Hereditary hyperferritinemia-cataract syndrome (HHCS) is an autosomal and dominant disease caused by heterogeneous mutations in the
iron responsive
element (IRE) of the 5' untranslated flanking region of
ferritin L-chain
mRNA, which reduce the binding to the trans
iron regulatory proteins and make L-chain synthesis constitutively upregulated. In the several families identified so far, the serum and tissue
L-ferritin levels are fivefold to 20-fold higher than in nonaffected control subjects,
iron metabolism is apparently normal, and the only relevant clinical symptom is early onset, bilateral
cataract. Some pathogenetic aspects of HHCS remain obscure, with particular reference to the isoferritins produced by HHCS cells, as well as the mechanism of
cataract formation. We analyzed lymphoblastoid cell lines obtained from two nonaffected control subjects and from HHCS patients carrying the substitution A40G (Paris-1), G41C (Verona-1), and the deletion of the residues 10-38 (Verona-2) in the IRE structure.
Enzyme-linked
immunosorbent assays specific for the H- and L-type
ferritins showed that
L-ferritin levels were up to 20-fold higher in HHCS than in control cells and were not affected by
iron supplementation or chelation. Sequential immunoprecipitation experiments of metabolically-labeled cells with specific
antibodies indicated that in HHCS cells about half of the L-chain was assembled in L-chain homopolymers, which did not incorporate
iron, and the other half was assembled in isoferritins with a high proportion of L-chain. In control cells, all
ferritin was assembled in functional heteropolymers with equivalent proportion of H- and L-chains. Cellular and
ferritin iron uptake was slightly higher in HHCS than control cells. In addition, we analyzed the lens recovered from
cataract surgery of a HHCS patient. We found it to contain about 10-fold more
L-ferritin than control lens. The
ferritin was fully soluble with a low
iron content. It was purified and partially characterized. Our data indicate that: (1) in HHCS cells a large proportion of
L-ferritin accumulates as nonfunctional L-chain 24 homopolymers; (2) the concomitant fivefold to 10-fold expansion of
ferritin heteropolymers, with a shift to L-chain-rich isoferritins, does not have major effects on cellular
iron metabolism; (3) L-chain accumulation occurs also in the lens, where it may induce
cataract formation by altering the delicate equilibrium between other water-soluble
proteins (ie,
crystallins) and/or the
antioxidant properties.