Selection Against Mutant Alleles in Blood Leukocytes is a Consistent Feature in Incontinentia Pigmenti Type 2

Incontinentia Pigmenti 2 (IP2) is an X-linked dominantdisorder with male lethality. Affected females display acharacteristic skin eruption that evolves through fourclassic stages, frequently accompanied by dental andretinal abnormalities. Non-random (skewed)X-inactivation in peripheral blood leukocytes and infibroblasts has been observed in females with IP2;however, sample sizes have been small and methods ofanalysis varied. We have examined X-inactivation in alarge group of multigenerational IP2 families, in smallerfamilies, and in isolated cases. Ninety-eight percent ofaffected females in multigenerational IP2 pedigreesshow completely skewed patterns of X-inactivation,while only ∼10% of a normal control population isskewed. Results both in small families and in newmutation cases with subsequent segregation consistentwith Xq28 linkage are similar. Isolated cases show alower percentage (85%) of skewed affected individuals;this difference may be due to inaccurate clinicalascertainment. The parent of origin of new mutationscould be determined in 15 families; paternal newmutations were twice as common as maternal. Fibro-blast subclones from a biopsy at the boundary of a skinlesion in a newborn IP2 patient were isolated, andclones with either one or the other X active wereidentified, demonstrating that cells with the activedisease-bearing X chromosome are still present instage I skin lesions.INTRODUCTIONFamilial Incontinentia Pigmenti (IP2) is a neurocutaneousgenodermatosis which segregates as an X-linked dominant traitwith prenatal male lethality (for review, see ref. 1). In affectedfemales, its most prominent phenotypic features involve the skinand its derivatives, the eye and the central nervous system. Thisdisorder is diagnosed in affected females at or soon after birth bythe presence of a progressive erythematous and vesicular rash. Thisrash becomes sequentially verrucous, pigmented, then atrophic, andmay leave adolescents and adults with areas of linear and reticularhypopigmentation. Other manifestations include cicatricial al opecia,hypodontia or anodontia, eosinophilia (in the early stages),peripheral vascular abnormalities and cicatrization of the retina,and other secondary ocular findings. The penetrance of the diseaseapproaches 100%, but its expressivity is highly variable, evenwithin families.Linkage of IP2 to Xq28 markers was demonstrated in 1989 ( 2);subsequently, IP2 was localized more precisely to the regionsurrounding and most likely telomeric to DXS52 (3). Morerecently, the region of interest has been narrowed to Xq28 distalto Factor VIII; the highest LOD scores are found with DXS1108(θ = 0.00) (4).Non-random X inactivation in patients with IP2 has beenreported by several groups. Migeon et al. (5), using G6PDvariants and an RFLP at the HPRT locus which detects adifferentially methylated HpaII site, observed that thechromosome bearing the normal allele at the IP locus is the activeX in the majority of skin fibroblasts in affected females in threefamilies. Variable skewing in peripheral blood leukocytes wasalso observed in these families, which was unexpected since thistissue had not previously been thought to be involved in thisdisease. Analysis of G6PD isozymes showed a more randomdistribution in red blood cells, suggesting that red cell progenitorsare less likely to be affected negatively by the IP mutation thanare white cell progenitors. Other investigators (6) used somaticcell hybrid analysis to compare skin biopsies taken from normaland hyperpigmented (stage III) areas of the skin; the same Xchromosome was found to be active in every clone, regardless ofwhether it was derived from normal or hyperpigmented skin.These authors concluded that invasion of affected skin by normalcells most likely takes place at the transition from theinflammatory (I) to the verrucous (II) stage, and that their resultssupport the model that normal cells have a proliferativeadvantage. A third study ( 7), again with RFLP analysis, producedcontradictory results. Extreme skewing was found in a minorityof cases, and in most of these cases it was not possible todetermine parental origin of the inactive X chromosome; in theone fully informative family with extreme skewing, the X