Hypotonia-cystinuria type 1 syndrome

Hypotonia-cystinuria type 1 syndrome (also known as HCS type 1 or hypotonia-cystinuria syndrome) is a rare contiguous gene deletion syndrome caused by deletions on chromosome 2p21 that encompass at least two adjacent genes: SLC3A1 (associated with cystinuria type A) and PREPL (associated with hypotonia). Because the deletion removes both genes simultaneously, affected individuals present with features of both conditions combined into a recognizable clinical syndrome. The hallmark features include neonatal or infantile hypotonia (low muscle tone), failure to thrive, feeding difficulties in early life, and cystinuria — a condition in which excessive amounts of the amino acid cystine are excreted in the urine, predisposing to recurrent kidney stone formation. Additional features may include growth hormone deficiency, minor facial dysmorphism, and developmental delay, though cognitive outcomes can be variable. The hypotonia typically improves with age but may persist to some degree. Management is multidisciplinary and largely supportive. Treatment of cystinuria involves high fluid intake, urinary alkalinization, and in some cases thiol-binding drugs (such as tiopronin or D-penicillamine) to reduce cystine stone formation. Growth hormone replacement therapy may be indicated when growth hormone deficiency is documented. Physical therapy and nutritional support are important in early life to address hypotonia and feeding difficulties. There is currently no cure or gene-specific therapy available for this syndrome.

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