Riboflavin Transporter Deficiency as a Cause of Progressive Encephalopathy

Background/Objective: Riboflavin transporter deficiency (RTD) is a rare neurodegenerative disease, with under 500 cases genetically confirmed since the early 2000s. Thus far, three separate subtypes of RTD2 are described-type 1, 2 and 3-but, previously, RTD was classified as two separate genetic defects: Brown-Vialetto-Van Laere syndrome and Fazio-Londe syndrome, caused by mutations in the SLC52A2 and SLC52A3 genes, respectively. The most prominent symptoms found in patients include encephalopathy, expressed as peripheral and cranial nerve neuropathy, which in turn lead to a series of complications: decreased muscle strength, hypotonia, visual impairment, sensorineural hearing loss, bulbar palsy, sensory ataxia and respiratory insufficiency secondary to diaphragmatic paresis. At the cellular level, riboflavin is modified into active flavin cofactors: FMN, mediating riboflavin phosphorylation through riboflavin kinase, and FAD, involved in FMN adenylation through the flavin dinucleotide 1 synthesis. FMN and FAD are two of approximately 100 proteins collectively described as the 'flavoproteome'. Most of them are mitochondrial oxidoreductases, catalyzing the electron transport in many metabolic reactions, as well as regulating important cell processes, such as the production of reactive oxygen species, protein conformation and damage repair. FMN and FAD are also responsible for the conversion of B6 and B9 vitamins into their active forms, which allows for healthy cell growth and immune function. Methods: In this article, the authors describe two children, a 6-year-old girl and her 5-year-old sister, both presenting with RTD2 caused by mutations in the SLC52A2 gene (c.916G>C (p.Gly306Arg); c.477C>G (p.Cys159Trp)), in whom the disease progression was successfully inhibited by vitamin B₂ supplementation in varying doses. Results: Their clinical image consists of psychomotor developmental delay, ataxia, horizontal nystagmus, hearing loss and a lack of visual fixation. Conclusions: The phenotype and clinical signs presented by the described sisters are further discussed in relation to the previously published reports of RTD2 cases.