The MT-ATP6 gene m.8993T>G pathogenic variant has been associated with Leigh syndrome, especially in patients exhibiting a high degree of heteroplasmy. Although patients may present with a wide phenotypic spectrum, characteristic findings include bilateral, symmetric hyperintensities in the basal ganglia and brainstem on brain MRI, particularly on T2-weighted and fluid-attenuated inversion recovery sequences. Additionally, the biochemical phenotype associated with this pathogenic variant often mimics that of multiple carboxylase deficiency and proximal urea cycle disorders. This report describes a male infant with an atypical neurological presentation of Leigh syndrome. At 2 months of age, he presented with status epilepticus of left temporal origin that was refractory to treatment. Initial brain MRI revealed a large region of non-enhancing signal abnormality in the left temporal lobe, raising concern for an infectious etiology. However, biochemical testing revealed hypocitrullinemia, elevated 3-hydroxyisovalerylcarnitine, elevated propionylcarnitine, and urinary excretion of lactate and pyruvate, prompting further investigation for MT-ATP6 mitochondrial disease. Mitochondrial DNA analysis confirmed the presence of a homoplasmic m.8993T>G pathogenic variant in the MT-ATP6 gene. Despite treatment with citrulline and high-dose biotin, the patient died 5 weeks later due to cardiorespiratory failure following a severe respiratory infection. Retrospective review of his newborn screening revealed two screens positive for low citrulline that were ultimately cleared on a third screen, delaying the diagnosis. This case underscores the importance of considering MT-ATP6 mitochondrial disease in the differential diagnosis of patients presenting with atypical neurological symptoms and biochemical abnormalities. It also highlights the value of newborn screening in identifying potential mitochondrial disorders, where early diagnosis and timely intervention may improve outcomes, even in severe cases.

Biotinidase (BTD, encoded by the BTD gene) deficiency is an autosomal recessive neurometabolic disease caused by abnormal BTD activity in the biotin cycle. The clinical symptoms of patients, which are mainly neurocutaneous, range from mild to severe based on the enzyme activity level. This study aimed to identify BTD gene mutations in suspected BTD deficiency patients for the first time in the southwest of Iran and evaluate their genotype-phenotype correlations. 11 clinically and biochemically suspected patients from nine unrelated families and their available family members were subjected to Sanger sequencing. Segregation analysis was performed for novel mutation. The effect of each mutation on protein stability and hydropathicity, as well as the pathogenicity prediction of all detected mutations were assessed using various in silico analysis tools. Six mutations including a novel and five previously reported mutations were identified in patients with different ethnicities. Three out of five known mutations were reported for the first time in Iran. Various common clinical manifestations and a rarely reported coexistence of celiac disease in biotinidase deficiency patients were observed. The novel missense variant c.787G > A (p.Glu263Lys) was detected in exon 4 of the BTD gene, within the biotinidase-like domain of the BTD protein. This variation was found in two cousins of a family, both developed the same initial clinical presentation. In silico analyses revealed that this missense substitution decreased protein stability and increased protein hydrophilicity. Additionally, the known frameshift mutation c.1264delG (p.val422serfster59), was the most frequent allele in the studied population. We also predicted and visualized the effects of the novel and frameshift mutations on protein structure. Our findings expanded the mutational spectrum of the BTD gene and provided valuable data on genotype-phenotype correlations, which helps in genetic counseling. Furthermore, the necessity of performing molecular analysis along with enzymatic analysis was highlighted by this study.

Holocarboxylase synthase (HCS) deficiency is an extremely rare metabolic disorder typically presenting as severe neonatal metabolic acidosis, lethargy, hypotonia, vomiting, and seizures. This report describes two siblings in a family with late-onset forms of HCS deficiency. The younger sister presented at the age of 11 years and manifested as acute metabolic acidosis, which promptly resolved following rehydration and biotin administration. The results of the organic urine profile confirmed multiple carboxylase deficiency, and genetic testing revealed a novel pathogenic variant in the HLCS gene (NM_000411.8) in the homozygous state: c.995A>G; p. (Gln332Arg). No further decompensation was observed for her during the 3-year follow-up period. His older brother was diagnosed at the age of 23 years-old through biochemical tests, without any history of acidotic decompensation. A mini-review of HCS deficiency with late onset (>1 year) or early onset (<1 month) revealed that splice variants are associated with late onset, while both variants p. (Leu216Arg) and p. (Leu237Pro) are associated with early onset. However, the majority of genotypes do not show a clear correlation with the timing of HCS deficiency onset. The most significant point here is the description of extremely late-onset cases of HCS deficiency. This can prompt metabolic investigations and raise suspicion of this rare disease in cases of unexplained metabolic acidosis, even beyond early childhood.

Biotin, also known as vitamin B7 or vitamin H, is a water-soluble B-complex vitamin and serves as an essential co-enzyme for five specific carboxylases. Holocarboxylase synthase (HCS) activates biotin and facilitates its covalent attachment to these enzymes, while biotinidase releases free biotin in the biotin cycle. The transport of biotin, primarily from the intestine, is mediated by the sodium-dependent multi-vitamin transporter (SMVT). Severe biotin deficiency leads to multiple carboxylase deficiency. Moreover, biotin is crucial to glucose and lipid utilization in cellular energy production because it modulates the expression of metabolic enzymes via various signaling pathways and transcription factors. Biotin also modulates the production of proinflammatory cytokines in the immune system through similar molecular mechanisms. These regulatory roles in metabolic and immune homeostasis connect biotin to conditions such as diabetes, dermatologic manifestations, and multiple sclerosis. Furthermore, deficiencies in biotin and SMVT are implicated in inflammatory bowel disease, affecting intestinal inflammation, permeability, and flora. Notably, HCS and probably biotin directly influence gene expression through histone modification. In this review, we summarize the current knowledge on the molecular aspects of biotin and associated molecules in diseases related to both acute inflammatory responses and chronic inflammation, and discuss the potential therapeutic applications of biotin.

Biotin (vitamin H or B7) is a coenzyme essential for all forms of life. Biotin has biological activity only when covalently attached to a few key metabolic enzyme proteins. Most organisms have only one attachment enzyme, biotin protein ligase (BPL), which attaches biotin to all target proteins. The sequences of these proteins and their substrate proteins are strongly conserved throughout biology. Structures of both the biotin ligase- and biotin-acceptor domains of mammals, plants, several bacterial species, and archaea have been determined. These, together with mutational analyses of ligases and their protein substrates, illustrate the exceptional specificity of this protein modification. For example, the Escherichia coli BPL biotinylates only one of the >4000 cellular proteins. Several bifunctional bacterial biotin ligases transcriptionally regulate biotin synthesis and/or transport in concert with biotinylation. The human BPL has been demonstrated to play an important role in that mutations in the BPL encoding gene cause one form of the disease, biotin-responsive multiple carboxylase deficiency. Promiscuous mutant versions of several BPL enzymes release biotinoyl-AMP, the active intermediate of the ligase reaction, to solvent. The released biotinoyl-AMP acts as a chemical biotinylation reagent that modifies lysine residues of neighboring proteins in vivo. This proximity-dependent biotinylation (called BioID) approach has been heavily utilized in cell biology.