Functional Xp disomy and hypomelanosis of Ito

A proposed explanation for female predominance in alveolar soft part sarcoma

BACKGROUND

Alveolar soft-part sarcoma (ASPS) is a rare malignant soft tissue tumor with both clinically and morphologically distinct features. It often involves the extremities of adolescents and young adults and shows a predilection for females. Recently, ASPS was found to have a nonreciprocal der(17)t(X;17) translocation with the corresponding fusion gene located in chromosome 17. Because females have an extra X-chromosome, their likelihood of developing an X;autosome translocation is theoretically double that of males, and thus, this extra X-chromosome is a likely explanation for female predominance of ASPS.

METHODS

The authors used data from the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) registry program, which included 87 ASPS cases (33 males and 54 females), and published ASPS cases, which included 317 cases (121 males and 196 females), to test our hypothesis. The authors compared the observed proportion of female cases with that expected under the two X-chromosomes-double-risk hypothesis including the consideration of X-inactivation status.

RESULTS

The hypothesis that the fusion gene is not subject to X-inactivation is supported by data (P = 0.6, 0.24, and 0.20 for SEER cases, published cases, and their combination, respectively). In contrast, the competing hypothesis that the fusion gene is subject to X-inactivation is rejected (P = 0.007, < 0.00001, and < 0.00001 for SEER cases, published cases, and their combination, respectively).

CONCLUSIONS

Therefore, the authors found a statistical association between the female predominance observed in ASPS and female possession of an extra X-chromosome/noninactivation of the ASPS X;autosome translocation fusion gene.

Abnormal pigmentation in hypomelanosis of Ito and pigmentary mosaicism: the role of pigmentary genes

There is increasing evidence that hypomelanosis of Ito and related disorders such as linear and whorled naevoid hypermelanosis are due to mosaicism for a variety of chromosomal abnormalities. This group of disorders is better termed 'pigmentary mosaicism'. In this review we explain how disparate chromosomal abnormalities might manifest as a common pigmentary phenotype. In particular, we provide evidence supporting the hypothesis that the chromosomal abnormalities reported in these disorders specifically disrupt expression or function of pigmentary genes.

Hypomelanosis of Ito: clinical syndrome or just phenotype?

The term hypomelanosis of Ito is applied to individuals with skin hypopigmentation along the lines of Blaschko. Even though originally described as a purely cutaneous disease, subsequent reports have included a 33% to 94% association with multiple extracutaneous manifestations mostly of the central nervous and musculoskeletal systems leading to frequent characterization as a neurocutaneous disorder. A number of reports claimed familial occurrence and supported single gene inheritance for hypomelanosis of Ito, but none has been proved. Miscellaneous chromosomal mosaicisms have been demonstrated in some but not all affected individuals. Thus, it has been suggested that hypomelanosis of Ito is not a single condition but rather a nonspecific manifestation (ie, a phenotype) of chromosomal mosaicism and that this term should now be dropped. In this article, we review these developments focusing on the neurologic and genetic aspects of hypomelanosis of Ito. Our personal experience with 41 hypomelanosis of Ito patients and literature review led us to conclude that (1) the term hypomelanosis of Ito has been often misapplied to individuals with nonspecific "patchy depigmentation of the skin" who had several conditions of different etiologies; (2) the white matter involvement seen at neuroimaging in most of our hypomelanosis of Ito patients was similar to that reported in well-defined neurocutaneous disorders, including Sjögren-Larsson syndrome and incontinentia pigmenti; (3) whatever figures we take for associated central nervous system abnormalities in hypomelanosis of Ito, these represent the most frequent extracutaneous findings and, therefore, the use of the term neurocutaneous disorder for hypomelanosis of Ito might well be appropriate.