|Dr. John DiGiovanna|
Discovering the precise genetic cause of one form of ichthyosis tells us not only a great deal about the affected individual, his or her family, and other individuals with the same disorder, but it also teaches us about normal epidermal biology. The term ichthyosis comes from the Greek ichthys, meaning fish, and refers to the clinical appearance of scaly skin. Ichthyosis can be present at birth or develop later in life, be limited to the skin, or occur in association with abnormalities of other organ systems. Cutaneous (skin) manifestations span a broad spectrum of severity. For many ichthyosis patients, diagnosis can be uncertain. Without a specific diagnosis, genetic counseling and predictions based on family history and pedigree can be unreliable. Accurate genetic counseling is important. Each child of a person with an autosomal dominant disorder has a 50% risk of inheriting the disorder. However, for an individual affected with an autosomal recessive ichthyosis, unless he or she marries a close relative, the risk of producing a similarly affected child is very low. Mutation identification can permit not only a precise clinical diagnosis, but also reliable genetic counseling and prenatal diagnosis. Once the mutation is identified, testing of the family members can provide accurate information about the risk of transmission.
Discovering the precise genetic cause of one form of ichthyosis tells us not only a great deal about the affected individual, his or her family, and other individuals with the same disorder, but it also teaches us about normal epidermal biology. Furthermore, it helps us to understand the entire spectrum of ichthyosis - not unlike solving a puzzle where fitting one piece helps to clarify the role of all of the other pieces, which have not yet found their place. To date, while a series of discoveries have identified the molecular basis for several ichthyosiform dermatoses, and these have enhanced diagnosis and genetic counseling, little progress has been realized in treatment and ultimate hope for a "cure."
Epidermolytic ichthyosis (EI) (formerly called epidermolytic hyperkeratosis (EHK) (bullous congenital ichthyosiform erythroderma; BCIE) is characterized by hyperkeratosis, often in association with peeling and blistering. While at least six clinical phenotypes (the outward expression of the disease) have been described, all share the same histopathology of hyperkeratosis with epidermal vacuolar degeneration.1 Most cases have been found to be caused by mutations in KRT1 or KRT10, the genes for the differentiation specific keratins 1 and 10. Mutations in either the KRT1 or KRT10 gene lead to keratinocyte (skin cell) fragility, and the clinical result is hyperkeratosis, or peeling, and easy blistering in the affected layers. Identification of the specific mutation can confirm the diagnosis and enables prenatal and/or preimplantation diagnosis.
Ichthyosis Bullosa of Siemens (IBS) is similar in clinical appearance to EHK, but with a more superficial peeling. As with EI, the epidermis is fragile, but the fragility is more superficial and confined to the granular layer. IBS is caused by mutations in KRT2e, the gene encoding keratin 2e, a differentiation specific keratin, which is expressed in the more superficial, granular layer of the epidermis. Therefore, the pathophysiology of these two disorders is weakness in the internal structure of the skin cells with subsequent epidermal fragility, and it differs by the location of the keratin whose function is abnormal.
Erythrokeratoderma Variabilis (EKV) is characterized by both hyperkeratosis (generalized or localized) and distinctive, sharply demarcated, migratory, red patches. The patches move over short periods of time (10 to 20 minutes) and may be precipitated by trauma or change in temperature. For patients with classic features, the diagnosis is clinically apparent; in others it can be elusive. The discovery of mutations in either GJB3 or GBJ4, the genes encoding connexins 31 and 30.3, in patients with EKV enables a definitive diagnosis and implicates defective communication between the epidermis and cutaneous vasculature (the system of blood vessels that supply the skin) as the cause.1 Structural proteins group to form channels that dock with a neighboring channel in an adjacent cell membrane. The resulting channels allow direct cell-to-cell communication, the transfer of physiologic signals, ions, and small nutrients, and coordination of cellular responses to internal and external stimuli. In addition, the identification of different mutations in the GJB3 gene in other patients with deafness (with and without skin disease) is an example of how different mutations in one gene can cause overlapping clinical syndromes with abnormalities in one or more organ systems. While these discoveries enhance diagnosis and a general understanding of the pathophysiology, they also highlight our poor understanding of the complexities of cell-to-cell communication and its mediators.
Keratitis-Ichthyosis-Deafness (KID) syndrome is characterized by keratitis (inflammation of the cornea), ichthyosiform skin changes, and neurosensory deafness. There are usually discrete, fixed erythematous (reddened) plaques, and there may be generalized hyperkeratosis. Hyperkeratosis of the hair follicles can result in scarring alopecia (baldness), and there may be an increased susceptibility to infection. The discovery that KID syndrome is caused by mutations in GJB2, encoding connexin 26, allows for molecular diagnosis and provides a framework for understanding how this group of clinical findings can be caused by disordered cell-to-cell communication in the skin, eyes, and ears.1
The collodion baby is born ensheathed in a shiny translucent membrane. This phenomenon can have a spectrum of severity at its presentation and a spectrum of clinical outcomes as the baby develops. These range from the mild "self healing" phenotype to the more severe types including congenital ichthyosiform erythroderma (CIE), lamellar ichthyosis (LI), and Netherton syndrome (NS), with or without the involvement of other organ systems. Some will develop a distinctive clinical phenotype (physical presentation of the disease) of severe LI or CIE. However, there are a spectrum of phenotypes with variable erythema (red skin) and scale that do not fit neatly into these categories. While several genes have been discovered to cause LI and CIE, thereby enabling a precise diagnosis for a few patients, an understanding of the genotype-phenotype correlation (the genetic make-up of the person versus the outward expression of the disease) for this spectrum of disorders has had limited progress. The recent suggestion that specific mutations in TGM1 may correlate with the mild phenotype suggests that some useful predictions may yet be realized.
Lamellar ichthyosis typically presents as a collodion baby. During the newborn period, the skin may be red, but over time it develops large, plate-like scales that appear to be arranged in a mosaic pattern. Tautness can lead to ectropion (flipping out of the eyelids), eclabium (turning out of the lips), and alopecia (baldness). Patients with this phenotype of severe LI often have little to no erythroderma (reddened skin) in adulthood. Mutations in TGM1, the gene encoding transglutaminase-1, were first found to cause LI, and probably account for most patients with this severe, classic phenotype. One study showed that the transfer of the TGM1 gene to human LI epidermis temporarily corrected the epidermal deficit, suggesting the potential for therapeutic gene delivery to human skin.1 However, the engineered keratinocytes did not retain the gene. More recently, mutations in adenosine triphosphate-binding cassette A12 (ABCA12) have been found in a few families with LI from North Africa.2 ABCA12 is a member of a superfamily of proteins that translocate substrates across membranes, and may be important in cellular lipid trafficking in keratinocytes.
Congenital ichthyosiform erythroderma (nonbullous congenital ichthyosiform erythroderma; CIE) usually presents as a collodion newborn, similar to LI. Infants with CIE have skin that remains red, usually with a fine, white scale, and there is a spectrum of involvement with ectropion, eclabium, and alopecia. In a few families with CIE, mutations have been found in one of two lipoxygenase genes (ALOXE3 and ALOX12B).2 These lipoxygenase enzymes catalyze the oxidation of fatty acids. A few patients with CIE are reported to have mutations in TGM1.
Netherton syndrome (NS) is characterized by ichthyosis, a hair shaft abnormality (trichorrhexis invaginata), and atopy (allergies or hypersensitivities). The typical cutaneous manifestation is ichthyosis linearis circumflexa with red, wavy, scaling plaques marked by characteristic, migratory doubled-edged scale at the margins. Newborns may present with generalized erythroderma or a collodion phenotype, and some may not survive infancy. Atopy may manifest as dermatitis or asthma with marked elevation in IgE. Some patient may have aminoaciduria (excess amino acids in the urine), mild developmental delay, and impaired cellular immunity. NS is caused by mutations in SPINK5, a gene encoding LEKT1, a serine protease inhibitor expressed in epithelial and lymphoid tissues.1 Before this was known, this diagnosis was often difficult to establish because of indistinct cutaneous findings. In addition to focusing on pathophysiologic abnormalities, this discovery allows accurate molecular diagnosis and permits prenatal and carrier testing.
Ichthyosis can occur with progressive neurologic disease. In Sjögren-Larsson syndrome, pruritic (itchy) ichthyosis, in association with the neurologic involvement (spasticity), should prompt an examination for glistening white dots in the retina and testing for fatty aldehyde dehydrogenase activity or for mutations in FALDH, the gene encoding that enzyme. Ichthyosis in association with progressive neurologic degeneration, as seen in Refsum disease, a disorder of phytanic acid catabolism, can be identified by measuring phytanic acid levels or identifying mutations in PAHX or PEX7 genes. The constellation of Photosensitivity, Ichthyosis, Brittle hair, Intellectual impairment, Delayed development, and Short stature (PIBIDS) is one of the clinical phenotypes of trichothiodystrophy, and should prompt the light microscopic examination of hair with polarizing lenses to demonstrate the typical tiger-tail banding. Amino acid analysis of hair can confirm low sulfur content and mutations may be found in the XPD or XPB genes, helicase components of the transcription factor TFIIH.1
These stepwise advances in molecular diagnosis are clarifying the relationship between clinical phenotypes and helping to solve the overall ichthyosis puzzle. I was recently consulted by consanguineous (related by blood) parents concerned about their child, who was born with a collodion membrane and, at 2 months old, has the clinical appearance of typical LI. Previously, cosanguineous cousins in their family also had a collodion baby who had a stormy course and did not survive. That baby had the clinical findings of NS and molecular diagnosis confirmed a paired SPINK5 mutation. Could there be two different genetic diseases with collodion presentation appearing within this one family? Or, do these children have the same genetic disorder with different clinical presentations? A few years ago, we would have probably guessed that the odds favored a single rare skin disorder causing a collodion presentation within one family. Therefore, we would have interpreted this scenario as representing variable expressivity - the genetic characteristic where the clinical phenotype of a disease varies because of other influences such as modifying genes or environmental factors. Our prognosis for the health of this baby and for future pregnancies would be problematic, because involvement in one baby appears to be confined to the skin, while the other baby died of multisystem disease. However, in this case, genetic testing found the child with clinical LI had a paired mutation in TGM1, providing molecular confirmation of the clinical diagnosis of LI rather than NS. Further analysis showed that the parents did not carry the SPINK5 mutation identified in their cousins who had produced a child with NS. Therefore, this child is not at risk for his cousin's poor outcome, and these parents are not at risk to transmit NS. This confirms segregation of two distinct ichthyoses with a collodion presentation within this one cosanguineous family, and clearly helps to define the clinical spectrum of both disorders. We do not have to infer an incorrect relationship between the clinical phenotypes of LI and NS.
These advances are enhancing our understanding of pathophysiology. The stratum corneum has been called our "outer skin" and the "beauty layer." It is our direct interface with the outside world. How does the epidermis form this tough, resilient, highly functional stratum corneum? These discoveries of the defects underlying the ichthyoses are helping to understand: (1) the specific processes involved in the formation of the stratum corneum, and (2) how the failure of each of these processes can lead to ichthyosis.
The process by which keratinocytes form the stratum corneum is complex. A useful model considers corneocytes as protein-rich "bricks" surrounded by a hydrophobic, lipid- enriched, intercellular "mortar"-like matrix. The importance of lipid formation and metabolism (mortar) was known early on, when steroid sulfatase, controlling the hydrolysis of cholesterol sulfate, was found deficient in X-linked ichthyosis. More recently, additional lipid (mortar) abnormalities have been discovered as FALDH in Sjögren-Larsson, lipoxygenases in CIE, and ABCA12 in LI. The importance of the structural integrity of the bricks was first highlighted by finding mutations in keratins leading to the cytoskeleton fragility that causes EHK. In addition, TGM1 as a cause of LI highlights the importance of proper formation of the cornified envelope, a laboratory analog of the "brick." Discovery of connexin mutations in EKV and KID syndrome tells us that failure of the intercellular communication can cause ichthyosis. We can also see ichthyosis with failure of transcription (PIBIDS), peroxisomal enzymes (Refsum disease) and a serine protease inhibitor (NS). Each of these discoveries is defining a piece of the ichthyosis puzzle.
So many discoveries, so little progress. Discovery of the mutations underlying a variety of Ichthyosiform dermatoses has revolutionized diagnosis, enabled accurate and reliable genetic counseling and carrier testing, and created opportunity for both prenatal and preimplantation genetic diagnosis. Insights into pathophsyiology have helped to clarify the relationships between the different ichthyosiform dermatoses and establish the foundation for novel approaches to treatment based on mechanisms or genetic engineering. While these exciting discoveries have led to major advances in diagnosis and understanding, they have not yet led to significant advances in treatment or progress towards the ultimate goal of "cure" for our patients. We are beginning to identify the pieces of the ichthyosis puzzle, and some of the pieces are beginning to fit together. We still have a long way to go. So many discoveries, and yet, so little progress.Resources
Genetic testing resources for clinicians can be found at www.genetest.org. For clinical information on genetic disorders for health professional, visit the Online Mendelian Inheritance in Man Web site at www.ncbi.nlm.nih.gov/omim/.
1 DiGiovanna JJ. Ichthyosiform dermatoses. In: Freedberg IM, Eisen AZ, Wolff K, Austen KF, Goldsmith LA, Katz SI, editors. Fitzpatrick?s Dermatology in General Medicine. 6th ed. New York: McGraw Hill; 2003. p. 481-505.
2 National Center for Biotechnology Information. Online Mendelian Inheritance in Man Web site. Available at www.ncbi.nlm.nih.gov. Accessed February 27, 2004.
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