Harlequin ichthyosis (ichq): a juvenile lethal mouse mutation with ichthyosiform dermatitis

Area of Science:

• Genetics
• Developmental Biology
• Dermatology

Background:

• The harlequin ichthyosis (ichq) mouse mutation arose spontaneously in 1989.
• Affected mice exhibit thick skin with compact, orthokeratotic scales, leading to early mortality (9-12 days).
• The phenotype onset correlates with hair fiber emergence at 5 days of age.

Purpose of the Study:

• To characterize the harlequin ichthyosis (ichq) mouse mutation.
• To investigate the underlying skin pathology and genetic basis.
• To evaluate its potential as a model for human harlequin ichthyosis.

Main Methods:

• Phenotypic analysis of affected mice, including histology and ultrastructural examination.
• Immunohistochemical analysis of keratinocyte differentiation markers (keratin 6, loricrin, filaggrin).
• Skin grafting onto immunodeficient mice and genetic mapping to chromosome 19.

Main Results:

• Mutant mice displayed marked proliferation of hair follicle root sheaths and compact cornified hair fiber sheaths.
• Ultrastructural analysis revealed large mitochondria with lamellar inclusions; keratohyalin granules were smaller.
• Overexpression of mouse-specific keratin 6 was observed in the hyperplastic epidermis; filaggrin expression was prominent.

Conclusions:

• The harlequin ichthyosis (ichq) mouse mutation presents a distinct phenotype with significant alterations in skin barrier formation and hair follicle development.
• The mutation is inherited as a fully penetrant autosomal recessive trait, mapped to mouse chromosome 19.
• This ichq mouse model demonstrates high similarity to human type 2 harlequin ichthyosis, making it a valuable research tool.