Familial incontinentia pigmenti (IP; MIM 308310) is a genodermatosis that segregates as an X-linked dominant disorder and is usually lethal prenatally in males. In affected females it causes highly variable abnormalities of the skin, hair, nails, teeth, eyes and central nervous system. The prominent skin signs occur in four classic cutaneous stages: perinatal inflammatory vesicles, verrucous patches, a distinctive pattern of hyperpigmentation and dermal scarring [1]. Cells expressing the mutated X chromosome are eliminated selectively around the time of birth, so females with IP exhibit extremely skewed X-inactivation [2]. The reasons for cell death in females and< pres: italics> in utero lethality in males are unknown. The locus for IP has been linked genetically to the< pres: italics> factor VIII gene in Xq28 (ref. 3). The gene for NEMO (NF-κB essential modulator)/IKKγ (IκB kinase-γ) has been mapped to a position 200 kilobases proximal to the< pres: italics> factor VIII locus [4]. NEMO is required for the activation of the transcription factor NF-κB and is therefore central to many immune, inflammatory and apoptotic pathways [5, 6, 7, 8, 9]. Here we show that most cases of IP are due to mutations of this locus and that a new genomic rearrangement accounts for 80% of new mutations. As a consequence, NF-κB activation is defective in IP cells.

The extreme skewing of X-inactivation observed in IP patients has implications for screening of candidate genes. Complementary DNA prepared from females with IP will not contain the mutated allele of the disease locus, so screening of candidate genes for mutation must be performed with genomic DNA. We used two approaches to screen< pres: italics> NEMO:(1) generation and sequencing of fibroblast cDNA from extremely rare cases that express only the mutated X chromosome (two male cases and one female abortus, see Methods); and (2) resolution of the< pres: italics> NEMO genomic structure for mutational analysis of individual exons. Reverse transcriptase polymerase chain reaction (RT-PCR) from messenger RNA derived from skin fibroblasts from four affected fetuses (D, K, IP85m and G) and controls (C1 and C2) was conducted with< pres: italics> NEMO-specific primers (Fig. 1). Male samples D and IP85m have only one X chromosome. Female sample K is unusual in that contrary X-inactivation skewing has resulted in expression of only the mutated (IP) X chromosome. Affected female G expresses both Xs, as expected for a fetal IP case that has not yet undergone selective elimination of affected cells.