Low‐level mosaicism in tuberous sclerosis complex in four unrelated patients: Comparison of clinical characteristics and diagnostic pathways

Tuberous sclerosis complex (TSC) is an autosomal dominant neurocutaneous syndrome caused by either TSC1 or TSC2 gene mutations. About 15% of TSC patients remain without genetic diagnosis by conventional analysis despite clinical evidence. It is important to identify somatic mosaics, as therapeutic options are now available in patients with TSC1 or TSC2 mutations. Here, we describe the clinical and genetic characteristics of four male TSC patients with low‐level mosaicism. Patients presented at ages between 9 months and 32 years. Clinical manifestations varied considerably and included brain lesions in all four patients, cardiac rhabdomyomas in two young patients, skin involvement in two patients, and retinal hamartomas and renal angiomyolipomas in three patients. One patient presented with epileptic seizures and psychomotor delay. Low levels of mosaicism for TSC1 or TSC2 mutation were found in different tissue samples employing next generation sequencing and multiple ligation‐dependent probe amplification. The five disease‐associated variants, including one second‐hit mutation, include three truncating mutations and one deletion in TSC2, and one truncating mutation in TSC1. Sanger sequencing, allele‐specific oligonucleotide PCR (ASO‐PCR), and droplet digital PCR were used to confirm and quantify the disclosed mutations. Genetic identification of low‐level mosaicism for TSC remains challenging but is important for optimal surveillance and management.