ARTICLE
In their article entitled Familial occurrence of nevus sebaceous of
Jadasshon: another case of paradominant inheritance? [1], Dr. Laino
and associates discuss some of the paradominant conditions and syndromes.
Although the article is not a review, the reference list seems to be incomplete.
In fact, no literature is cited with regard to the studies concerning
the paradominant inheritance occurring for the ectomesodermal association
between polythelia (supernumerary nipple) and Becker's nevus [2, 3].
Supernumerary nipples (SN) are developmental anomalies located along
the embryonic mammary lines, and usually occur sporadically, although
familial aggregation is reported [4]. The inheritance of SN is heterogeneous,
since three possible modes have been observed: autosomal dominant, X-linked
dominant and recessive [5]. This anomaly would be secondary to non-expression
of a single major gene regulating the normal regression of redundant accumulations
(or anlagen) along the ectodermal mammary ridge, which normally occurs
during the third month of embryonic development. In sporadic cases, the
genetic defect might be caused by a somatic mutation occurring early in
embryogenesis, giving rise to a clone of mosaic cells that lose heterozygosity
and are homozygous or hemizygous for the underlying allele mutation [3].
Becker's nevus is usually reported as a sporadic anomaly, but occasionally
it can show familial aggregation [6], which suggests the influence of
hereditary factors. Convincing evidence for a paradominant inheritance
has been fully elucidated by Happle [7].
A common paradominant trait has been also hypothesized in the ectomesodermal
association linking SN with Becker's nevus. In these patients both the
defects were sporadic and preponderantly ipsilateral (in 88% [2] and 93%
[3] of the surveys, respectively), and polythelia within Becker's nevus
is exceptionally described [8]. The two anomalies could originate either
from one common postzygotic mutation arising during the very initial
stages of embryonic development, or from two single (synchronous
or asynchronous) mutations inside their specific embryonic developmental
fields (skin and mammary anlagen) [9]. Phenotypically, SN and Becker's
nevus appear together when a postzygotic mutation occurs in those individuals
already carrying one mutation [3, 8], exactly as is expected to occur
in the spectrum of Becker nevus syndrome [10-12]. Future clinicoepidemiological
research and investigations at the molecular genetic level should show
whether these hypotheses hold true.
References
1. Laino L, et al. Familial occurrence of nevus sebaceous
of Jadasshon: another case of paradominant inheritance? Eur J Dermatol
2001; 11: 97-8.
2. Urbani CE, Betti R. Supernumerary nipples and Becker's nevus:
a previously undescribed association. Report of 9 patients including a
subset with uropathies. Eur J Dermatol 1995; 5: 685-7.
3. Urbani CE, Betti R. Supernumerary nipples occurring together
with Becker's nevus: an association involving one common paradominant
trait? Hum Genet 1997; 100: 388-90.
4. Leung AKC. Familial supernumerary nipple. Am J Med Genet
1988; 31: 631-5.
5. Urbani CE, Betti R. Familial aberrant mammary tissue: a clinicoepidemiological
survey of 18 cases. Dermatology 1995; 190: 207-9.
6. Panizzon R, Schnyder UW. Familial Becker's nevus. Dermatologica
1988; 176: 275-6.
7. Happle R. Paradominant inheritance: a possible explanation
for Becker's pigmented hairy nevus. Eur J Dermatol 1992; 2: 39-40.
8. Urbani CE, Betti R. Polythelia within Becker's nevus. Dermatology
1998; 196: 251-2.
9. Urbani CE, Betti R. Accessory mammary tissue in clinical practice.
McGraw-Hill, 1st ed., Milan, 1996: 41-8.
10. Happle R, Koopman RJJ. Becker nevus syndrome. Am J Med
Genet 1997 : 357-61.
11. Urbani CE, Betti R. Supernumerary nipple in association with
Becker nevus vs Becker nevus syndrome: a semantic problem only.
Am J Med Genet 1998; 77: 76-7.
12. Happle R, Koopman RJJ. Becker nevus syndrome and supernumerary
nipples. Am J Med Genet 1998; 77: 78.
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