Human 17β-hydroxysteroid dehydrogenase type 10 (HSD10) is a rare X-linked mitochondrial disorder caused by mutations in the HSD17B10 gene. It is typically associated with neurodegeneration and cardiomyopathy in severe cases. The neonatal form often has a poor prognosis; however, its clinical spectrum remains unclear. Herein, we present a neonatal-onset case of HSD10 disease with a previously unreported HSD17B10 variant presenting with early-onset cardiomyopathy triggered by a viral infection. A male infant presented with lactic acidosis and hypoglycemia on the first day of life. Initial treatment improved metabolic status. He was diagnosed with HSD10 disease using a targeted panel of nuclear and mitochondrial genes, which identified a novel hemizygous variant, NM_004493.3: c.34G>A p.(Val12Met). Cardiac function was normal at five months, but neurodevelopmental regression occurred at six months following vaccination. At seven months, viral myocarditis was diagnosed following rhinovirus/enterovirus infection. Echocardiography revealed a reduced left ventricular ejection fraction and ventricular dilation. Despite supportive therapy, cardiac function deteriorated, leading to death at nine months. A patient with a novel HSD17B10 variant showed early-onset cardiomyopathy in a neonatal case of HSD10 disease and rapid and fatal deterioration of cardiac function. This report highlights the importance of clarifying genotype-phenotype correlations to guide the diagnosis and management of rare mitochondrial diseases.

Hydroxysteroid 17-beta dehydrogenase type 10 (HSD10) deficiency (HSD10 disease) is a rare X-linked neurodegenerative condition caused by abnormalities in the HSD17B10 gene. A total of 10 mutations have been reported in the literature since 2000. Described phenotypes include a severe neonatal or progressive infantile form with hypotonia, choreoathetosis, seizures, cardiomyopathy, neurodegeneration, and death, as well as an attenuated form with variable regression. Here we present the second report of a c.194T>C (p.V65A) mutation in two half-brothers with a clinical phenotype characterized by neurodevelopmental delay, choreoathetosis, visual loss, cardiac findings, and behavioral abnormalities, with regressions now noted in the older sibling. Neither has experienced a metabolic crisis. Both of the siblings had normal tandem mass spectroscopy analysis of their newborn screening samples. The older brother's phenotype may be complicated by the presence of a 3q29 microduplication. Diagnosis requires a high index of suspicion, as the characteristic urine organic acid pattern may escape detection. The exact pathogenic mechanism of disease remains to be elucidated, but may involve the non-dehydrogenase functionalities of the HSD10 protein. Our report highlights clinical features of two patients with the less fulminant phenotype associated with a V65A mutation, compares the reported phenotypes to date, and reviews recent findings regarding the potential pathophysiology of this condition.Summary Sentence Hydroxysteroid 17-beta dehydrogenase type 10 (HSD10) disease (HSD10 disease) is a rare X-linked neurodegenerative condition with a variable clinical phenotype; diagnosis requires a high index of suspicion.