Paradominant inheritance of twin spotting: phacomatosis pigmentovascularis as a further possible example

ARTICLE

Auteur(s) : Retno DANARTI, Rudolf HAPPLE

To the Editor
Recently Baba et al. [1] described two sisters with cutis tricolor, a disorder characterized by nevoid areas of hyper- and hypopigmented macules and thus representing a possible example of didymosis [2]. To explain the unusual familial occurrence of this trait, Baba et al. discussed the possibility of paradominant inheritance. In both sisters compound heterozygosity would be present at the underlying gene locus, and postzygotic recombination would have occurred at an early developmental stage giving rise to two different cell populations homozygous for either allele. We have found in the literature another possible example of paradominant transmission of didymosis. Phacomatosis pigmentovascularis usually occurs sporadically and has likewise been proposed to represent a twin-spot phenomenon [3]. Suzuki et al. [4] reported phacomatosis pigmentovascularis of the type III in two siblings. A 14-year-old boy had nevus flammeus, Becker nevus, nevus spilus, and nevus depigmentosus. His 15-year-old sister had nevus flammeus, nevus anemicus, nevus spilus, and nevus depigmentosus. This unusual familial constellation may be explained in the following way: In the boy, nevus spilus and nevus depigmentosus reflect allelic didymosis, whereas the associated nevus flammeus represents nonallelic didymosis. In the girl, nevus flammeus and nevus anemicus reflect allelic didymosis, and the coexistence of nevus spilus and nevus depigmentosus can likewise be taken as allelic didymosis. On the other hand, the occurrence of vascular versus pigmentary lesions would reflect non-allelic didymosis. In the girl, both compound heterozygosity and double heterozygosity would involve one pair of homologous chromosomes. In both siblings an early postzygotic recombination affecting this chromosomal region would have given rise to loss of heterozygosity resulting in more than two different mosaic spots.
In conclusion, the report by Suzuki et al. [4] can be taken as a first example of paradominant inheritance of phacomatosis pigmentovascularis. Hence, this family observation lends further support to the assumption that on rare occasions twin spotting may occur as a hereditary trait. n

References

1. Baba M, Seçkin D, Akçali C, Happle R. Familial cutis tricolor: A possible example of paradominant inheritance. Eur J Dermatol 2003; 13: 343-5.

2. Happle R, Barbi G, Eckert D, Kennerknecht I. “Cutis tricolor”: Congenital hyper- and hypopigmented macules associated with a sporadic multisystem birth defect: An unusual example of twin spotting ? J Med Genet 1997; 34: 676-8.

3. Happle R, Steijlen PM. Phacomatosis pigmentovascularis gedeutet als ein Phänomen der Zwillingsflecken. Hautarzt 1989; 40: 721-4.

4. Suzuki K, Ishizaki H, Takahashi H. Phacomatosis pigmentovascularis IIIb associated with porokeratosis, acanthosis nigricans and endocrinopathy in brother and sister (In Japanese, with an English summary). Skin Res (Hifu) 1990; 32: 65-70.