Intermittent episodes of acute severe encephalomyopathy and early death in two siblings caused by biallelic likely pathogenic variants in FASTKD2: Expanding phenotype and literature review.

Introduction: Combined oxidative phosphorylation (OXPHOS) deficiency 44 (COXPD44; MIM# 618855) is caused by biallelic pathogenic variants in FAS-activated serine-threonine kinase domain 2 (FASTKD2) (MIM# 612322). COXPD44 is characterized by variable clinical features-developmental delay, chronic epileptic encephalopathy, seizure disorder/status epilepticus and cerebellar ataxia. We ascertained one sib with episodic acute encephalomyopathy triggered by acute gastroenteritis and associated with haematological abnormalities, rhabdomyolysis leading to acute kidney injury, hypotensive shock leading to early death and a similarly affected sib with early death. Both siblings were normal neurologically in between the acute episodes.

Material and methods: Whole exome sequencing (WES) was performed in the elder sibling. Mitochondrial respiratory chain enzyme activity assaywas performed in fibroblast cells and muscle tissue of the elder sibling. Also, Adenosine triphosphate (ATP) determination assay was done in fibroblast cells of the elder sibling.

Results: WES revealed compound heterozygous missense likely pathogenic variants in FASTKD2. Mitochondrial respiratory chain enzyme activity in muscle tissue showed reduced complex IV activity and ATP determination assay showed a reduction of ATP in skin fibroblasts.

Conclusion: Herein, we report two siblings with novel clinical phenotype associated with COXPD44. Our report further validates the biallelic variants in FASTKD2 associated with the variable phenotypes and mitochondrial OXPHOS defect.