Human Genetic Diseases Linked to the Absence of NEMO: An Obligatory Somatic Mosaic Disorder in Male.

De novo somatic mutations are well documented in diseases such as neoplasia but are rarely reported in rare diseases. Hovewer, severe genetic diseases that are not compatible with embryonic development are caused exclusively by deleterious mutations that could only be found as mosaic and not as inherited mutations. We will review here the paradigmatic case of Incontinentia Pigmenti, a rare X-linked dominant disease caused by deficiency of the NEMO (also called IKKgamma) protein, which plays a pivotal role in tissue homeostasis. The loss-of-function mutations of NEMO are embryonically lethal in males while females survive because of unbalanced X-inactivation due to NEMO wild type (WT) expressing cells survival despite of NEMO mutant expressing cells. The few surviving IP males are obligatory mosaic mutants with the typical clinical presentation of IP in female. Indeed, the IP pathogenesis in the female and most likely also in the male somatic mosaics is based on the cellular effects of an impaired NEMO activity, but in the context of the interaction of genetically different cells in the affected tissue, which might underline the inflammatory status.

Importance of extracutaneous organ involvement in determining the clinical severity and prognosis of incontinentia pigmenti caused by mutations in the IKBKG gene.

Incontinentia pigmenti (IP) is a rare X-linked skin disease caused by mutations in the IKBKG gene, which is required for activation of the nuclear factor-kappa B signalling pathway. Multiple systems can be affected with highly variable phenotypic expressivity. We aimed to clarify the clinical characteristics observed in molecularly confirmed Korean IP patients. The medical records of 25 females confirmed as IP by molecular genetic analysis were retrospectively reviewed. The phenotypic score of extracutaneous manifestations was calculated to assess the disease severity. The IKBKG gene partial deletion or intragenic mutations were investigated using long-range PCR, multiplex ligation-dependent probe amplification and direct sequencing methods. Among the 25 individuals, 18 (72%) were sporadic cases. All patients showed typical skin manifestations at birth or during the neonatal period. Extracutaneous findings were noted in 17 (68%) patients; ocular manifestations (28%), neurological abnormalities (28%), hair abnormalities (20%), dental anomalies (12%), nail dystrophy (8%). The common exon 4-10 IKBKG deletion was observed in 20 (80%) patients. In addition, five intragenic sequence variants were identified, including three novel variants. The phenotype scores were highly variable, ranging from abnormal skin pigmentation only to one or more extracutaneous features, although no significant difference was observed for each clinical characteristic between the group with sequence variants and that with common large deletion. Our cohort with IP showed heterogeneity of extracutaneous manifestations and high incidence of sporadic cases. Long-term monitoring with multidisciplinary management is essential for evaluating the clinical status, providing adequate genetic counselling and understanding the genotype-phenotype correlation in IP.

Incontinentia pigmenti in adults.

Incontinentia Pigmenti (IP; MIM 308300) is an X-linked dominant genodermatosis caused by pathogenic variant in IKBKG. The phenotype in adults is poorly described compared to that in children. Questionnaire survey of 99 affected women showed an age at diagnosis from newborn to 41 years, with 53 diagnosed by 6 months of age and 30 as adults. Stage I, II, and III lesions persisted in 16%, 17%, and 71%, respectively, of those who had ever had them. IP is allelic to two forms of ectodermal dysplasia. Many survey respondents reported hypohidrosis and/or heat intolerance and most had Stage IV findings. This suggests that "Stage IV" may be congenitally dysplastic skin that becomes more noticeable with maturity. Fifty-one had dentures or implants with 26 having more invasive jaw or dental surgery. Half had wiry or uncombable hair. Seventy-three reported abnormal nails with 27 having long-term problems. Cataracts and retinal detachment were the reported causes of vision loss. Four had microphthalmia. Respondents without genetic confirmation of IP volunteered information suggesting more involved phenotype or possibly misassigned diagnosis. Ascertainment bias likely accounts for the low prevalence of neurocognitive problems in the respondents.

Rescue of recurrent deep intronic mutation underlying cell type-dependent quantitative NEMO deficiency.

X-linked dominant incontinentia pigmenti (IP) and X-linked recessive anhidrotic ectodermal dysplasia with immunodeficiency (EDA-ID) are caused by loss-of-function and hypomorphic IKBKG (also known as NEMO) mutations, respectively. We describe a European mother with mild IP and a Japanese mother without IP, whose 3 boys with EDA-ID died from ID. We identify the same private variant in an intron of IKBKG, IVS4+866 C>T, which was inherited from and occurred de novo in the European mother and Japanese mother, respectively. This mutation creates a new splicing donor site, giving rise to a 44-nucleotide pseudoexon (PE) generating a frameshift. Its leakiness accounts for NF-κB activation being impaired but not abolished in the boys' cells. However, aberrant splicing rates differ between cell types, with WT NEMO mRNA and protein levels ranging from barely detectable in leukocytes to residual amounts in induced pluripotent stem cell-derived (iPSC-derived) macrophages, and higher levels in fibroblasts and iPSC-derived neuronal precursor cells. Finally, SRSF6 binds to the PE, facilitating its inclusion. Moreover, SRSF6 knockdown or CLK inhibition restores WT NEMO expression and function in mutant cells. A recurrent deep intronic splicing mutation in IKBKG underlies a purely quantitative NEMO defect in males that is most severe in leukocytes and can be rescued by the inhibition of SRSF6 or CLK.

Lack of interaction between NEMO and SHARPIN impairs linear ubiquitination and NF-κB activation and leads to incontinentia pigmenti.

BACKGROUND: Incontinentia pigmenti (IP; MIM308300) is a severe, male-lethal, X-linked, dominant genodermatosis resulting from loss-of-function mutations in the IKBKG gene encoding nuclear factor κB (NF-κB) essential modulator (NEMO; the regulatory subunit of the IκB kinase [IKK] complex). In 80% of cases of IP, the deletion of exons 4 to 10 leads to the absence of NEMO and total inhibition of NF-κB signaling. Here we describe a new IKBKG mutation responsible for IP resulting in an inactive truncated form of NEMO. OBJECTIVES: We sought to identify the mechanism or mechanisms by which the truncated NEMO protein inhibits the NF-κB signaling pathway. METHODS: We sequenced the IKBKG gene in patients with IP and performed complementation and transactivation assays in NEMO-deficient cells. We also used immunoprecipitation assays, immunoblotting, and an in situ proximity ligation assay to characterize the truncated NEMO protein interactions with IKK-α, IKK-ß, TNF receptor-associated factor 6, TNF receptor-associated factor 2, receptor-interacting protein 1, Hemo-oxidized iron regulatory protein 2 ligase 1 (HOIL-1), HOIL-1-interacting protein, and SHANK-associated RH domain-interacting protein. Lastly, we assessed NEMO linear ubiquitination using immunoblotting and investigated the formation of NEMO-containing structures (using immunostaining and confocal microscopy) after cell stimulation with IL-1ß. RESULTS: We identified a novel splice mutation in IKBKG (c.518+2T>G, resulting in an in-frame deletion: p.DelQ134_R256). The mutant NEMO lacked part of the CC1 coiled-coil and HLX2 helical domain. The p.DelQ134_R256 mutation caused inhibition of NF-κB signaling, although the truncated NEMO protein interacted with proteins involved in activation of NF-κB signaling. The IL-1ß-induced formation of NEMO-containing structures was impaired in fibroblasts from patients with IP carrying the truncated NEMO form (as also observed in HOIL-1-/- cells). The truncated NEMO interaction with SHANK-associated RH domain-interacting protein was impaired in a male fetus with IP, leading to defective linear ubiquitination. CONCLUSION: We identified a hitherto unreported disease mechanism (defective linear ubiquitination) in patients with IP.

Cerebral Arteriopathy in a Newborn With Incontinentia Pigmenti.

BACKGROUND: Incontinentia pigmenti is a rare X-linked neurocutaneous disorder that can present in the neonatal period with seizures and encephalopathy. Brain magnetic resonance imaging and magnetic resonance angiography may reveal cerebral infarction and arteriopathy. PATIENT DESCRIPTION: We describe a neonate with the typical rash of incontinentia pigmenti along with seizures and brain magnetic resonance imaging abnormalities. RESULTS: Brain magnetic resonance imaging, magnetic resonance angiography, and magnetic resonance spectroscopy at age one week revealed chronic and acute brain injury, arteriopathy of the small and medium-sized cerebral vessels, and elevation of a lactate metabolite peak. By age six months, her magnetic resonance angiogram had normalized. At age 2.5 years, she has well-controlled complex partial seizures, global developmental delay, and residual hemiplegia. CONCLUSION: Despite extensive cerebral arteriopathy in association with incontinentia pigmenti, this girl had a relatively stable early clinical course, steady developmental progress over time, and seizures that have been well controlled. Later brain imaging revealed resolution of the arteriopathy.

Brain endothelial TAK1 and NEMO safeguard the neurovascular unit.

Inactivating mutations of the NF-κB essential modulator (NEMO), a key component of NF-κB signaling, cause the genetic disease incontinentia pigmenti (IP). This leads to severe neurological symptoms, but the mechanisms underlying brain involvement were unclear. Here, we show that selectively deleting Nemo or the upstream kinase Tak1 in brain endothelial cells resulted in death of endothelial cells, a rarefaction of brain microvessels, cerebral hypoperfusion, a disrupted blood-brain barrier (BBB), and epileptic seizures. TAK1 and NEMO protected the BBB by activating the transcription factor NF-κB and stabilizing the tight junction protein occludin. They also prevented brain endothelial cell death in a NF-κB-independent manner by reducing oxidative damage. Our data identify crucial functions of inflammatory TAK1-NEMO signaling in protecting the brain endothelium and maintaining normal brain function, thus explaining the neurological symptoms associated with IP.

EDA-ID and IP, two faces of the same coin: how the same IKBKG/NEMO mutation affecting the NF-κB pathway can cause immunodeficiency and/or inflammation.

Anhidrotic Ectodermal Dysplasia with ImmunoDeficiency (EDA-ID, OMIM 300291) and Incontinentia Pigmenti (IP, OMIM 308300) are two rare diseases, caused by mutations of the IKBKG/NEMO gene. The protein NEMO/IKKγ is essential for the NF-κB activation pathway, involved in a variety of physiological and cellular processes, such as immunity, inflammation, cell proliferation, and survival. A wide spectrum of IKBKG/NEMO mutations have been identified so far, and, on the basis of their effect on NF-κB activation, they are considered hypomorphic or amorphic (loss of function) mutations. IKBKG/NEMO hypomorphic mutations, reducing but not abolishing NF-κB activation, have been identified in EDA-ID and IP patients. Instead, the amorphic mutations, abolishing NF-κB activation by complete IKBKG/NEMO gene silencing, cause only IP. Here, we present an overview of IKBKG/NEMO mutations in EDA-ID and IP patients and describe similarities and differences between the clinical/immunophenotypic and genetic aspects, highlighting any T and B lymphocyte defect, and paying particular attention to the cellular and molecular defects that underlie the pathogenesis of both diseases.

Identification of a new NEMO/TRAF6 interface affected in incontinentia pigmenti pathology.

NF-kappaB Essential MOdulator (NEMO) has been shown to play a critical role in NF-kappaB activation, as the regulatory subunit of IkappaB kinase. Upon cell stimulation, NEMO can be modified through phosphorylation, sumoylation or ubiquitination. In the latter case, not much is known regarding the exact function of this posttranslational modification. One of the E3 ligase responsible for K63-linked NEMO polyubiquitination is TRAF6, which participates in several signaling pathways controlling immunity, osteoclastogenesis, skin development and brain functions. We previously observed a potentially important interaction between NEMO and TRAF6. In this study, we defined in more detail the domains required for this interaction, uncovering a new binding site for TRAF6 located at the amino-terminus of NEMO and recognized by the coiled-coil domain of TRAF6. This site appears to work in concert with the previously identified NEMO ubiquitin-binding domain which binds polyubiquitinated chains, suggesting a dual mode of TRAF6 recognition. We also showed that E57K mutation of NEMO found in a mild form of the genetic disease incontinentia pigmenti, resulted in impaired TRAF6 binding and IL-1beta signaling. In contrast, activation of NF-kappaB by TNF-alpha was not affected. These data demonstrate that NEMO/TRAF6 interaction has physiological relevance and might represent a new target for therapeutic purposes.

Incontinetia pigmenti-related myopathy or unsolved "double trouble"?

Incontinentia pigmenti is an X-linked dominant or sporadic multisystemic disorder with involvement of skin, eyes and central nervous system which results from mutations in the gene for NF-kappaB essential modulator (NEMO). We report on a patient with genetically confirmed Bloch-Sulzberger syndrome, who presented with a progressive myopathy and cardiomyopathy. Genetic analyses revealed an intragenic deletion (Intron3 and Exon10) of the NEMO/IKKgamma/IKKAP/IKBKG gene. Further complete sequencing of genes encoding for desmin, lamin A/C, emerin, and FHL1 showed no evidence of pathogenic mutations. A pathological expansion of CCTG repeats of the ZNF9 gene (PROMM) was ruled out by PCR amplification analysis. MLPA-analysis showed no evidence for duplications or deletions of the dystrophin gene. This report highlights the unusual combination of a genetically confirmed incontinentia pigmenti and a proximal myopathy and dilatative cardiomyopathy of unknown origin. We discuss that the striated muscle involvement (i) might be based on the observed intragenic deletion of the NEMO gene, or (ii) on an additional gene defect leading to an adult onset myopathy. Further studies on neuromuscular involvement in patients with incontinentia pigmenti are needed to clarify this issue.

Microdeletion/duplication at the Xq28 IP locus causes a de novo IKBKG/NEMO/IKKgamma exon4_10 deletion in families with Incontinentia Pigmenti.

The Incontinentia Pigmenti (IP) locus contains the IKBKG/NEMO/IKKgamma gene and its truncated pseudogene copy, IKBKGP/deltaNEMO. The major genetic defect in IP is a heterozygous exon4_10 IKBKG deletion (IKBKGdel) caused by a recombination between two consecutive MER67B repeats. We analyzed 91 IP females carrying the IKBKGdel, 59 of whom carrying de novo mutations (65%). In eight parents, we found two recurrent nonpathological variants of IP locus, which were also present as rare polymorphism in control population: the IKBKGPdel, corresponding to the exon4_10 deletion in the pseudogene, and the MER67Bdup, that replicates the exon4_10 region downstream of the normal IKBKG gene. Using quantitative DNA analysis and microsatellite mapping, we established that both variants might promote the generation of the pathological IKBKGdel. Indeed, in family IP-516, the exon4_10 deletion was repositioned in the same allele from the pseudogene to the gene, whereas in family IP-688, the MER67Bdup generated the pathological IKBKGdel by recombination between two direct nonadjacent MER67Bs. Moreover, we found an instance of somatic recombination in a MER67Bdup variant, creating the IKBKGdel in an IP male. Our data suggest that the IP locus undergoes recombination producing recurrent variants that might be "at risk" of generating de novo IKBKGdel by NAHR during either meiotic or mitotic division.

Identification of TRAF6-dependent NEMO polyubiquitination sites through analysis of a new NEMO mutation causing incontinentia pigmenti.

The regulatory subunit NEMO is involved in the mechanism of activation of IkappaB kinase (IKK), the kinase complex that controls the NF-kappaB signaling pathway. During this process, NEMO is modified post-translationally through K63-linked polyubiquitination. We report the molecular characterization of a new missense mutation of NEMO (A323P) which causes a severe form of incontinentia pigmenti (OMIM#308300), an inherited disease characterized predominantly by skin inflammation. The A323P mutation was found to impair TNF-, IL-1-, LPS- and PMA/ionomycin-induced NF-kappaB activation, as well as to disrupt TRAF6-dependent NEMO polyubiquitination, due to a defective NEMO/TRAF6 interaction. Mutagenesis identified the affected ubiquitination sites as three lysine residues located in the vicinity of A323. Unexpectedly, these lysines were ubiquitinated together with two previously identified lysines not connected to TRAF6. Mutation of all these ubiquitination sites severely impaired NF-kappaB activation induced by stimulation with IL-1, LPS, Nod2/RICK or serum/LPA. In contrast, mutation at all of these sites had only a limited effect on stimulation by TNF. These findings indicate that post-translational modification of NEMO through K63-linked polyubiquitination is a key event in IKK activation and that perturbation of this step may cause human pathophysiology.

NEMO, NFkappaB signaling and incontinentia pigmenti.

The identification of mutations in the NEMO gene in humans with incontinentia pigmenti and several other genetic conditions has led to an appreciation of the multiple roles of signaling through the NFkappaB pathway, and how erroneous signalling contributes to disease. The finding that the disease results from a common, recurrent mutation was surprising given the high variability in patients' phenotypes and illustrates the role of X inactivation and selection in females. Recent advances in mouse models and in understanding the multiple roles of NEMO in the cell provide additional avenues to define the various roles of NEMO in NFkappaB signaling.

Skin lesion development in a mouse model of incontinentia pigmenti is triggered by NEMO deficiency in epidermal keratinocytes and requires TNF signaling.

NF-kappaB essential modulator (NEMO), the regulatory subunit of the IkappaB kinase, is essential for NF-kappaB activation. Mutations disrupting the X-linked NEMO gene cause incontinentia pigmenti (IP), a human genetic disease characterized by male embryonic lethality and by a complex pathology affecting primarily the skin in heterozygous females. The cellular and molecular mechanisms leading to skin lesion pathogenesis in IP patients remain elusive. Here we used epidermis-specific deletion of NEMO in mice to investigate the mechanisms causing the skin pathology in IP. NEMO deletion completely inhibited NF-kappaB activation and sensitized keratinocytes to tumor necrosis factor (TNF)-induced death but did not affect epidermal development. Keratinocyte-restricted NEMO deletion, either constitutive or induced in adult skin, caused inflammatory skin lesions, identifying the NEMO-deficient keratinocyte as the initiating cell type that triggers the skin pathology in IP. Furthermore, genetic ablation of tumor necrosis factor receptor 1 (TNFRI) rescued the skin phenotype demonstrating that TNF signaling is essential for skin lesion pathogenesis in IP. These results identify the NEMO-deficient keratinocyte as a potent initiator of skin inflammation and provide novel insights into the mechanism leading to the pathogenesis of IP.

The NF-kappaB signalling pathway in human diseases: from incontinentia pigmenti to ectodermal dysplasias and immune-deficiency syndromes.

The transcription factor NF-kappaB regulates the expression of numerous genes controlling the immune and stress responses, inflammatory reaction, cell adhesion, and protection against apoptosis. Incontinentia pigmenti (IP) is the first genetic disorder to be ascribed to NF-kappaB dysfunction. IP is an X-linked dominant genodermatosis antenatally lethal in males. A complex rearrangement of the NEMO (NF-kappaB essential modulator) gene accounts for 85% of IP patients, and results in undetectable NEMO protein and absent NF-kappaB activation. On the other hand, hypohidrotic/anhidrotic ectodermal dysplasia (HED/EDA) has been ascribed to at least three genes also involved in NF-kappaB activation: ectodysplasin (EDA1), EDA-receptor (EDAR) and EDAR-associated death domain (EDARADD). During hair follicle morphogenesis, EDAR is activated by ectodysplasin, and uses EDARADD as an adapter to build a signal transducing complex that leads to NF-kappaB activation. Hence, several forms of HED/EDA also result from impaired activation of the NF-kappaB cascade. Finally, hypomorphic NEMO mutations have been found to cause anhidrotic ectodermal dysplasia with immunodeficiency (EDA-ID), whilst stop codon mutations cause a more severe phenotype associating EDA-ID with osteopetrosis and lymphoedema (OL-EDA-ID). The immunological and infectious features observed in patients result from impaired NF-kappaB signalling, including cellular response to LPS, IL-1beta, IL-18, TNF-alpha, Tlr2 and CD40 ligand. Consistently, mouse knockout models have shown the essential role of NF-kappaB in the immune, inflammatory and apoptotic responses. Unravelling the molecular bases of other forms of EDA not associated with mutations in NEMO will possibly implicate other components of the NF-kappaB signalling pathway.

NF-kappaB signaling and human disease.

Despite substantial progress in understanding the NF-kappaB signaling pathway, the connections between this pathway and human disease are only now being elucidated. Genes that function within or upstream of the NF-kappaB pathway have been found to cause four distinct disorders and two allelic conditions. Investigation of these genes and disorders has brought significant insight into the role of NF-kappaB in various aspects of physiological development.

Evaluation of the norrie disease gene in a family with incontinentia pigmenti.

Incontinentia pigmenti (IP) is an ectodermal multisystem disorder which can affect dental, ocular, cardiac and neurologic structures. The ocular changes of IP can have a very similar appearance to the retinal detachment of X-linked familial exudative vitreoretinopathy, which has been shown to be caused by the mutations in the Norrie disease gene. Therefore, it is of interest to determine whether similar mutations in the gene can account for the retinal pathology in patients with IP. To test our hypothesis, we have analyzed the entire Norrie disease gene for a family with IP, by single strand conformational polymorphism followed by DNA sequencing. The sequencing data revealed no disease-specific sequence alterations. These data suggest that ocular findings of IP are perhaps associated with different genes and there is no direct relationship between the genotype and phenotype.

[Incontinentia pigmenti]. / Incontinentia pigmenti.

Incontinentia pigmenti (IP) is a hereditary syndrome characterized by specific skin lesions occurring mostly during the neonatal period (96% of the cases before 6 weeks of age). These skin lesions have four steps of evolution: inflammatory or erythemato-bullous stage (very often associated with peripheral blood hyper-eosinophilia), proliferative or verruco-lichenoid stage, pigmentary or terminal stage characterized by "fountain" or "firework" features (with a picture of pigmentary incontinence at histological examination), sometimes there is a fourth stage referred to as "involutive". Ocular and neurological involvement is the main determinant in the prognosis. Eye lesions include corneal flecks, cataracts, uveitis or optical atrophy with retrolental fribroplasia. The neurological involvement includes pyramidal syndrome, cerebral ataxia, microcephalia, and mental retardation. The disease has mainly an X-linked dominant transmission and is usually lethal for males. Rare cases are observed in boys, some being associated with Klinefelter syndrome. Research is ongoing to identify the IP gene on the X chromosome. In the family form of IP, the gene has been located on chromosome Xq28, which allows prenatal diagnosis using trophoblast biopsy.