Type I transglutaminase accumulation in the endoplasmic reticulum may be an underlying cause of autosomal recessive congenital ichthyosis

Exome Analysis Identifies a Novel Compound Heterozygous Alteration in TGM1 Gene Leading to Lamellar Ichthyosis in a Child From Saudi Arabia: Case Presentation

Background: Lamellar ichthyosis is an autosomal recessive type of rare skin disorders characterized with defective epidermis leading hyperkeratosis with brownish-gray scales over the body. These patients are born as collodion babies and may also exhibit additional features like erythema, ectropion, and eclabium. This disease is mainly caused by homozygous and compound heterozygous alterations in transglutaminase 1 encoding gene (TGM1), which is located on 14q12. Case presentation: This study reports the genetic analysis of a 4-year Saudi girl presenting lamellar ichthyosis. She was the first child of unrelated parents. The family had no previous history of the disease phenotype. She was born as a collodion baby without any prenatal complications. At the time of this study she had developed rough scaly skin on her legs, arms and trunk regions with thick palms and soles. Whole exome sequencing (WES) followed by Sanger sequence validation identified a novel compound heterozygous variant in TGM1 gene. The paternal variant was a missense transition (c.1141G>A; p.Ala381Thr) present at exon 7, while maternal variant (c.758-1G>C) was present at the intron4-exon5 boundary. To the best of our knowledge these variants had not been reported before in TGM1 gene. Conclusion: In isolated and inbred populations, homozygous variants are identified more frequently; however, our results suggest that compound heterozygous variants should also be considered especially when the marriages are not consanguineous.

ARCN1 Mutations Cause a Recognizable Craniofacial Syndrome Due to COPI-Mediated Transport Defects

Cellular homeostasis is maintained by the highly organized cooperation of intracellular trafficking systems, including COPI, COPII, and clathrin complexes. COPI is a coatomer protein complex responsible for intracellular protein transport between the endoplasmic reticulum and the Golgi apparatus. The importance of such intracellular transport mechanisms is underscored by the various disorders, including skeletal disorders such as cranio-lenticulo-sutural dysplasia and osteogenesis imperfect, caused by mutations in the COPII coatomer complex. In this article, we report a clinically recognizable craniofacial disorder characterized by facial dysmorphisms, severe micrognathia, rhizomelic shortening, microcephalic dwarfism, and mild developmental delay due to loss-of-function heterozygous mutations in ARCN1, which encodes the coatomer subunit delta of COPI. ARCN1 mutant cell lines were revealed to have endoplasmic reticulum stress, suggesting the involvement of ER stress response in the pathogenesis of this disorder. Given that ARCN1 deficiency causes defective type I collagen transport, reduction of collagen secretion represents the likely mechanism underlying the skeletal phenotype that characterizes this condition. Our findings demonstrate the importance of COPI-mediated transport in human development, including skeletogenesis and brain growth.

Unfolded protein response in keratinocytes: impact on normal and abnormal keratinization

The unfolded protein response (UPR) is a signaling pathway from the endoplasmic reticulum (ER) to the nucleus that protects cells from stress caused by misfolded or unfolded proteins. As such, ER stress is an ongoing challenge for all cells, given the central biologic importance of secretion as part of normal physiologic functions. Mild UPR is activated by mild ER stress, which occurs under normal conditions. Abnormal UPR is activated by severe ER stress, which occurs under pathological conditions. Abnormal UPR activation is associated with a number of diseases, including diabetes mellitus and Alzheimer's disease. Within skin tissues, keratinocytes in the epidermis are especially dependent upon a mild UPR for normal differentiation in the course of their differentiation into secretory cells in the uppermost granular layers. Association between abnormal UPR activation and hereditary keratoses, including Darier's disease, keratosis linearis with ichthyosis congenita and keratoderma syndrome, erythrokeratoderma variabilis, and ichthyosis follicularis with atrichia and photophobia syndrome, have been elucidated recently. This review describes the UPR in normal and abnormal keratinization and discusses the regulation of abnormal UPR activation by chemical chaperones as a potential treatment for one of the hereditary keratoses.

Increase in endoplasmic reticulum-associated tissue transglutaminase and enzymatic activation in a cellular model of Parkinson's disease

Parkinson's disease (PD) is characterized by accumulation of α-synuclein aggregates and degeneration of melanized, catecholaminergic neurons. The tissue transglutaminase (tTG) enzyme catalyzes molecular protein cross-linking. In PD, tTG levels are increased and cross-linking has been identified as an important factor in α-synuclein aggregation. In our quest to link tTGs distribution in the human brain to the hallmarks of PD pathology, we recently reported that catecholaminergic neurons in PD disease-affected brain areas display typical endoplasmic reticulum (ER) granules showing tTG immunoreactivity. In the present study, we set out to elucidate the nature of the interaction between tTG and the ER in PD pathogenesis, using retinoic-acid differentiated SH-SY5Y cells exposed to the PD-mimetic 1-methyl-4-phenylpyridinium (MPP(+)). Alike our observations in PD brain, MPP(+)-treated cells displayed typical TG-positive granules, that were also induced by other PD mimetics and by ER-stress inducing toxins. Additional immunocytochemical and biochemical investigation revealed that tTG is indeed associated to the ER, in particular at the cytoplasmic face of the ER. Upon MPP(+) exposure, additional recruitment of tTG toward the ER was found. In addition, we observed that MPP(+)-induced tTG activity results in transamidation of ER membrane proteins, like calnexin. Our data provide strong evidence for a, so far unrecognized, localization of tTG at the ER, at least in catecholaminergic neurons, and suggests that in PD activation of tTG may have a direct impact on ER function, in particular via post-translational modification of ER membrane proteins.