Ichthyoses comprise a heterogeneous group of genodermatoses characterized by abnormal desquamation over the whole body, for which the genetic causes of several human forms remain unknown. We used a spontaneous dog model in the golden retriever breed, which is affected by a lamellar ichthyosis resembling human autosomal recessive congenital ichthyoses (ARCI), to carry out a genome-wide association study. We identified a homozygous insertion-deletion (indel) mutation in PNPLA1 that leads to a premature stop codon in all affected golden retriever dogs. We subsequently found one missense and one nonsense mutation in the catalytic domain of human PNPLA1 in six individuals with ARCI from two families. Further experiments highlighted the importance of PNPLA1 in the formation of the epidermal lipid barrier. This study identifies a new gene involved in human ichthyoses and provides insights into the localization and function of this yet uncharacterized member of the PNPLA protein family.
Dogs differ greatly in their morphological characteristics including various tail phenotypes. Congenitally short-tailed dogs are present in many breeds; however, the causative mutation located in the T-box transcription factor T gene (C189G) had only been described in the bobtailed Pembroke Welsh Corgis. We investigated here the presence of the T gene mutation in 23 other breeds (360 dogs, including 156 natural short tailed) in which natural bobtailed dogs exist. In the 17 breeds in which the C189G mutation was observed, there was a perfect correlation between this mutation and the short-tail phenotype. However, 6 breeds did not carry the known substitution or any other mutations in the T gene coding regions. No dogs were found to be homozygous for the C189G mutation, suggesting that the homozygous condition is lethal. In order to study the effect of the T gene mutation on litter size, we compared the number of puppies born from short-tailed parents to that born from long-tailed parents. In the Swedish Vallhund breed, we observed a 29% decrease in the litter size when both parents were short tailed. Given that the T gene mutation is not present in all breeds of short-tailed dog, there must be yet other genetic factors affecting tail phenotypes to be discovered.
A number of genodermatoses are characterized by distinct morphological markers which have been and are used for classification and diagnosis as well as for identifying causative gene mutations and pathogenetic pathways. Various types of animal models and organotypic cell cultures have been established to provide further insight into disease mechanisms and treatment. Selected examples are: (i) a spontaneous rat model for dominant epidermolysis bullosa revealing similar variability of anchoring fibril expression as in human patients; (ii) PNPLA1 as a new gene involved in autosomal recessive congenital ichthyosis pathology identified from a Golden Retriever breed spontaneously affected by lamellar ichthyosis; (iii) knockout mice for lipoxygenases expressing differential skin barrier defects and compensatory hyperkeratosis; (iv) a long-term skin-humanized mouse model for transglutaminase 1-deficient lamellar ichthyosis, obviously in some respects advantageous to organotypic cell cultures and successfully having been used for enzyme substitution therapy; (v) Persian cat with classical Ehlers-Danlos syndrome. Investigation of such monogenetic disease models can help to understand causal correlations between pathology and clinical manifestations and provide insights towards developing and evaluating novel causal therapies.
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