What is mosaicism?
Mosaicism describes an individual composed of two or more genetically different populations of cells existing side by side within the skin. The two cell lines develop very very early in the life of the embryo.
Many genetic skin disorders reflect cutaneous mosaicism.
There are two major genetic categories:
- Functional mosaicism resulting from inactivation of the X chromosome
- Genomic mosaicism resulting from autosomal mutation (mutation of one of the 44 non-sexual chromosomes).
What are the different patterns of cutaneous mosaicism?
Five patterns of cutaneous mosaicism have been described.
|Type 1a||Blaschko lines, narrow bands||Incontinentia pigmenti|
|Type 1b||Blaschko lines, broad bands||McCune-Albright syndrome|
|Type 2||Checkerboard pattern||Becker naevus / vascular malformation (portwine stain)|
|Type 3||Phylloid pattern (leaf-like)||Mosaic trisomy13|
|Type 4||Large patches without midline separation||Large congenital melanocytic naevus|
|Type 5||Lateralisation||CHILD syndrome|
Some conditions may occur in various mosaic patterns, for example, segmental vitiligo. Patients may present with unique 'birthmarks' due to mutations of known or unknown genes.
Proteus syndrome is an example of mosaicism in which the abnormal AKT1 gene in some cells produces a growth activating protein. Various forms of overgrowth and naevi occur and may progress throughout life.
What is the relationship between Blaschko lines and cutaneous mosaicism?
Cutaneous lesions following Blaschko lines are a manifestation of cutaneous mosaicism. They represent lines of division of the two cell lines within the embryonic skin tissue.
Cutaneous mosaicism does not always follow Blaschko lines, which may relate to the time the mosaicism arises during embryonic development.
What investigations can be undertaken in cutaneous mosaicism?
Patients with mosaic disorders may be referred to a geneticist. The affected tissue can be tested to detect somatic mutations to explain the cause of the skin disease. This usually involves whole-exome sequencing.