Riboflavin Transporter Deficiency Type 2: Expanding the Phenotype of the Lebanese Founder Mutation p.Gly306Arg in the SLC52A2 Gene

<b>Background</b>: Riboflavin transporter deficiency type 2 is an ultra-rare, yet treatable, inborn error of metabolism. This autosomal recessive disorder is caused by pathogenic mutations in the <i>SLC52A2</i> gene leading to progressive ataxia, polyneuropathy, and hearing and visual impairment. The early initiation of riboflavin therapy can prevent or mitigate the complications. To date, only 200 cases have been reported, mostly in consanguineous populations. The p.Gly306Arg founder mutation, identified in patients of Lebanese descent, is the most frequently reported worldwide. It was described in a homozygous state in a total of 21 patients. Therefore, studies characterizing the phenotypic spectrum of this mutation remain scarce. <b>Methods</b>: A retrospective review of charts of patients diagnosed with riboflavin transporter deficiency type 2 at a tertiary-care reference center in Lebanon was performed. Clinical, biochemical, and molecular profiles were analyzed and compared to reported cases in the literature. <b>Results</b>: A total of six patients from three unrelated families were diagnosed between 2018 and 2023. All patients exhibited the homozygous founder mutation, p.Gly306Arg, with variable phenotypes, even among family members. The median age of onset was 3 years. Diagnosis was achieved by exome sequencing at a median age of 5 years, as clinical and biochemical profiles were inconsistently suggestive. The response to riboflavin was variable. One patient treated with high-dose riboflavin recovered his motor function, while the others were stabilized. <b>Conclusions</b>: This study expands the current knowledge of the phenotypic spectrum associated with the p.Gly306Arg mutation in the <i>SLC52A2</i> gene. Increased awareness among physicians of the common manifestations of this rare disorder is crucial for early diagnosis and treatment. In the absence of a consistent clinical or biochemical phenotype, the use of next-generation sequencing as a first-tier diagnostic test may be considered.