What is the pathogenesis of acne?

For a long time, the mantra of acne pathogenesis debates has been that acne vulgaris lesions develop when (supposedly largely androgen-mediated) increased sebum production, ductal hypercornification, and propionibacteria come together with local inflammatory process in the unlucky affected individual. And yet, the exact sequence, precise interdependence, and choreography of pathogenic events in acne, especially the 'match that lights the fire' have remained surprisingly unclear, despite the venerable tradition of acne research over the past century. However, exciting recent progress in this--conceptually long somewhat stagnant, yet clinically, psychologically, and socioeconomically highly relevant--everyday battlefield of skin pathology encourages one to critically revisit conventional concepts of acne pathogenesis. Also, this provides a good opportunity for defining more sharply key open questions and intriguing acne characteristics whose underlying biological basis has far too long remained uninvestigated, and to emphasize promising new acne research avenues off-the-beaten-track--in the hope of promoting the corresponding development of innovative strategies for acne management.

What is the 'true' function of skin?

Conventional textbook wisdom portrays the skin as an organ that literally enwraps whatever each of us stands for as a more or less functional, individual member of the mammalian species, and has it that the skin primarily establishes, controls and transmits contacts with the external world. In addition, the skin has long been recognized to protect the organism from deleterious environmental impacts (physical, chemical,microbiological), and is well-known as crucial for the maintenance of temperature, electrolyte and fluid balance. Now, ever more studies are being published that show the skin to also operate as a huge and highly active biofactory for the synthesis,processing and/or metabolism of an astounding range of e.g. structural proteins, glycans, lipids and signaling molecules. Increasingly, it becomes appreciated that the skin, furthermore, is an integral component of the immune, nervous and endocrine systems, with numerous lines of cross-talk between these systems established intracutaneously (e.g. Ann NY Acad Sci Vol 885, 1999; Endocrine Rev 21:457-487, 2000; Physiol Rev 80:980-1020, 2001; Exp Dermatol 10: 349-367, 2001). All these emerging cutaneous functions beyond the classical image of the skin as a barrier and sensory organ are immediately relevant for many of the quandaries that clinical dermatology, dermatopathology, and dermatopharmacology are still struggling with to-date, and offer the practising dermatologist attractive new targets for therapeutic intervention. Yet, many of these skin functions are not even mentioned in dermatology textbooks and await systematic therapeutic targeting. Following a suggestion by Enno Christophers, the current 'Controversies' feature brings together an unusually diverse council of biologists and clinicians, who share their thought-provoking views with the readers and allow us to peek into the future of research in cutaneous biology, not the least by reminding us of the -- often ignored -- evolutionary and embryonal origins of our favorite organ. Hopefully, this unique discussion feature will foster an understanding of the 'true' skin functions that is both more comprehensive and more profound than conventional teaching on this topic, and will stimulate more than 'skin-deep' reflections on the full range of skin functions.

Mutations of the gene encoding the transmembrane transporter protein ABC-C6 cause pseudoxanthoma elasticum.

We recently published the precise chromosomal localization on chromosome 16p13.1 of the genetic defect underlying pseudoxanthoma elasticum (PXE), an inherited disorder characterized by progressive calcification of elastic fibers in skin, eye, and the cardiovascular system. Here we report the identification of mutations in the gene encoding the transmembrane transporter protein, ABC-C6 (also known as MRP-6), one of the four genes located in the region of linkage, as cause of the disease. Sequence analysis in four independent consanguineous families from Switzerland, Mexico, and South Africa and in one non-consanguineous family from the United States demonstrated several different mis-sense mutations to cosegregate with the disease phenotype. These findings are consistent with the conclusion that PXE is a recessive disorder that displays allelic heterogeneity, which may explain the considerable phenotypic variance characteristic of the disorder.

Acute and subchronic toxicity of sucralose.

The toxicity of sucralose has been evaluated in acute and subchronic toxicity studies. Acute oral toxicity studies in male and female mice and male rats documented no deaths or treatment-related signs at doses of 16g/kg for mice and 10g/kg for rats. Sucralose was administered to male and female rats for 4 and 8 weeks at dietary concentrations of 1.0, 2.5 or 5.0%. Achieved dose ranges (mg/kg/day) for the respective dietary levels were 737-1287, 1865-3218 and 2794-6406. There were no toxicologically significant effects observed at the 1.0% or 2.5% dietary levels. However, decreases in food consumption, body weight gain and selected organ weights and ratios as well as splenic and thymic histopathologic changes occurred in rats administered 5.0% for 4 or 8 weeks. A gavage study wherein doses of 0, 750, 1500 or 3000mg/kg/day were administered to male and female rats for 26 weeks investigated further the observations from the dietary study as well as general subchronic toxicity. The gavage study documented no sucralose-related toxicity. These results implicate the reduced palatability and digestibility of diets containing high concentrations of sucralose seen in the diet study as the cause for the decreased food consumption and other accompanying alterations. Dose selection for chronic toxicity studies in rats took into consideration the effect of high concentrations of sucralose on digestion and food consumption and the limitations that would be imposed on subsequent studies. In male and female dogs, no sucralose-related adverse effects were observed following the dietary administration of 0.3, 1.0 or 3.0% for 12 months achieving doses of approximately 90, 300 and 900mg/kg/day respectively. These studies establish that sucralose is non-toxic in rodents following acute oral administration. The rat no-observed-adverse-effect level ranged between 2.5 and 5.0% following subchronic dietary administration. A 3.0% dietary concentration equivalent to a dose of 900mg/kg/day produced no adverse effects in beagle dogs when fed for 12 months.

A 500-kb region on chromosome 16p13.1 contains the pseudoxanthoma elasticum locus: high-resolution mapping and genomic structure.

We have recently mapped the genetic defect underlying pseudoxanthoma elasticum (PXE), an inherited disorder characterized by progressive calcification of elastic fibers in skin, eye, and cardiovascular system, to chromosome 16p 13.1. Here we report further data on the fine-mapping and genomic structure of this locus. Haplotype analysis of informative PXE families narrowed the locus to an interval of less than 500 kb located between markers D16B9621 and D16S764. Three overlapping YAC clones were found to cover this region through YAC-STS content mapping. An overlapping BAC contig was then constructed to cover this interval and the surrounding region. About 80% of this chromosomal region has been fully sequenced using the BAC shotgun technique. Gene content and sequence analysis predicted four genes (MRP1, MRP6, PM5, and a novel transcript) and two pseudogenes (ARA and PKDI) within this interval. By screening a somatic cell hybrid panel we were able to precision-map the breakpoint of Cy185 and the starting point of a chromosomal duplication within 20 kb of BAC A962B4. The present data further refine the localization of PXE, provide additional physical cloning resources, and will aid in the eventual identification of the genetic defect causing PXE.

Hot spot mutations in keratin 2e suggest a correlation between genotype and phenotype in patients with ichthyosis bullosa of Siemens.

Ichthyosis bullosa of Siemens (IBS) is a rare disorder of cornification characterized by blister formation in the upper suprabasal layers of the epidermis. Molecular analysis of IBS has identified mutations in the keratin 2e (K2e) gene, which is located in the type II keratin gene cluster on chromosome 12q. We have studied two IBS families and have identified heterozygous point mutations in codon 493 of the K2e gene in both families. Whereas a non-conservative amino acid substitution at position 117 of the 2B region of K2e (E117K) was associated with a severe phenotype in family 1, family 2 showed mild clinical features as a result of a conservative substitution (E117D). These data suggest a phenotype-genotype correlation in these families.

How realistic is cutaneous gene therapy?

Recent progress with innovative, experimental gene therapy approaches in animals, and recent improvements in our understanding and manipulation of stem cells, gene expression and gene delivery systems, have raised plenty of hopes in essentially all branches of clinical medicine that hitherto untreatable or poorly manageable diseases will soon become amenable to treatment. Few other organ systems have received such enthusiastic reviews in recent years as to the chances and prospects of gene therapy as the skin, with its excellent accessibility and its pools of--seemingly--readily manipulated epithelial stem cells (cf. Cotsarelis et al., Exp Dermatol 1999: 8: 80-88). However, as in other sectors of clinical medicine, the actual implementation of general gene therapy strategies in clinical practice has been faced with a range of serious difficulties (cf. Smith, Lancet 1999: 354 (suppl 1): 1-4; Lattime & Gerson (eds.), Gene Therapy of Cancer, Academic Press, San Diego, 1999). Thus, it is critically important to carefully distinguish unfounded hype from justified hope in this embryonal area of dermatologic therapy, to discuss in detail what can be realistically expected from cutaneous gene therapy approaches in the next few years, and importantly, what kind of promises should not be made to our patients at this time.

Treatment of cutaneous sarcoidosis using phonophoresis.

Sarcoidosis is a multiple-system disorder of unknown origin characterized histologically by epithelioid granulomas with little or no necrosis. We describe a 32-year-old female patient with a history of systemic sarcoidosis and common variable immunodeficiency with recurrent, multiple, soft, erythematous and violaceous nodules on the back of her left hand. Her lesions responded to phonophoresis after unsuccessful treatment with topical and intralesional corticosteroids. However, nodules appeared on other parts of her body after phonophoresis was stopped, which suggests that phonophoresis had a localized rather than systemic effect.

Evidence that apoptosis and terminal differentiation of epidermal keratinocytes are distinct processes.

Although there are clear parallels between apoptosis and epidermal terminal differentiation it is unclear whether terminal differentiation of keratinocytes is a form of apoptosis. We found that apoptosis was rare in adherent cultures of normal keratinocytes, even when growth factors were removed. When keratinocytes were placed in suspension for 24-96 h the majority of cells were induced to undergo terminal differentiation, as assessed by involucrin expression and cornified envelope assembly, but few cells underwent apoptosis, as assessed by morphological examination, TUNEL labelling and by DNA laddering. Withdrawal of serum and growth factors stimulated apoptosis of suspended keratinocytes but led to some reduction in the number of cells that underwent terminal differentiation. At 96 h the majority of cells retained their nuclei in the presence or absence of serum and growth factors. In normal epidermis only occasional cells within the granular layer had apoptotic nuclei, determined by TUNEL labelling and light and electron microscopy. In affected epidermis of psoriasis, Darier's disease and pityriasis rubra pilaris, diseases characterized by perturbation of growth, differentiation or adhesion, light microscopy revealed no higher proportion of apoptotic nuclei than in normal epidermis. However, the majority of viable epidermal layers in diseased skin were positive by TUNEL labelling, suggesting that TUNEL is not always a specific marker of apoptosis in keratinocytes. We conclude that in vivo and in culture keratinocyte terminal differentiation and apoptosis are distinct cellular events, subject to different stimuli.

Induction of TR4 orphan receptor by retinoic acid in human HaCaT keratinocytes.

Human TR4 orphan receptor (TR4) can modulate the transcriptional activity of the reporter gene containing an AGGTCA direct repeat-hormone response element. Here we studied the potential role of TR4 in human HaCaT keratinocytes. Using a chloramphenicol acetyl-transferase reporter gene assay, it was shown that TR4 can suppress retinoic acid-induced transactivation by 47.3% in human HaCaT keratinocytes. Electrophoretic mobility shift assay indicated that this suppression may be due to TR4 binding with higher affinity to the retinoic acid response element than retinoid receptors. Western blot analysis further suggested that retinoic acid can increase the expression of TR4 protein in human HaCaT keratinocytes, indicating that TR4 acts as a negative feedback modulator for retinoic acid action. Interestingly, TR4 expression is increased in normal human keratinocytes when substituting a low calcium medium with a high calcium medium. Together, our data suggested, for the first time, that an orphan receptor, such as TR4, may play an important part in retinoid-mediated signaling pathways in human keratinocytes, providing a new insight into keratinocyte biology.

A novel mutation in the 1A domain of keratin 2e in ichthyosis bullosa of Siemens.

Ichthyosis bullosa of Siemens (IBS) is a rare autosomal dominant skin disorder with clinical features similar to epidermolytic hyperkeratosis (EHK). Both diseases have been linked to the type II keratin cluster on chromosome 12q. Hyperkeratosis and blister formation are relatively mild in IBS compared with EHK, and the lysis of keratinocytes is restricted to the upper spinous and granular layers of the epidermis of IBS patients, whereas in EHK lysis occurs in the lower spinous layer. Recently, mutations in the helix initiation and termination motifs of keratin 2e (K2e) have been described in IBS patients. The majority of the mutations reported to date lie in the 2B region. In this report, we have examined a large kindred in which the disease was originally diagnosed as EHK and mapped to the type II keratin cluster on chromosome 12q. Molecular analysis revealed a novel amino acid substitution at the beginning of the conserved 1A region of the rod domain (I4N) of K2e, resulting from a T to A transversion in codon 188.

Coexistence of incontinentia pigmenti and neonatal herpes simplex virus infection.

We present a female infant with classic clinical and histologic features of stage I incontinentia pigmenti with coexistent neonatal herpes simplex virus infection. The diagnosis of a heritable cutaneous condition does not exclude the possibility of a coexistent infection and, given the similar clinical presentation of neonatal vesicular eruptions, accurate diagnoses may require skin biopsy and culture.