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MEDNIK Syndrome: a New Entry in the Spectrum of Inborn Errors of Copper Metabolism

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MEDNIK Syndrome: a New Entry in the Spectrum of Inborn Errors of Copper Metabolism www.jpnim.com Open Access eISSN: 2281-0692 Journal of Pediatric and Neonatal Individualized Medicine 2020;9(2):e090202 doi: 10.7363/090202 Received: 2020 Apr 23; accepted: 2020 May 03; published online: 2020 May 14 Editorial MEDNIK syndrome: a new entry in the spectrum of inborn errors of copper metabolism Gavino Faa1,2, Clara Gerosa1, Massimo Castagnola3 1Institute of Pathology, Department of Medical Sciences and Public Health, AOU Cagliari, University of Cagliari, Cagliari, Italy 2Temple University, Philadelphia, PA, USA 3Laboratory of Proteomics, IRCSS Fondazione Santa Lucia, Rome, Italy “There is nothing Either Good or bad But thinking Makes it so” William Shakespeare, Hamlet, II, II Keywords MEDNIK syndrome, inborn errors, copper, metabolism. Corresponding author Clara Gerosa, Divisione di Anatomia Patologica, Dipartimento di Scienze Mediche e Sanità Pubblica, AOU Cagliari, University of Cagliari, Cagliari, Italy; email: [email protected]. How to cite Faa G, Gerosa C, Castagnola M. MEDNIK syndrome: a new entry in the spectrum of inborn errors of copper metabolism. J Pediatr Neonat Individual Med. 2020;9(2):e090202. doi: 10.7363/090202. 1/5 www.jpnim.com Open Access Journal of Pediatric and Neonatal Individualized Medicine • vol. 9 • n. 2 • 2020 Copper homeostasis, including intestinal absorp­ copper metabolism, including liver copper storage tion, blood transport, uptake, trafficking, and associated with intrahepatic cholestasis and low excretion, is regulated by multiple genes encoding copper serum levels [12]. The similarities between for specific copper transporters, which coordinate MEDNIK syndrome and Wilson disease were copper bioavailability [1]. Mutations in genes coding confirmed by the finding in the same subject of for copper pumps or copper chaperons are responsible low ceruloplasmin serum levels. On the other hand, for copper overload or deficiency, with relevant studies in mutant fibroblasts carrying the sameAP1S1 consequences on cell structure [2] and human health mutations evidenced aberrant intracellular trafficking [3]. The spectrum of genetic disorders of copper of ATP7A protein, revealing strong similarities metabolism includes multiple entities, characterized between MEDNIK syndrome and Menkes disease. by different clinical presentations, with liver and brain Thanks to the fundamental work of Martinelli and as target organs. Wilson disease and Menkes disease Dionisi­Vici, MEDNIK syndrome was included are the most widely studied copper­related genetic in the human defects of copper metabolism, with disorders in humans. Wilson disease is an autosomal clinical and biochemical signs of both Wilson and recessive disorder caused by a mutation in the ATP7B Menkes disease [14]. Regarding the molecular gene, encoding for a copper pump localized in the pathways involved in MEDNIK syndrome, AP1S1 Trans­Golgi Network (TGN) or at the biliary pole has been identified as the pivotal gene involved in of the hepatocyte, that is strongly expressed in the the correct functioning of the AP­1 complex, which normal liver [4]. Malfunction or dislocation of ATP7B is responsible for the crosstalk between the TGN and is at the basis of copper storage in the liver [5] and the other endosomes. in the brain, in which copper overload is associated The discovery of MEDNIK syndrome highlights with storage of other trace elements such as iron [6]. the role of AP complexes in copper metabolism. Menkes disease, also known as kinky hair disease, is Indeed, as mentioned above, AP1S1 encodes for an X­linked, often fatal, neurodegenerative disease the small σ1 subunit of the AP­1, a member of the caused by a mutation in the ATP7A gene, encoding for family of the heterotetrameric AP complexes. This another copper pump, ATP7A, whose malfunction is family accounts for five members, from AP­1 to AP­ responsible for impaired copper absorption in the gut 5. Each complex consists of two large subunits (γ, and decreased copper passage through the blood­brain α, δ, ε, ζ and β1­β5), a medium subunit (μ1­μ5), and barrier [7]. The Occipital Horn Syndrome is the milder a small subunit (σ1­σ5). Therefore, the structures of variant of Menkes syndrome. Moreover, a distal motor the five adaptor protein complex are AP­1: γ, β1, neuropathy caused by a novel missense mutation in μ1, σ1; AP­2: α, β2, μ2, σ2; AP­3: δ, β3, μ3, σ3; the ATP7A gene has been recently described [8]. AP­4: ε, β4, μ4, σ4; AP­5: ζ, β5, μ5, σ5. Moreover, In recent years, a new autosomal recessive several adaptor protein complexes display different mucocutaneous syndrome characterized by Mental isoforms, depending on the different organs and retardation, Enteropathy, Deafness, peripheral Neu­ tissue. For instance, two γ (γ1/γ2), and three σ (σ1A/ ropathy, Ichthyosis, and Keratodermia (MEDNIK) has σ1B/σ1C) isoforms are known for AP­1, two α (α1/ been described in several French­Canadian families. α2) isoforms for AP­2 and two β (β3A/β3B), two μ Previously defined as atypical erythrokeratodermia in (μ3A/μ3B) and two σ (σ3A/σ3B) isoforms for AP­3 1972 [9], and as erythrodermia variabilis­3 in 2005 [14]. Two appendage domains (sometimes called [10], in 2008 MEDNIK syndrome has been associated ears) are linked by covalent chains to μ, and β subunits with a splice mutation in the Adaptor Protein­1 S1 and the small σ subunit has a role in stabilizing the (AP1S1) gene, encoding for the small subunit σ1A whole structure. AP­1, AP­3, and AP­4 all localize of the adaptor protein (AP)­1 complex [11]. This at the TGN or endosomes to facilitate protein syndrome has been subsequently described in an sorting within the endosomal system. In contrast, Italian patient [12] and a Turkish child [13]. AP­2 localizes at the plasma membrane to facilitate In its first report, MEDNIK syndrome was endocytosis. While the scaffolding proteins that associated with a disarrangement of vesicular facilitate AP­3 and AP­4 vesicle formation remain trafficking between organelles, ending with major elusive, AP­1 and AP­2 complexes interact with changes in the development of skin and spinal clathrin and incorporate their cargos into clathrin­ cord, without any evidence of involvement of coated vesicles [15]. The sorting signals addressing copper transport. An elegant study carried out in these vesicles towards the proper target are typically an Italian patient affected by MEDNIK syndrome small peptide motifs encoded in the cytoplasmic tail revealed, for the first time, severe perturbations of of transmembrane proteins. Although non­canonical 2/5 Faa • Gerosa • Castagnola Journal of Pediatric and Neonatal Individualized Medicine • vol. 9 • n. 2 • 2020 www.jpnim.com Open Access signals exist, basolateral sorting signals frequently cytosolic domains of the proteins by the σ1 subunit conform to two types of sorting motifs: tyrosine­ of the clathrin adaptor AP­1. Their studies suggest based motifs that conform to the YxxØ consensus that in addition to failure to localize the TGN at sequence (in which Ø is a bulky hydrophobic amino low Cu2+ levels, the altered polarity of ATP7B acid F, I, L, M, or V), and di­leucine motifs often (and perhaps also ATP7A) connected to variations displaying [DE]xxxL[LI] sequences. Jains and of the small σ1 subunit of AP­1 in polarized cell colleagues have shown [16] that polarized sorting of types might underlie some of the copper metabolism the Cu2+ transporter ATP7B to the somatodendritic defects in MEDNIK syndrome. domain of rat hippocampal neurons is mediated Fig. 1 shows the possible molecular connections by recognition of dileucine­based signals in the between the AP1S1 gene and the intracellular Figure 1. ATP7B trafficking inside hepatocytes. On the right side, ATP7B in physiological conditions is mainly localized to the Trans-Golgi Network (TGN). When copper stores increase, ATP7B undergoes vesicular transport regulated by AP-1 and is transported to the biliary canaliculus (BC) membrane. In this location, ATP7B acts as a copper transporter, allowing excess copper excretion into bile. On the left side, the consequences of AP1S1 gene mutations are summarised. The absence of AP-1 causes aberrant localisation of ATB7B, leading to copper accumulation in the hepatocyte and reduced incorporation of copper ions in apuceruloplasmin, ending with low ceruloplasmin serum levels. Mednik syndrome: a new entry in the spectrum of inborn errors of copper metabolism 3/5 www.jpnim.com Open Access Journal of Pediatric and Neonatal Individualized Medicine • vol. 9 • n. 2 • 2020 trafficking of the copper pump ATP7B, playing a Table 1. Spectrum of inborn errors of copper metabolism. fundamental role in copper homeostasis. Protein Gene Disease The hypothesis that MEDNIK syndrome might Menkes disease represent the first example of diseases of copper Occipital Horn Copper metabolism associated with mutations in genes ATP7A (MC1, Syndrome transporting encoding for the subunits of AP complexes [14] MNK) ATPase 1 X-linked Distal has been recently confirmed by the report of Hereditary Motor three patients with a mutation in the AP1B1 gene, Neuropathy Copper encoding for the β1 subunit of the AP­1 complex ATP7B (PWD, transporting Wilson disease WC1, WND) [17]. In these patients, a MEDNIK­like syndrome ATPase 2 was observed, with low serum copper and Acetyl CoA SLC33A1 Huppke-Brendel ceruloplasmin levels, suggesting dysfunction of transporter (ACATN, AT1) syndrome copper pumps, but in the absence of liver disease Copper chaperone for due to copper overload. CCS CCS deficiency As all the cell machineries, copper pumps ATP7A superoxide dismutase and ATP7B need a correct vesicular transport, in σ1 subunit of the AP1S1 (AP19, MEDNIK order to be addressed towards their right subcellular AP-1 complex CLAPS1) syndrome compartment. MEDNIK syndrome evidences that a β1 subunit of the AP1B1 (ADTB1, MEDNIK-like mutation in the AP1S1 gene causes a perturbation AP-1 complex CLAPB2) syndrome of copper pump recruitment with subsequent heavy perturbation of copper metabolism. Given the role played by copper during development and, Declaration of interest in particular, in the development of the nervous system, missense variations of the AP1S1 gene The Authors declare that there is no conflict of interest.
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