A newly identified ferritin L-subunit variant results in increased proteasomal subunit degradation, impaired complex assembly, and severe hypoferritinemia

Ferritin is a hetero-oligomeric nanocage, composed of 24 subunits of two types, FTH1 and FTL. It protects the cell from excess reactive iron, by storing iron in its cavity. FTH1 is essential for the recruitment of iron into the ferritin nanocage and for cellular ferritin trafficking, whereas FTL contributes to nanocage stability and iron nucleation inside the cavity. Here we describe a female patient with a medical history of severe hypoferritinemia without anemia. Following inadequate heavy IV iron supplementation, the patient developed severe iron overload and musculoskeletal manifestations. However, her serum ferritin levels rose only to normal range. Genetic analyses revealed an undescribed homozygous variant of FTL (c.92A > G), which resulted in a Tyr31Cys substitution (FTL^Y31C). Analysis of the FTL structure predicted that the Y31C mutation will reduce the variant's stability. Expression of the FTL^Y31C variant resulted in significantly lower cellular ferritin levels compared with the expression of wild-type FTL (FTL^WT). Proteasomal inhibition significantly increased the initial levels of FTL^Y31C, but could not protect FTL^Y31C subunits from successive degradation. Further, variant subunits successfully incorporated into hetero-polymeric nanocages in the presence of sufficient levels of FTH1. However, FTL^Y31C subunits poorly assembled into nanocages when FTH1 subunit levels were low. These results indicate an increased susceptibility of unassembled monomeric FTL^Y31C subunits to proteasomal degradation. The decreased cellular assembly of FTL^Y31C-rich nanocages may explain the low serum ferritin levels in this patient and emphasize the importance of a broader diagnostic approach of hypoferritinemia without anemia, before IV iron supplementation.