Mutations in SREBF1, Encoding Sterol Regulatory Element Binding Transcription Factor 1, Cause Autosomal-Dominant IFAP Syndrome.

IFAP syndrome is a rare genetic disorder characterized by ichthyosis follicularis, atrichia, and photophobia. Previous research found that mutations in MBTPS2, encoding site-2-protease (S2P), underlie X-linked IFAP syndrome. The present report describes the identification via whole-exome sequencing of three heterozygous mutations in SREBF1 in 11 unrelated, ethnically diverse individuals with autosomal-dominant IFAP syndrome. SREBF1 encodes sterol regulatory element-binding protein 1 (SREBP1), which promotes the transcription of lipogenes involved in the biosynthesis of fatty acids and cholesterols. This process requires cleavage of SREBP1 by site-1-protease (S1P) and S2P and subsequent translocation into the nucleus where it binds to sterol regulatory elements (SRE). The three detected SREBF1 mutations caused substitution or deletion of residues 527, 528, and 530, which are crucial for S1P cleavage. In vitro investigation of SREBP1 variants demonstrated impaired S1P cleavage, which prohibited nuclear translocation of the transcriptionally active form of SREBP1. As a result, SREBP1 variants exhibited significantly lower transcriptional activity compared to the wild-type, as demonstrated via luciferase reporter assay. RNA sequencing of the scalp skin from IFAP-affected individuals revealed a dramatic reduction in transcript levels of low-density lipoprotein receptor (LDLR) and of keratin genes known to be expressed in the outer root sheath of hair follicles. An increased rate of in situ keratinocyte apoptosis, which might contribute to skin hyperkeratosis and hypotrichosis, was also detected in scalp samples from affected individuals. Together with previous research, the present findings suggest that SREBP signaling plays an essential role in epidermal differentiation, skin barrier formation, hair growth, and eye function.

Genotype-phenotype correlations emerging from the identification of missense mutations in MBTPS2.

Missense mutations affecting membrane-bound transcription factor protease site 2 (MBTPS2) have been associated with Ichthyosis Follicularis with Atrichia and Photophobia (IFAP) syndrome with or without BRESHECK syndrome, with keratosis follicularis spinulosa decalvans, and Olmsted syndrome. This metalloprotease activates, by intramembranous trimming in conjunction with the protease MBTPS1, regulatory factors involved in sterol control of transcription and in cellular stress response. In this study, 11 different MBTPS2 missense mutations detected in patients from 13 unrelated families were correlated with the clinical phenotype, with their effect on cellular growth in media without lipids, and their potential role for sterol control of transcription. Seven variants were novel [c.774C>G (p.I258M); c.758G>C (p.G253A); c.686T>C (p.F229S); c.1427T>C (p.L476S); c.1430A>T (p.D477V); c.1499G>A (p.G500D); c.1538T>C (p.L513P)], four had previously been reported in unrelated sibships [c.261G>A (p.M87I); c.1286G>A (p.R429H); c.1424T>C (p.F475S); c.1523A>G (p.N508S)]. In the enzyme, the mutations cluster in transmembrane domains. Amino-acid exchanges near the active site are more detrimental to functionality of the enzyme and, clinically, associated with more severe phenotypes. In male patients, a genotype-phenotype correlation begins to emerge, linking the site of the mutation in MBTPS2 with the clinical outcome described as IFAP syndrome with or without BRESHECK syndrome, keratosis follicularis spinulosa decalvans, X-linked, Olmsted syndrome, or possibly further X-linked traits with an oculocutaneous component.

Intronic mutations affecting splicing of MBTPS2 cause ichthyosis follicularis, alopecia and photophobia (IFAP) syndrome.

Ichthyosis follicularis, alopecia and photophobia (IFAP) syndrome is an X-linked genodermatosis with congenital atrichia being the most prominent feature. Recently, we have shown that functional deficiency of MBTPS2 (membrane-bound transcription factor protease site 2) - a zinc metalloprotease essential for cholesterol homeostasis and endoplasmic reticulum stress response - causes the disease. Here, we present results obtained by analysing two intronic MBTPS2 mutations, c.671-9T>G and c.225-6T>A, using in silico and cell-based splicing assays. Accordingly, the c.225-6T>A transversion generated a new splice acceptor site, which caused extension of exon 3 by four bases and subsequently introduced a premature stop codon. Both, minigene experiments and RT-PCR analysis with patient-derived mRNA, demonstrated that the c.671-9T>G mutation resulted in skipping of exon 6, most likely because of disruption of the polypyrimidin tract or a putative intronic splicing enhancer (ISE). Our combined biocomputational and experimental analysis strongly suggested that both intronic alterations are disease-causing mutations.

Keratosis Follicularis Spinulosa Decalvans is caused by mutations in MBTPS2.

Keratosis Follicularis Spinulosa Decalvans (KFSD) is a rare genetic disorder characterized by development of hyperkeratotic follicular papules on the scalp followed by progressive alopecia of the scalp, eyelashes, and eyebrows. Associated eye findings include photophobia in childhood and corneal dystrophy. Due to the genetic and clinical heterogeneity of similar disorders, a definitive diagnosis of KFSD is often challenging. Toward identification of the causative gene we reanalyzed a large Dutch KFSD family. SNP arrays (1 M) redefined the locus to a 2.9-Mb region at Xp22.12-Xp22.11. Screening of all 14 genes in the candidate region identified MBTPS2 as the candidate gene carrying a c.1523A>G (p.Asn508Ser) missense mutation. The variant was also identified in two unrelated X-linked KFSD families and cosegregated with KFSD in all families. In symptomatic female carriers, skewed X-inactivation of the normal allele matched with increased severity of symptoms. MBTPS2 is required for cleavage of sterol regulatory element-binding proteins (SREBPs). In vitro functional expression studies of the c.1523A>G mutation showed that sterol responsiveness was reduced by half. Other missense mutations in MBTPS2 have recently been identified in patients with IFAP syndrome. We postulate that both phenotypes are in the spectrum of one genetic disorder with a partially overlapping phenotype.

CHILD syndrome in 3 generations: the importance of mild or minimal skin lesions.

BACKGROUND: CHILD syndrome (congenital hemidysplasia with ichthyosiform nevus and limb defects, Online Mendelian Inheritance in Man 308050) is an X-linked dominant trait with lethality for male embryos. The disorder is caused by mutations in NSDHL (Online Mendelian Inheritance in Man 300275), a gene playing an important role in the cholesterol biosynthetic pathway. Most reports deal with sporadic cases, and only 5 cases of mother-to-daughter transmission have been documented. We present here a family with mild features of CHILD syndrome in 3 generations. Molecular analysis was used to confirm the diagnosis. OBSERVATIONS: We studied 14 members of a family with CHILD syndrome. The 23-year-old proposita, her mother, 2 aunts, and her grandmother presented with mild or minimal skin lesions that had been present since infancy. Analysis of the NSDHL gene showed missense mutation c.370G-->A in these 5 patients. This mutation was absent in the 9 clinically unaffected family members tested. CONCLUSIONS: In this family, we recognized CHILD syndrome with mild or minimal features in 3 generations because we were able to verify our clinical diagnosis by means of molecular analysis. We assume that many cases that so far have been considered sporadic may in fact be familial when a meticulous physical examination of female family members is combined with molecular testing.

Multiple familial trichoepithelioma caused by mutations in the cylindromatosis tumor suppressor gene.

The recessive oncogene cylindromatosis (CYLD) mapping on 16q12-q13 is generally implicated in familial cylindromatosis, whereas a gene region for multiple familial trichoepithelioma has been assigned to 9p21. Markers from both chromosome intervals were subjected to linkage analysis in a large family with multiple hereditary trichoepithelioma (TE) from Algeria. Linkage to 9p21 was excluded, whereas CYLD remained as a candidate. Mutation analysis identified a single bp germ-line deletion expected to result in truncation or absence of the encoded protein, which segregated with the multiple TE phenotype. In individual tumors, loss of heterozygosity at 16q or a somatic point mutation in the CYLD gene was detected. Hence, mutations of the tumor suppressor gene CYLD at 16q12-q13 may give rise to familial TE indistinguishable from the phenotype assigned to 9p21.

A novel missense mutation of NSDHL in an unusual case of CHILD syndrome showing bilateral, almost symmetric involvement.

The CHILD syndrome (MIM 308050), an acronym for congenital hemidysplasia with ichthyosiform nevus and limb defects, is an X-linked dominant trait with lethality for male embryos. Recently, we elucidated the underlying gene defect by demonstrating point mutations in NSDHL (NAD[P]H steroid dehydrogenase-like protein) at Xq28 in 6 patients with classic CHILD syndrome. The most striking clinical feature is an inflammatory nevus that usually shows a unique lateralization with strict midline demarcation. Ipsilateral defects involve all skeletal structures and internal organs such as the brain, the lung, the heart, or the kidney. As an exception to this rule, in some cases the CHILD nevus may occur in a more or less bilateral distribution. In 1997 Fink-Puches et al described a case of CHILD nevus with an almost symmetric arrangement. To test the correctness of the diagnosis, we now examined blood lymphocytes of this patient by single-strand conformation analysis and genomic sequencing. We identified a novel missense mutation in NSDHL that potentially may impair protein function. We conclude that a diagnosis of CHILD syndrome can be based on clinical features such as the highly characteristic morphology of the CHILD nevus. A symmetric distribution of this nevus can exceptionally be seen in patients with CHILD syndrome, and this bilateral involvement should not mislead the clinician to any other diagnosis. Apparently, the effect of random X-inactivation is responsible for different patterns of cutaneous involvement in female carriers of NSDHL mutations.