Mutations in the inhibitor of kappa light polypeptide gene enhancer in B-cells, kinase gamma (IKBKG), also called nuclear factor-kappaB (NF-kB) essential modulator (NEMO), gene are the most common single cause of incontinentia pigmenti (IP) in females and anhydrotic ectodermal dysplasia with immunodeficiency (EDA-ID) in males. The IKBKG gene, located in the Xq28 chromosomal region, encodes for the regulatory subunit of the inhibitor of kappaB (IkB) kinase (IKK) complex required for the activation of the NF-kB pathway. Therefore, the remarkably heterogeneous and often severe clinical presentation reported in IP is due to the pleiotropic role of this signaling transcription pathway. A recurrent exon 4_10 genomic rearrangement in the IKBKG gene accounts for 60 to 80% of IP-causing mutations. Besides the IKBKG rearrangement found in IP females (which is lethal in males), a total of 69 different small mutations (missense, frameshift, nonsense, and splice-site mutations) have been reported, including 13 novel ones in this work. The updated distribution of all the IP- and EDA-ID-causing mutations along the IKBKG gene highlights a secondary hotspot mutation in exon 10, which contains only 11% of the protein. Furthermore, familial inheritance analysis revealed an unexpectedly high incidence of sporadic cases (>65%). The sum of the observations can aid both in determining the molecular basis of IP and EDA-ID allelic diseases, and in genetic counseling in affected families. Hum Mutat 29(5), 595–604, 2008.

Incontinentia pigmenti (IP) is an X‐linked‐dominant Mendelian disorder caused by mutation in the IKBKG/NEMO gene, encoding for NEMO/IKKgamma, a regulatory protein of nuclear factor kappaB (NF‐kB) signaling. In more than 80% of cases, IP is due to recurrent or nonrecurrent deletions causing loss‐of‐function (LoF) of NEMO/IKKgamma. We review how the local architecture of the IKBKG/NEMO locus with segmental duplication and a high frequency of repetitive elements favor de novo aberrant recombination through different mechanisms producing genomic microdeletion. We report here a new microindel (c.436_471delinsT, p.Val146X) arising through a DNA‐replication‐repair fork‐stalling‐and‐template‐switching and microhomology‐mediated‐end‐joining mechanism in a sporadic IP case. The LoF mutations of IKBKG/NEMO leading to IP include small insertions/deletions (indel) causing frameshift and premature stop codons, which account for 10% of cases. We here present 21 point mutations previously unreported, which further extend the spectrum of pathologic variants: 14/21 predict LoF because of premature stop codon (6/14) or frameshift (8/14), whereas 7/21 predict a partial loss of NEMO/IKKgamma activity (two splicing and five missense). We review how the analysis of IP‐associated IKBKG/NEMO hypomorphic mutants has contributed to the understanding of the pathophysiological mechanism of IP disease and has provided important information on affected NF‐kB signaling. We built a locus‐specific database listing all IKBKG/NEMO variants, accessible at http://IKBKG.lovd.nl.

Human sex chromosomes, which are morphologically and genetically different, share few regions of homology. Among them, only pseudoautosomal regions (PARs) pair and recombine during meiosis. To better address the complex biology of these regions, we sequenced the telomeric 400 kb of the long arm of the human X chromosome, including 330 kb of the human Xq/YqPAR and the telomere. Sequencing reveals subregions with distinctive regulatory and evolutionary features. The proximal 295 kb contains two genes inactivated on both the inactive X and Y chromosomes [SYBL1 and a novel homologue (HSPRY3) of Drosophilasprouty]. The GC-rich distal 35 kb, added in stages and much later in evolution, contains the X/Y expressed gene IL9R and a novel gene, CXYorf1, only 5 kb from the Xq telomere. These properties make Xq/YqPAR a model for studies of region-specific gene inactivation, telomere evolution, and involvement in sex-limited conditions.

IKBKG/NEMO gene mutations cause an X-linked, dominant neuroectodermal disorder named Incontinentia Pigmenti (IP). Located at Xq28, IKBKG/NEMO has a unique genomic organization, as it is part of a segmental duplication or low copy repeat (LCR1–LCR2, >99% identical) containing the gene and its pseudogene copy (IKBKGP). In the opposite direction and outside LCR1, IKBKG/NEMO partially overlaps G6PD, whose mutations cause a common X-linked human enzymopathy. The two LCRs in the IKBKG/NEMO locus are able to recombine through non-allelic homologous recombination producing either a pathological recurrent exon 4–10 IKBKG/NEMO deletion (IKBKGdel) or benign small copy number variations. We here report that the local high frequency of micro/macro-homologies, tandem repeats and repeat/repetitive sequences make the IKBKG/NEMO locus susceptible to novel pathological IP alterations. Indeed, we describe the first two independent instances of inter-locus gene conversion, occurring between the two LCRs, that copies the IKBKGP pseudogene variants into the functional IKBKG/NEMO, causing the de novo occurrence of p.Glu390ArgfsX61 and the IKBKGdel mutations, respectively. Subsequently, by investigating a group of 20 molecularly unsolved IP subjects using a high-density quantitative polymerase chain reaction assay, we have identified seven unique de novo deletions varying from 4.8 to ∼115 kb in length. Each deletion removes partially or completely both IKBKG/NEMO and the overlapping G6PD, thereby uncovering the first deletions disrupting the G6PD gene which were found in patients with IP. Interestingly, the 4.8 kb deletion removes the conserved bidirectional promoterB, shared by the two overlapping IKBKG/NEMO and G6PD genes, leaving intact the alternative IKBKG/NEMO unidirectional promoterA. This promoter, although active in the keratinocytes of the basal dermal layer, is down-regulated during late differentiation. Genomic analysis at the breakpoint sites indicated that other mutational forces, such as non-homologous end joining, Alu-Alu-mediated recombination and replication-based events, might enhance the vulnerability of the IP locus to produce de novo pathological IP alleles.