Combined oxidative phosphorylation deficiency 4 (COXPD4) is a rare mitochondrial condition caused by biallelic deleterious variants in the nuclear-encoded gene TUFM. To date, most individuals with COXPD4 have presented with encephalopathy, hypotonia, and abnormal brain imaging. Many of the reported individuals died in infancy. We aim to expand the clinical and biochemical phenotype of COXPD4 by reporting on an adult with this condition. Our proband has a homozygous TUFM c.1025T>G, p.(Val342Gly) variant. He has sensorineural hearing loss, hyperlactatemia with mild illness, and reduced activity in mitochondrial complexes I, III, and IV on endomyocardial biopsy. He presents with hypertrophic cardiomyopathy and chronic kidney failure, which have not previously been reported in this condition. Our findings suggest not all individuals with COXPD4 present with significant neurological involvement and highlight the importance of considering COXPD4 as part of the differential diagnosis of hypertrophic cardiomyopathy.

Combined oxidative phosphorylation deficiency 4 (COXPD4, OMIM #610678) is a very rare mitochondrial disorder caused by biallelic variants in TUFM gene. The condition is characterized by microcephaly, severe early-onset lactic acidosis, and progressive, often fatal, infantile encephalopathy. To date, only 8 patients with biallelic TUFM variants have been reported. We present a case of a female infant with microcephaly who died from severe lactic acidosis at 7 months of age. Genetic analysis revealed homozygous c.1016G>A (p.Arg339Gln) variant in the TUFM gene, which has previously been reported in three other COXPD4 cases. This is the fourth publication describing the same variant in this rare disorder, suggesting that it is a recurrent variant in COXPD4 patients. Arg339Gln variant was found in all patients from Turkey and is considered a potential founder mutation. This report aims to contribute to the phenotypic spectrum of COXPD4, explore the frequency and clinical presentation of the reported variants, enhance the understanding of genotype-phenotype correlations, and raise awareness of rare mitochondrial disorders.

Mitochondrial dysfunction is a critical factor leading to a wide range of clinically heterogeneous and often severe disorders due to its central role in generating cellular energy. Mutations in the TUFM gene are known to cause combined oxidative phosphorylation deficiency 4 (COXPD4), a rare mitochondrial disorder characterized by a comprehensive quantitative deficiency in mitochondrial respiratory chain (MRC) complexes. The development of a reliable animal model for COXPD4 is crucial for elucidating the roles and mechanisms of TUFM in disease pathogenesis and benefiting its medical management. In this study, we construct a zebrafish tufm-/- mutant that closely resembles the COXPD4 syndrome, exhibiting compromised mitochondrial protein translation, dysfunctional mitochondria with oxidative phosphorylation defects, and significant metabolic suppression of the tricarboxylic acid cycle. Leveraging this COXPD4 zebrafish model, we comprehensively validate the clinical relevance of TUFM mutations and identify probucol as a promising therapeutic approach for managing COXPD4. Our data offer valuable insights for understanding mitochondrial diseases and developing effective treatments.

The mitochondrial elongation factor Tu (EF-Tu), encoded by the TUFM gene, is a GTPase, which is part of the mitochondrial protein translation mechanism. If it is activated, it delivers the aminoacyl-tRNAs to the mitochondrial ribosome. Here, a patient was described with a homozygous missense variant in the TUFM [c.1016G>A (p.Arg339Gln)] gene. To date, only six patients have been reported with bi-allelic pathogenic variants in TUFM, leading to combined oxidative phosphorylation deficiency 4 (COXPD4) characterized by severe early-onset lactic acidosis, encephalopathy, and cardiomyopathy. The patient presented here had the phenotypic features of TUFM-related disease, lactic acidosis, hypotonia, liver dysfunction, optic atrophy, and mild encephalopathy. We aimed to expand the clinical spectrum of pathogenic variants of TUFM.

Translation of mitochondrial-specific DNA is required for proper mitochondrial function and energy production. For this purpose, an elaborate network of dedicated molecular machinery including initiation, elongation and termination factors exists. We describe a patient with an unusual phenotype and a novel homozygous missense variant in TUFM (c.344A>C; p.His115Pro), encoding mtDNA translation elongating factor Tu (EFTu). To date, only four patients have been reported with bi-allelic mutations in TUFM, leading to combined oxidative phosphorylation deficiency 4 (COXPD4) characterized by severe early-onset lactic acidosis and progressive fatal infantile encephalopathy. The patient presented here expands the phenotypic features of TUFM-related disease, exhibiting lactic acidosis and dilated cardiomyopathy without progressive encephalopathy. This warrants the inclusion of TUFM in differential diagnosis of metabolic cardiomyopathy. Cases that further refine genotype-phenotype associations and characterize the molecular basis of mitochondrial disorders allow clinicians to predict disease prognosis, greatly impacting patient care, as well as provide families with reproductive planning options.