Phacomatosis pigmentokeratotica: a patient with the rare melanocytic-epidermal twin nevus syndrome

Phacomatosis spilosebacea: A new name for a distinctive binary genodermatosis

Phacomatosis pigmentokeratotica (PPK) is defined by the association of papular nevus spilus arranged in a flag-like pattern and sebaceous nevus following Blaschko's lines. A systematic search of the worldwide literature retrieved 95 well-established PPK cases. An additional 30 cases were excluded for a number of reasons. Based on this study, we propose to rename PPK phacomatosis spilosebacea (PSS). Mosaic mutations of the HRAS gene are the only proven cause of PSS. The extracutaneous abnormalities of PSS result from various degrees of intermingling of Schimmelpenning syndrome and papular nevus spilus syndrome. PSS seems to be a condition at particularly high risk of developing basal cell carcinoma, urogenital malignancies, and vitamin D-resistant hypophosphatemic rickets. Extracutaneous abnormalities were detected in approximately 75% of PSS cases.

Phacomatosis Pigmentokeratotica: A Mosaic RASopathy with Malignant Potential

Phacomatosis pigmentokeratotica (PPK) is a rare epidermal nevus syndrome characterized by the co-occurrence of a nevus sebaceous arranged along the lines of Blaschko with a speckled lentiginous nevus (SLN). We report a novel KRAS mutation in a patient with a large nevus sebaceous and an SLN who subsequently developed a vaginal botryoid rhabdomyosarcoma, an association not previously reported in the literature. This case expands our knowledge of the genetic basis for phacomatosis, in which mutations in HRAS have been previously described, although this report provides evidence that activating mutations in KRAS or HRAS may cause PPK. This report confirms that PPK is a mosaic RASopathy with malignant potential and raises the question of whether screening for other RAS-associated malignancies should be performed for all children with PPK.

Phacomatosis pigmentokeratotica is a "pseudodidymosis"

In phacomatosis pigmentokeratotica, papular nevus spilus coexists with nevus sebaceus. The disorder was thought to be a didymosis with early postzygotic recombination. In this issue, however, Groesser and co-workers provide a new concept. Both nevi originate from a single heterozygous HRAS mutation in a pluripotent progenitor cell. This new understanding has implications for other proposed examples of didymosis in humans.

Phacomatosis pigmentokeratotica without extracutaneous abnormalities: a case study involving a preterm baby

Phacomatosis pigmentokeratotica (PPK) is a rare syndrome defined by the association of an organoid nevus occasionally with sebaceous differentiation, a speckled lentiginous nevus, and other extracutaneous anomalies. A preterm male infant of only 830 g at 27 week gestational age had an organoid nevus showing sebaceous differentiation. Also, he had multiple speckled-lentiginous nevus. Correlating the observed clinical presentation with the histopathological findings, the diagnosis of PPK was established. There have been less than 10 cases of PPK without extracutaneous manifestation. We present an uncommon case of a preterm patient with PPK who had no extracutaneous abnormalities.

Phacomatosis pigmentokeratotica: a further case without extracutaneous anomalies and review of the condition

Epidermal nevus syndrome is the term for the association of an epidermal nevus and extracutaneous anomalies, including neurologic, ophthalmic, and skeletal defects. Epidermal nevus syndromes include different disorders that share the feature of mosaicism. Phacomatosis pigmentokeratotica (PPK) is a distinctive new epidermal nevus syndrome first described in 1996 characterized by the presence of multiple organoid nevi with sebaceous differentiation, a speckled lentiginous nevus, and skeletal and neurologic abnormalities. Only a handful of cases of PPK without extracutaneous manifestations have been reported. We report here an individual with PPK with only cutaneous signs and confirm this distinctive syndrome has two subtypes according to the presence or absence of extracutaneous involvement.

The group of epidermal nevus syndromes Part I. Well defined phenotypes

The epidermal nevus syndromes represent a group of distinct disorders that can be distinguished by the type of associated epidermal nevus and by the criterion of presence or absence of heritability. Well defined syndromes characterized by organoid epidermal nevi include Schimmelpenning syndrome, phacomatosis pigmentokeratotica, nevus comedonicus syndrome, angora hair nevus syndrome, and Becker nevus syndrome. The molecular basis of these disorders has so far not been identified. By contrast, the group of syndromes characterized by keratinocytic nevi comprises three phenotypes with a known molecular etiology in the form of CHILD (congenital hemidysplasia with ichthyosiform nevus and limb defects) syndrome, type 2 segmental Cowden disease, and fibroblast growth factor receptor 3 epidermal nevus syndrome (García-Hafner-Happle syndrome), whereas Proteus syndrome is still of unknown origin. From this overview, it is clear that a specific type of these disorders cannot be classified by the name "epidermal nevus syndrome" nor by the terms "organoid nevus syndrome" or "keratinocytic nevus syndrome."

LEARNING OBJECTIVES

After completing this learning activity, participants should be able to distinguish nine different epidermal nevus syndromes by their characteristic features, understand the practical significance of avoiding terms like "epidermal nevus syndrome" or "keratinocytic nevus syndrome" to define any specific entity within this group of disorders, and differentiate between nonhereditary traits and those bearing a genetic risk because of either Mendelian or non-Mendelian inheritance.

Phacomatosis pigmentokeratotica associated with hemihypertrophy and a rhabdomyosarcoma of the abdominal wall

Phacomatosis pigmentokeratotica (PPK) represents a specific "twin nevus" syndrome in which a speckled lentiginous nevus (SLN) is associated with an organoid nevus with sebaceous differentiation. A boy with a large nevus sebaceus on the left face and upper part of the trunk, a giant segmental SLN extending from the abdomen to the feet bilaterally, and right hemihypertrophy developed an embryonal rhabdomyosarcoma of the right abdominal wall at age 6 months. A variety of musculoskeletal, neurologic, and ocular anomalies have been observed in patients with PPK, reflecting the individual manifestations of both SLN and Schimmelpenning syndromes. This report adds hemihypertrophy to the spectrum of extracutaneous manifestations of PPK and, to our knowledge, represents the first observation of a rhabdomyosarcoma arising in contiguity with an SLN in a patient with PPK. The development of a rhabdomyosarcoma in our patient likely reflects both increased propensity for growth (as evidenced by the hemihypertrophy) and the pluripotent nature of neural-crest derived cells within the field defect that underlies an SLN.

Two Distinct Types of Speckled Lentiginous Nevi Characterized by Macular versus Papular Speckles

BACKGROUND

Speckled lentiginous nevus (SLN; synonym: nevus spilus) is a darkly spotted light-brown macule that mostly occurs as an isolated lesion of rather limited dimensions but sometimes may involve large areas of the body. So far, this skin disorder has been considered to represent one clinical entity.

OBJECTIVE

Because SLN is occasionally associated with complex birth defects such as phacomatosis pigmentovascularis, phacomatosis pigmentokeratotica, or SLN syndrome, we tested our idea that two different types of SLN may exist, each one associated with a particular syndrome.

METHODS

A review of case reports on SLN published during the years 1970-2004 was performed.

RESULTS

This evaluation of cases provided evidence that two different types of SLN exist, in the form of macular versus papular SLN, each one being related to a specific syndrome. Macular SLN is characterized by a tannish-brown background with darker flat speckles. The distribution of speckles is rather even and resembles a polka-dot pattern. Histopathologically, this type of SLN is characterized by what has been called a 'jentigo' pattern in the darker speckles and by some nests of melanocytes at the dermoepidermal junction at the tips of the papillae, whereas the background pigmentation shows the microscopical features of a lentigo. Papular SLN is characterized by a light-brown macule superimposed by multiple melanocytic nevi in the form of papules or nodules that show a more uneven distribution reminiscent of a star map. Small dark macules may likewise be present. Histopathologically, the papular component consists of dermal or compound melanocytic nevi. A separation of the two types of SLN is important because our analysis showed that macular SLN is a hallmark of a particular type of phacomatosis pigmentovascularis, whereas papular SLN is typically present in phacomatosis pigmentokeratotica as well as in SLN syndrome.

CONCLUSIONS

Macular SLN and papular SLN appear to be two distinct cutaneous entities. This dichotomy may turn out to be important when such nevi will be analyzed at the molecular level.

[Patterns on the skin. New aspects of their embryologic and genetic causes]

The concept of cutaneous mosaicism has now been proven by numerous studies. In this way, the idea that all nevi represent mosaics has so far been confirmed. We can distinguish five different patterns of mosaicism in human skin in the form of lines of Blaschko, checkerboard pattern, phylloid pattern, patchy pattern without midline separation, and lateralization. Two different etiological categories are epigenetic and genomic mosaicism. According to present knowledge all epigenetic mosaics are caused by the activity of retrotransposons. In addition to the well-known examples of functional X-chrosomosome mosaicism, the concept of autosomal functional mosaicism, as observed in mice and dogs, has recently been established. Most likely, this mechanism may also explain the exceptional familial occurrence of pigmentary mosaicism in man. Phylloid hypomelanosis and speckled lentiginous nevus syndrome are recently described phenotypes that can be added to the list of lethal mutations surviving by mosaicism. The concept of type 2 segmental manifestation of autosomal dominant skin diseases has recently been confrirmed at the molecular level in a case of Hailey-Hailey disease. The concept of didymosis may explain why two different nevi can occur in spatial and temporal proximity, e.g., cutis tricolor or phacomatosis pigmentovascularis. The mechanism of paradominant inheritance may explain why some nevi that usually occur sporadically may affect, by way of exception, several members of a family. Examples are sebaceous nevus and Becker nevus. In autosomal recessive skin disorders, an event of loss of homozygosity may occur at an early developmental stage, giving rise to healthy skin areas arranged in a nevoid distribution. For example, when dermatologists examine a patient with xeroderma pigmentosum they should pay particular attention to such areas of revertant mosaicism.

Loss of heterozygosity in human skin

Loss of heterozygosity (LOH) is a genetic mechanism by which a heterozygous somatic cell becomes either homozygous or hemizygous because the corresponding wild-type allele is lost. LOH has today been recognized as a major cause of malignant growth. This article gives a comprehensive review of skin disorders in which an origin from LOH has been either documented at the molecular level or postulated on the basis of clinical evidence. LOH has been shown to cause basal cell carcinoma, squamous cell carcinoma, and malignant melanoma, but this mechanism can likewise be taken as an important model to explain the origin of many other skin diseases such as benign hamartomas; type 2 segmental manifestation of autosomal dominant skin disorders; a pronounced segmental manifestation of acquired skin disorders with a polygenic background, superimposed on symmetric lesions of the usual type; paired mutant patches in the form of either allelic or nonallelic twin spotting; and the exceptional familial occurrence of some nevi, reflecting paradominant transmission. completion of this learning activity, readers should be familiar with the mechanism of LOH and its general significance for the biology of plants, animals, and humans. Participants should understand that this mechanism plays a crucial role not only in cutaneous malignant growth but also in the development of benign skin disorders, and they should be able to examine such diseases with a prepared mind to gain a better understanding of their origin.