A significant number of inherited neurodegenerative metabolic diseases (NMDs) arise from altered lipid metabolism, including impaired degradation of sphingolipids and dysfunction in organelle-related machineries involved in lipid processing and trafficking. These lipid dysregulations profoundly impact cellular membranes, signaling pathways, and myelin integrity, contributing to the complex and multisystemic clinical phenotypes characteristic of NMD, which often complicate diagnosis and delay treatment initiation. Here, we present a high-throughput, multiplex LC-MS/MS method for the analysis of an extended panel of NMD biomarkers in plasma and dried blood spots. One-step sample extraction and targeted LC-MS/MS acquisitions in positive and negative ionization allowed the simultaneous measurement of 13 diagnostic biomarkers associated with GM1 and GM2 gangliosidosis, Fabry, Gaucher, and Krabbe diseases, acid sphingomyelinase deficiency, Niemann-Pick disease type C, X-linked adrenoleukodystrophy, peroxisomal biogenesis disorders (Zellweger syndrome), metachromatic leukodystrophy, and mental retardation, enteropathy, deafness, neuropathy, ichthyosis, keratoderma (MEDNIK)/MEDNIK-like syndromes, a disorder of cellular trafficking. The method was analytically and clinically validated, confirming the diagnosis of all targeted NMDs in samples from 89 patients. Additionally, the method allowed the differentiation of X-linked adrenoleukodystrophy from peroxisomal biogenesis disorder and revealed the elevation of C18- and C16-sulfatides in Krabbe disease and MEDNIK syndrome, respectively. This multiplex assay enhances diagnostic efficiency and expands the discovery of novel biomarkers, enabling the quantification of diagnostic markers for a wide range of NMDs. The method is suitable for diagnosis of NMD, as a first- or second-tier test in neonatal screening, as confirmatory testing of variant of unknown significance in genetic panels and for longitudinal monitoring in treatable diseases.

IDEDNIK syndrome (formerly MEDNIK syndrome, OMIM #609313) is a rare autosomal recessive neurocutaneous disorder characterized by dysregulated copper metabolism and multisystem involvement. The primary causative gene, AP1S1, encodes the σ1A subunit of the adaptor protein complex AP-1, while mutations in AP1B1, encoding the β1 subunit, can cause a similar phenotype. Pathogenic mutations impair intracellular vesicle trafficking, disrupting the precise sorting and transport of multiple proteins, including the copper-transporting ATPases ATP7A and ATP7B. This results in defective copper homeostasis and a clinical phenotype overlapping features of Menkes and Wilson's diseases. Hallmark manifestations include intellectual disability, enteropathy, deafness, peripheral neuropathy, ichthyosis, and keratoderma. Laboratory findings often reveal reduced serum copper and ceruloplasmin levels, with some patients exhibiting elevated hepatic or urinary copper. Cranial MRI typically demonstrates cerebral atrophy. No curative therapy is currently available; management is multidisciplinary, focusing on symptomatic relief and complication prevention. Oral zinc acetate has been reported to improve certain clinical features and biochemical parameters. This review provides a comprehensive update on the genetics, pathogenesis, clinical spectrum, diagnosis, management, and future directions for this debilitating disease.

MEDNIK syndrome (Mental Retardation, Enteropathy, Deafness, Neuropathy, Ichthyosis and Keratodermia) is a severe hyper-rare condition resulting from the biallelic variants in the AP1S1 gene, implicated in intracellular trafficking and copper homeostasis. Only 18 affected individuals (seven AP1S1 pathogenic variants overall) have been reported to date, with a high early lethality due to life-threatening congenital enteropathy. Seven patients have been empirically treated with zinc. Due to the paucity of literature data, little is known about the clinical course of individuals affected by MEDNIK syndrome, and the possible early association with epilepsy needs to be investigated. We present the first case of Italian origin affected by MEDNIK syndrome carrying a new homozygous AP1S1 stop variant, presenting with congenital severe enteropathy and feeding-related seizures, thus representing an early, singular manifestation of the disease. We describe her clinical course and the zinc acetate therapeutic experience. We also reviewed the literature focusing on clinical manifestations (especially neurological), brain neuroimaging and the symptom evolution of patients with AP1S1-related MEDNIK syndrome and discuss possible future therapeutic attempts.

Background: MEDNIK syndrome is a rare copper metabolism disorder caused by AP1S1 variants. Herein, we report the clinical and genetic characteristics of MEDNIK syndrome in two siblings. Methods: The clinical treatment process for MEDNIK syndrome and over 4 years of follow-up data were analysed in two siblings. Microscopic observations of the patients' hair were conducted. Gene sequencing, three-dimensional structural reconstruction of protein sequences, and in vitro mRNA splicing experiments were performed. Results: The proband and his sister exhibited developmental delays, seizures, yellow hair, sparse teeth and a high forehead. Furthermore, the sister initially presented with intractable diarrhoea and severe pneumonia. Both siblings showed varying degrees of developmental delays during follow-up, and the proband also showed symptoms of attention deficit hyperactivity disorder. The microscopic hair examination revealed a deficiency in intermediate pigment, a pale colour and an intermittent or absent medulla. Genetic sequencing revealed a homozygous AP1S1 mutation at the splicing site (NM_001283.3): c.430-1G>A. The in vitro mRNA splicing experiments confirmed a single base-pair deletion in the fifth exon of the mRNA sequence of the mutated plasmid, resulting in a frameshift mutation (p.Glu144ArgfsTer83). The mutation was inherited from both parents and classified as pathogenic according to the American College of Medical Genetics and Genomics guidelines, based on clinical features and family analysis. Conclusion: Both children with MEDNIK syndrome exhibited heterogeneous clinical phenotypes. Sparse teeth may be a previously unnoticed feature of MEDNIK syndrome. The pathogenic c.430-1G>A homozygous variant enriches the mutation spectrum of AP1S1. This mutation causes a frameshift mutation in the protein, altering the protein structure and affecting protein function. IDEDNIK syndrome is characterized by enteropathy, poor weight gain, growth deficiency, skin manifestations (ichthyosis, erythroderma, and keratoderma), sparse hair, global developmental delay, mild-to-severe intellectual disability, and deafness. Additional manifestations can include liver disease, recurrent infections, and hematologic and ocular manifestations (photophobia, corneal scarring, and keratitis). Reduced serum ceruloplasmin and total copper levels are common. Some individuals have findings on brain MRI (cerebral atrophy, basal ganglia abnormalities, and thin corpus callosum). Death prior to age two years occurs in some individuals due to severe enteropathy or sepsis; in others survival into adulthood is reported. The diagnosis of IDEDNIK syndrome is established in a proband by identification of biallelic pathogenic variants in AP1B1 or AP1S1 by molecular genetic testing. Targeted therapy: Treatment with oral zinc acetate therapy to reduce liver copper overload has been reported to improve behavioral disturbances, skin manifestations, and cognitive function in some individuals. Zinc sulfate may be an alternative, less expensive treatment option. Experience is limited with this targeted therapy. Supportive care: Dietary modification and potential parenteral supplementation for enteropathy; feeding therapy; gastrostomy tube placement as needed; treatment options for skin manifestations include low-dose oral acitretin, skin emollients and topical lactic acid, frequent emollient application and short courses of topical cortical steroids or pimecrolimus ointment, and 50% urea ointments; developmental and educational support; hearing aids as needed for sensorineural hearing loss; community hearing services; standard treatment of seizures and peripheral neuropathy by an experienced neurologist; supportive treatment as needed for liver disease; standard treatment for recurrent infections; supportive treatment as needed for hematologic manifestations, and occasionally transfusion may be necessary; standard treatment of cataracts and other ocular manifestations per ophthalmologist; treatment of cryptorchidism per urologist; treatment of hypothyroidism and growth hormone deficiency per perinatologist; social work and family support. Surveillance: At each visit, assess growth parameters, nutritional status, safety of oral intake, diarrhea, skin and hair manifestations, developmental progress and educational needs, mobility and self-help needs, seizures and peripheral neuropathy, behavioral issues, liver function tests, complete blood count, evidence of aspiration and respiratory infections, and family needs. Audiology evaluation as recommended by audiologist; ophthalmology evaluation for keratitis, cataract, and accommodative esotropia as recommended by ophthalmologist; assess thyroid function and for growth hormone deficiency as recommended by endocrinologist. Evaluation of relatives at risk: Clarify the genetic status of apparently asymptomatic older and younger at-risk sibs in order to identify as early as possible those who would benefit from prompt initiation of zinc acetate treatment. IDEDNIK syndrome is inherited in an autosomal recessive manner. If both parents are known to be heterozygous for an AP1B1 or AP1S1 pathogenic variant, each sib of an affected individual has at conception a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Once the AP1B1 or AP1S1 pathogenic variants have been identified in an affected family member, carrier testing for at-risk relatives and prenatal/preimplantation genetic testing are possible.

MEDNIK syndrome is a rare autosomal recessive disease characterized by mental retardation, enteropathy, deafness, peripheral neuropathy, ichthyosis, and keratoderma, and caused by variants in the adaptor-related protein complex 1 subunit sigma 1 (AP1S1) gene. This gene encodes the σ1A protein, which is a subunit of the adaptor protein complex 1 (AP-1), a key component of the intracellular protein trafficking machinery. Previous work identified three AP1S1 nonsense, frameshift and splice-site variants in MEDNIK patients predicted to encode truncated σ1A proteins, with consequent AP-1 dysfunction. However, two AP1S1 missense variants (c.269 T > C and c.346G > A) were recently reported in patients who presented with severe enteropathy but no additional symptoms of MEDNIK. This condition was described as a novel non-syndromic form of congenital diarrhea caused specifically by the AP1S1 missense variants. In this study, we report two patients with the same c.269 T > C variant, who, contrary to the previous cases, presented as complete MEDNIK syndrome. These data substantially revise the presentation of disorders associated with AP1S1 gene variants and indicate that all the identified pathogenic AP1S1 variants result in MEDNIK syndrome. We also provide a series of functional analyses that elucidate the impact of the c.269 T > C variant on σ1A function, contributing to a better understanding of the molecular pathogenesis of MEDNIK syndrome. KEY MESSAGES: A missense AP1S1 c.269 T > C (σ1A L90P) variant causes full MEDNIK syndrome. The σ1A L90P variant is largely unable to assemble into the AP-1 complex. The σ1A L90P variant fails to bind [DE]XXXL[LI] sorting motifs. The σ1A L90P variant results in loss-of-function of the protein.