Complex of human Melanotransferrin and SC57.32 Fab fragment reveals novel interdomain arrangement with ferric N-lobe and open C-lobe.

Melanotransferrin (MTf) is an iron-binding member of the transferrin superfamily that can be membrane-anchored or secreted in serum. On cells, it can mediate transferrin-independent iron uptake and promote proliferation. In serum, it is a transcytotic iron transporter across the blood-brain barrier. MTf has been exploited as a drug delivery carrier to the brain and as an antibody-drug conjugate (ADC) target due to its oncogenic role in melanoma and its elevated expression in triple-negative breast cancer (TNBC). For treatment of TNBC, an MTf-targeting ADC completed a phase I clinical trial (NCT03316794). The structure of its murine, unconjugated Fab fragment (SC57.32) is revealed here in complex with MTf. The MTf N-lobe is in an active and iron-bound, closed conformation while the C-lobe is in an open conformation incompatible with iron binding. This combination of active and inactive domains displays a novel inter-domain arrangement in which the C2 subdomain angles away from the N-lobe. The C2 subdomain also contains the SC57.32 glyco-epitope, which comprises ten protein residues and two N-acetylglucosamines. Our report reveals novel features of MTf and provides a point of reference for MTf-targeting, structure-guided drug design.

Structure of human DPEP3 in complex with the SC-003 antibody Fab fragment reveals basis for lack of dipeptidase activity.

Dipeptidase 3 (DPEP3) is one of three glycosylphosphatidylinositol-anchored metallopeptidases potentially involved in the hydrolytic metabolism of dipeptides. While its exact biological function is not clear, DPEP3 expression is normally limited to testis, but can be elevated in ovarian cancer. Antibody drug conjugates targeting DPEP3 have shown efficacy in preclinical models with a pyrrolobenzodiazepine conjugate, SC-003, dosed in a phase I clinical trial (NCT02539719). Here we reveal the novel atomic structure of DPEP3 alone and in complex with the SC-003 Fab fragment at 1.8 and 2.8 Å, respectively. The structure of DPEP3/SC-003 Fab complex reveals an eighteen-residue epitope across the DPEP3 dimerization interface distinct from the enzymatic active site. DPEP1 and DPEP3 extracellular domains share a conserved, dimeric TIM (ß/α)8-barrel fold, consistent with 49% sequence identity. However, DPEP3 diverges from DPEP1 and DPEP2 in key positions of its active site: a histidine to tyrosine variation at position 269 reduces affinity for the ß zinc and may cause substrate steric hindrance, whereas an aspartate to asparagine change at position 359 abolishes activation of the nucleophilic water/hydroxide, resulting in no in vitro activity against a variety of dipeptides and biological substrates (imipenem, leukotriene D4 and cystinyl-bis-glycine). Hence DPEP3, unlike DPEP1 and DPEP2, may require an activating co-factor in vivo or may remain an inactive, degenerate enzyme. This report sheds light on the structural discriminants between active and inactive membrane dipeptidases and provides a benchmark to characterize current and future DPEP3-targeted therapeutic approaches.

Atypical central retinal artery occlusion: an uncommon cause of retinopathy and visual loss in dermatomyositis.

Several ophthalmic manifestations in patients with dermatomyositis have been described but involvement of the retina is rare. A 39-year-old man diagnosed with dermatomysitis had developed sudden painless visual loss in the left eye. He was non-diabetic and non-hypertensive. He was on treatment with systemic prednisolone (50 mg/day) and methotrexate (7.5 mg/day) for dermatomyositis. His visual acuity was finger counting at 2 feet in the left eye and 6/6 in the right eye. Left fundus showed slight blurring of the optic disc margins, large peripapillary flame-shaped haemorrhages and extensive area of opacification suggestive of central retinal artery occlusion (CRAO) with sparing of the area supplied by the cilioretinal artery. The exact pathomechanism of CRAO in dermatomyositis remains poorly elucidated. The infarctive events following endothelial damage and platelet thrombi occurring in arterioles and capillaries of retina, optic disc and choriocapillaries causing ischemia/infarction have been suggested as possible reasons. However, central artery occlusion from high intraocular pressure from marked (heliotrope) orbital edema appears more plausible in our case. The degree of retinal damage is variable when CRAO lasts between 105 and 240 min and irreversible profound visual loss may occur from its occlusion lasting for more than 240 min. A prompt ophthalmological opinion/management is recommended to limit its progression.

Keratitis-Ichthyosis-Deafness syndrome: A rare congenital disorder.

Keratitis-Icthyosis-Deafness syndrome is a rare congenital disorder characterized by keratitis, ichthyosis, and deafness. We report a 13 year old female child who presented with diffuse alopecia of the scalp and body. There was erythrokeratoderma of face and discrete hyperkeratotic hyperpigmented papulo plaque lesions on the body. Patient also had reticulate hyperkeratosis of palms and soles. There was history of recurrent episodes of folliculitis over the scalp and body. There was no evidence of any malignancy. Eye involvement in the form of bilateral vascularising keratitis was present. There was bilateral mixed hearing loss.