Congenital triangular alopecia may be categorized as a paradominant trait

ARTICLE

Auteur(s) : Rudolf HAPPLE

Department of Dermatology, Philipp University of Marburg, Deutschhausstraße 9, D-35033  Marburg, Germany

Article accepted on 15/5/2003

Article accepted on 15/5/2003

Congenital triangular alopecia (CTA), also called temporal triangular alopecia, consists of a bald patch involving the temporal region in a more or less triangular shape (Fig. 1) [1, 2]. CTA is always present at birth (Fig. 2) but usually first noticed during infancy. The lesion is mostly unilateral, but bilateral involvement may likewise occur. The genetic basis of the disorder is not clear. Some authors asserted that this trait is congenital but not heritable [3]. The phenotype has so far got no entry in McKusick’s Catalog of Human Genes and Genetic Disorders (OMIM) [4]. Here, arguments are presented in favor of the idea that CTA is a paradominant trait.

The concept of paradominant inheritance

Individuals heterozygous for the hypothetical CTA mutation would usually be healthy, and the mutation could therefore be transmitted unperceived through many generations. The trait would only become manifest when, at an early developmental stage, an additional postzygotic mutation gave rise to loss of the corresponding wildtype allele, resulting in a cell clone either homozygous or hemizygous for the mutation [5].

Exceptional familial occurrence of a “sporadic” trait

CTA usually occurs sporadically [2]. Tosti [1] has reported 14 cases, and all of these were sporadic. By way of exception, however, several familial cases have been described [6-8]. Patrizi et al. [8] proposed to include CTA in the group of “epidermal nevi” and they hypothesized that this nevus could be due to a postzygotic mutation, “which may sometimes be genetically transmitted, as in these familial cases”. Some authors assumed that CTA may be an autosomal dominant trait [7]. However, because CTA virtually always occurs sporadically, it is rather difficult to conceive as a Mendelian trait. Paradominant inheritance may be a more plausible explanation [9, 10].

Unilateral versus bilateral involvement

CTA usually involves one side of the scalp but a bilateral arrangement may also occur [3]. Tosti [1] noted bilateral involvement in one out of 14 cases of CTA. This is reminiscent of what is observed in lateral telangiectatic nevi occurring as part of Sturge-Weber-Klippel-Trenaunay syndrome. Such vascular nevi do likewise occur mostly in a unilateral form, although bilateral lesions are also noted. Remarkably, this phenotype has likewise been proposed to represent an example of paradominant inheritance [11, 12].

Clinical evidence for loss of heterozygosity

Kim et al. [13] have reported the occurrence of CTA in three cases of phacomatosis pigmentovascularis. Because there is ample evidence that the anomalies of phacomatosis pigmentovascularis originate from loss of heterozygosity [12], it seems reasonable to assume that CTA can likewise arise from this genetic mechanism.

Conclusion

CTA has been described in association with neurological defects [7], and this may explain why the phenotype cannot be transmitted as an autosomal dominant trait. Apparently, homozygosity or hemizygosity for the hypothetical CTA mutation would act as a lethal factor when the entire zygote is involved. In other words, CTA would be a mosaic phenotype of which a nonmosaic counterpart does not exist.
Paradominant inheritance would explain why CTA usually occurs sporadically; why in the exceptional familial cases the number of affected members is rather limited; why the trait tends to be unilateral but in few cases may show bilateral involvement; and why CTA is found in phacomatosis pigmentovascularis and thus may be a component of didymosis (twin spotting) [12]. Future molecular studies may show whether this concept holds true. < 

References

1. Tosti A. Congenital triangular alopecia. J Am Acad Dermatol 1987; 16: 991-3.

2. Trakimas C, Sperling LC, Skelton HG, Smith KJ, Buker JL. Clinical and histologic findings in temporal triangular alopecia. J Am Acad Dermatol 1994; 31: 205-9.

3. Böhm A, Hoting E. Alopecia triangularis congenita. Akt Dermatol 1992; 18: 155-6.

4. Online Mendelian Inheritance in Man (OMIM):
http://www.ncbi.nlm.nih.gov/omim/

5. Happle R. Paradominant inheritance: a possibile explanation for Becker’s pigmented hairy nevus. Eur J Dermatol 1992; 2: 39-40.

6. Bargman H. Congenital triangular alopecia. Can Med Ass J 1984; 131: 1253-4.

7. Ruggieri M, Rizzo R, Pavone P, Baieli S, Sorge G, Happle R. Temporal triangular alopecia in association with mental retardation and epilepsy in a mother and daughter. Arch Dermatol 2000; 136: 426-7.

8. Patrizi A, Morrone P, Fiorentini C, Bianchi T. An additional familial case of temporal triangular alopecia. Pediatr Dermatol 2001; 18: 263-4.

9. Happle R, König A. Familial naevus sebaceus may be explained by paradominant transmission. Br J Dermatol 1999; 141: 377.

10. Laino L, van Steensel MA, Innocenzi D, Camplone G. Familial occurrence of nevus sebaceus of Jadassohn: another case of paradominant inheritance ? Eur J Dermatol 2001; 11: 97-8.

11. Happle R. Klippel-Trenaunay syndrome: is it a paradominant trait ? Br J Dermatol 1999; 141: 465.

12. Happle R. Loss of heterozygosity in human skin. J Am Acad Dermatol 1999; 41: 143-61.

13. Kim HJ, Park KB, Yang JM, Park SH, Lee ES. Congenital triangular alopecia in phakomatosis pigmentovascularis: report of 3 cases. Acta Derm Venereol 2000; 80: 215-6.