodevelopmental delay with severe hypotonia (3/19) on the other. Reversible or irreversible neurological decompensation after a febrile illness was common in the cohort, and there were invariable white matter abnormalities on neuroimaging. The study suggests that MMDS1 and HSP could be the two ends of the NFU1-related phenotypic continuum.

Expanded molecular features of carnitine acyl-carnitine translocase (CACT) deficiency by comprehensive molecular analysis.

Carnitine-acylcarnitine translocase (CACT) deficiency is a rare autosomal recessive disease of fatty acid oxidation, mainly affecting long chain fatty acid utilization. The disease usually presents at neonatal period with severe hypoketotic hypoglycemia, hyperammonemia, cardiomyopathy and/or arrhythmia, hepatic dysfunction, skeletal muscle weakness, and encephalopathy. Definitive diagnosis of CACT deficiency by molecular analysis of the SLC25A20 gene has recently become clinically available. In contrast to biochemical analysis, sequence analysis is a more rapid and reliable method for diagnosis of CACT deficiency. In this study, we used Sanger sequencing and target array CGH to identify molecular defects in the SLC25A20 gene of patients with clinical features and an acylcarnitine profile consistent with CACT deficiency. Eight novel mutations, including a large 25.9 kb deletion encompassing exons 5 to 9 of SLC25A20 were found. Review of the published cases revealed that CACT deficiency is a pan-ethnic disorder with a broad mutation spectrum. Mutations are distributed along the entire gene without a hot spot. Two thirds of them are nonsense, frame-shift, or splice site mutations resulting in premature stop codons. This study underscores the importance of comprehensive molecular analysis, including sequencing and targeted array CGH of the SLC25A20 gene when CACT deficiency is suspected.

Somatic mosaicism for PDHA1 mutation in a male with pyruvate dehydrogenase complex deficiency.

Pyruvate dehydrogenase complex deficiency is a clinically heterogeneous disorder. Most cases are due to mutations in an X-linked PDHA1 gene encoding the E1alpha subunit of the multienzyme complex. Females with mutations in the PDHA1 gene may be asymptomatic or have a milder phenotype as a result of skewed X-inactivation, while males are typically more severely affected. We report a case of PDHA1 mosaicism in a male patient who had a milder phenotype.

The N370S/R496H genotype in type 1 Gaucher disease - Natural history and implications for pre symptomatic diagnosis and counseling.

Type 1 Gaucher disease (GD1) patients with the N370S/R496H (N409S/R535H) genotype are increasingly identified through carrier and newborn screening panels. However, limited information is available on the phenotype associated with this genotype. Here, we report our experience with 14 patients with this genotype. Our data suggests that most patients with N370S/R496H present with mild manifestations and often do not require treatment. This information is important for counseling newly diagnosed patients and GD1 carrier couples.

Mitochondrial diseases in North America: An analysis of the NAMDC Registry.

OBJECTIVE: To describe clinical, biochemical, and genetic features of participants with mitochondrial diseases (MtDs) enrolled in the North American Mitochondrial Disease Consortium (NAMDC) Registry. METHODS: This cross-sectional, multicenter, retrospective database analysis evaluates the phenotypic and molecular characteristics of participants enrolled in the NAMDC Registry from September 2011 to December 2018. The NAMDC is a network of 17 centers with expertise in MtDs and includes both adult and pediatric specialists. RESULTS: One thousand four hundred ten of 1,553 participants had sufficient clinical data for analysis. For this study, we included only participants with molecular genetic diagnoses (n = 666). Age at onset ranged from infancy to adulthood. The most common diagnosis was multisystemic disorder (113 participants), and only a minority of participants were diagnosed with a classical mitochondrial syndrome. The most frequent classical syndromes were Leigh syndrome (97 individuals) and mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (71 individuals). Pathogenic variants in the mitochondrial DNA were more frequently observed (414 participants) than pathogenic nuclear gene variants (252 participants). Pathogenic variants in 65 nuclear genes were identified, with POLG1 and PDHA1 being the most commonly affected. Pathogenic variants in 38 genes were reported only in single participants. CONCLUSIONS: The NAMDC Registry data confirm the high variability of clinical, biochemical, and genetic features of participants with MtDs. This study serves as an important resource for future enhancement of MtD research and clinical care by providing the first comprehensive description of participant with MtD in North America.