Hypomyelinating leukodystrophy-ataxia-hypodontia-hypomyelination syndrome

Hypomyelinating leukodystrophy-ataxia-hypodontia-hypomyelination syndrome, also known as 4H syndrome (Hypomyelination, Hypodontia, and Hypogonadotropic Hypogonadism) or leukodystrophy hypomyelinating 7 (HLD7), is a rare inherited disorder affecting the central nervous system, teeth, and endocrine system. It is caused by biallelic mutations in the POLR3A or POLR3B genes, which encode subunits of RNA polymerase III, an enzyme essential for transcribing small non-coding RNAs critical for cell function. The condition belongs to the broader group of POLR3-related leukodystrophies. The hallmark features include diffuse cerebral hypomyelination visible on brain MRI, cerebellar ataxia (progressive difficulties with coordination and balance), hypodontia or oligodontia (missing or abnormally developed teeth), and hypogonadotropic hypogonadism leading to delayed or absent puberty. Affected individuals typically present in childhood with progressive motor difficulties including spasticity, tremor, and gait abnormalities. Cognitive function may be relatively preserved early in the disease course but can decline over time. Additional features may include short stature and myopia. There is currently no cure or disease-modifying treatment for 4H syndrome. Management is supportive and multidisciplinary, involving neurologists, endocrinologists, dentists, and rehabilitation specialists. Hormone replacement therapy may be used to address hypogonadotropic hypogonadism, dental prosthetics can address missing teeth, and physical and occupational therapy help maintain motor function. The disease is progressive, and the rate of neurological decline varies among individuals.

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