A novel point mutation in the steroid sulfatase gene in X-linked ichthyosis

A novel nonsense mutation in the STS gene in a Pakistani family with X-linked recessive ichthyosis: including a very rare case of two homozygous female patients

Background

X-linked ichthyosis (XLI; OMIM# 308100) is a recessive keratinization disorder characterized by the presence of dark brown, polygonal, adherent scales on different parts of the body surface. It almost exclusively affects males and the estimated prevalence ranges from 1:2000–6000 in males worldwide. Extracutaneous manifestations are frequent including corneal opacities, cryptorchidism, neuropsychiatric symptoms or others. Up to 90% of XLI cases are caused by recurrent hemizygous microdeletion encompassing entire STS gene on chromosome Xp22.3, while only a minority of patients shows partial deletions or loss of function point mutations in STS. Larger deletions also involving contiguous genes are identified in syndromic patients.

Methods

Here, we report clinical and genetic findings of a large Pakistani family having 16 affected individuals including 2 females with XLI. Molecular karyotyping and direct DNA sequencing of coding region of the STS gene was performed.

Results

The clinical manifestations in affected individuals involved generalized dryness and scaling of the skin with polygonal, dark scales of the skin on scalp, trunk, limbs, and neck while sparing face, palms and soles. There were no associated extra-cutaneous features such as short stature, cryptorchidism, photophobia, corneal opacities, male baldness, and behavioral, cognitive, or neurological phenotypes including intellectual disability, autism or attention deficit hyperactivity disorder. Molecular karyotyping was normal and no copy number variation was found. Sanger sequencing identified a novel hemizygous nonsense mutation (c.287G > A; p.W96*), in exon 4 of STS gene in all affected male individuals. In addition, two XLI affected females in the family were found to be homozygous for the identified variant.

Conclusions

This study is useful for understanding the genetic basis of XLI in the patients studied, for extending the known mutational spectrum of STS, diagnosis of female carriers and for further application of mutation screening in the genetic counseling of this family.

X-linked ichthyosis: Clinical and molecular findings in 35 Italian patients

Recessive X-linked ichthyosis (XLI), the second most common ichthyosis, is caused by mutations in the STS gene encoding the steroid sulfatase enzyme. A complete deletion of the STS gene is found in 85%-90% of cases. Rarely, larger deletions involving contiguous genes are detected in syndromic patients. We report the clinical and molecular genetic findings in a series of 35 consecutive Italian male patients. All patients underwent molecular testing by MLPA or aCGH, followed, in case of negative results, by next-generation sequencing analysis. Neuropsychiatric, ophthalmological and paediatric evaluations were also performed. Our survey showed a frequent presence of disease manifestations at birth (42.8%). Fold and palmoplantar surfaces were involved in 18 (51%) and 7 (20%) patients, respectively. Fourteen patients (42%) presented neuropsychiatric symptoms, including attention-deficit hyperactivity disorder and motor disabilities. In addition, two patients with mental retardation were shown to be affected by a contiguous gene syndrome. Twenty-seven patients had a complete STS deletion, one a partial deletion and 7 carried missense mutations, two of which previously unreported. In addition, a de novo STS deletion was identified in a sporadic case. The frequent presence of palmoplantar and fold involvement in XLI should be taken into account when considering the differential diagnosis with ichthyosis vulgaris. Our findings also underline the relevance of involving the neuropsychiatrist in the multidisciplinary management of XLI. Finally, we report for the first time a de novo mutation which shows that STS deletion can also occur in oogenesis.

X linked recessive ichthyosis: Current concepts

In the present review, we describe the most important aspects of the X-linked ichthyosis (XLI) and make a compilation of the some historic details of the disease. The aim of the present study is an update of the XLI. Historical, clinical, epidemiological, and molecular aspects are described through the text. Recessive XLI is a relatively common genodermatosis affecting different ethnic groups. With a high spectrum of the clinical manifestations due to environmental factors, the disease has a genetic heterogeneity that goes from a point mutation to a large deletion involving several genes to produce a contiguous gene syndrome. Most XLI patients harbor complete STS gene deletion and flanked sequences; seven intragenic deletions and 14 point mutations with a complete loss of the steroid sulfatase activity have been reported worldwide. In this study, we review current knowledge about the disease.

Cognitive, behavioural and psychiatric phenotypes associated with steroid sulfatase deficiency

The enzyme steroid sulfatase (STS) desulfates a variety of steroid compounds thereby altering their activity. STS is expressed in the skin, and its deficiency in this tissue has been linked to the dermatological condition X-linked ichthyosis. STS is also highly expressed in the developing and adult human brain, and in a variety of steroidogenic organs (including the placenta and gonads); therefore it has the potential to influence brain development and function directly and/or indirectly (through influencing the hormonal milieu). In this review, we first discuss evidence from human and animal model studies suggesting that STS deficiency might predispose to neurobehavioural abnormalities and certain psychiatric disorders. We subsequently discuss potential mechanisms that may underlie these vulnerabilities. The data described herein have potential implications for understanding the complete spectrum of clinical phenotypes associated with X-linked ichthyosis, and may indicate novel pathogenic mechanisms underlying psychological dysfunction in developmental disorders such as attention deficit hyperactivity disorder and Turner syndrome.

Expression patterns of the STAG gene in intact and regenerating planarians (Dugesia japonica)

We examined the spatial and temporal expression of the planarian Dugesia japonica STAG-related gene (DjStag), in both intact and regenerating planarians, by whole-mount in situ hybridization and relative quantitative real-time PCR. The first localized transcripts of DjStag were detected in the blastemas three days after amputation, in all regenerates including those from head, tail and trunk pieces. The maximum level of expression of DjStag transcripts occurred at five days after cutting. After regeneration for seven days, DjStag was weakly expressed. A similar decrease occurs regardless of the orientation of the cut. The expression pattern did not differ significantly in the different types of regeneration. Relative quantitative real-time PCR analysis of DjStag mRNA indicated that the expression of DjStag mRNA was increased after amputation compared to that in normal intact planarians, and the maximum level of expression of DjStag transcripts occurred at five days after amputation. All results suggest that DjStag, implicated in planarian regeneration, plays a role in maintaining the ability of pluripotent stem cells to regenerate lost tissue in planarians.

X-linked ichthyosis: an oculocutaneous genodermatosis

X-linked ichthyosis (XLI) is an X-linked recessive disorder of cutaneous keratinization with possible extracutaneous manifestations. It was first described as a distinct type of ichthyosis in 1965. XLI is caused by a deficiency in steroid sulfatase activity, which results in abnormal desquamation and a retention hyperkeratosis. XLI is usually evident during the first few weeks of life as polygonal, loosely adherent translucent scales in a generalized distribution that desquamate widely. These are quickly replaced by large, dark brown, tightly adherent scales occurring primarily symmetrically on the extensor surfaces and the side of the trunk. In addition, extracutaneous manifestations such as corneal opacities, cryptorchidism, and abnormalities related to contiguous gene syndromes may be observed. Diagnosis of XLI is usually made clinically, as the histopathology is nonspecific, but confirmation may be obtained through either biochemical or genetic analysis. Treatment should focus on cutaneous hydration, lubrication, and keratolysis and includes topical moisturizers and topical retinoids.

Filaggrin mutations are genetic modifying factors exacerbating X-linked ichthyosis

Mutations inactivating the STS gene cause X-linked ichthyosis (XLI), whereas null mutations in the FLG gene cause ichthyosis vulgaris. Two brothers presented with XLI. One had a typical fine scaling, and the other was much more severely affected. Both patients carried STS missense mutation T165I. Furthermore, the more severely affected patient also carried heterozygous FLG mutation R501X, which was absent from his mildly affected brother. These data suggest that disrupting epidermal differentiation via different pathways can increase phenotypic severity. Owing to the high population frequency of FLG mutations, filaggrin is a possible genetic modifier in other genodermatoses.

Cutaneous androgen metabolism: basic research and clinical perspectives

The skin, especially the pilosebaceous unit composed of sebaceous glands and hair follicles, can synthesize androgens de novo from cholesterol or by locally converting circulating weaker androgens to more potent ones. As in other classical steroidogenic organs, the same six major enzyme systems are involved in cutaneous androgen metabolism, namely steroid sulfatase, 3beta-hydroxy-steroid dehydrogenase, 17beta-hydroxysteroid dehydrogenase, steroid 5alpha-reductase, 3alpha-hydroxysteroid dehydrogenase, and aromatase. Steroid sulfatase, together with P450 side chain cleavage enzyme and P450 17-hydroxylase, was found to reside in the cytoplasm of sebocytes and keratinocytes. Strong steroid sulfatase immunoreactivity was observed in the lesional skin but not in unaffected skin of acne patients. 3beta-hydroxysteroid dehydrogenase has been mainly immunolocalized to sebaceous glands, with the type 1 being the key cutaneous isoenzyme. The type 2 17beta-hydroxysteroid dehydrogenase isoenzyme predominates in sebaceous glands and exhibits greater reductive activity in glands from facial areas compared with acne nonprone areas. In hair follicles, 17beta-hydroxysteroid dehydrogenase was identified mainly in outer root sheath cells. The type 1 5alpha-reductase mainly occurs in the sebaceous glands, whereby the type II isoenzyme seems to be localized in the hair follicles. 3alpha-hydroxysteroid dehydrogenase converts dihydrotestosterone to 3alpha-androstanediol, and the use of 3alpha-androstanediol glucuronide serum level to reflect the hyperandrogenic state in hirsute women may be a reliable parameter, especially for idiopathic hirsutism. In acne patients it is still controversial if 3alpha-androstanediol glucuronide or androsterone glucuronide could serve as suitable serum markers for measuring androgenicity. Aromatase, localized to sebaceous glands and to both outer as well as inner root sheath cells of anagen terminal hair follicles, may play a "detoxifying" role by removing excess androgens. Pharmacologic development of more potent specific isoenzyme antagonists may lead to better clinical treatment or even prevention of androgen-dependent dermatoses.

Somatic and germinal mosaicism for the steroid sulfatase gene deletion in a steroid sulfatase deficiency carrier

Steroid sulfatase deficiency results in X-linked ichthyosis, an inborn error of metabolism in which the principal molecular defect is the complete deletion of the steroid sulfatase gene and flanking markers. Mosaicism for the steroid sulfatase gene has not yet been reported in X-linked ichthyosis. In this study we describe an X-linked ichthyosis patient with complete deletion of the steroid sulfatase gene and his mother with somatic and germinal mosaicism for this molecular defect. The family (X-linked ichthyosis patient, grandmother, mother, and sister) was analyzed through steroid sulfatase enzyme assay, polymerase chain reaction, DNA markers, and fluorescence in situ hybridization of the steroid sulfatase gene. Steroid sulfatase activity was undetectable in the X-linked ichthyosis patient, very low in the mother, and normal in the grandmother and sister. The X-linked ichthyosis patient showed a 2 Mb deletion of the steroid sulfatase gene and flanking regions from 5'DXS1139 to 3'DXF22S1. The mother showed one copy of the steroid sulfatase gene in 98.5% of oral cells and in 80% of leukocytes. The grandmother and sister showed two copies of the steroid sulfatase gene. The origin of the X chromosome with the deletion of the steroid sulfatase gene corresponded to the grandfather of the proband. We report the first case of somatic and germinal mosaicism of the steroid sulfatase gene in an X-linked ichthyosis carrier and propose DNA slippage as the most plausible mechanism in the genesis of this mosaicism.

Carrier identification by FISH analysis in isolated cases of X-linked ichthyosis

X-linked ichthyosis (XLI) is an inborn error of metabolism due to steroid sulfatase (STS) deficiency. STS assay and FISH are useful in diagnosing carrier status of XLI. Biochemical analysis appears to indicate that most sporadic cases are inherited. Since this method does not seem to be completely reliable in recognizing XLI-carriers, the aim of the present study was to corroborate by FISH whether or not most sporadic cases of XLI had de novo mutations. XLI patients were classified through STS assay and PCR amplification of 5'-3' ends of the STS gene. XLI patients had undetectable levels of STS activity and complete deletion of the STS gene. Patients' mothers were studied through STS assay and FISH. Nine out of 12 mothers presented an STS activity compatible with XLI-carrier state. These mothers also had only one copy of the STS gene, indicating that they carry the primary gene defect. One mother had normal STS activity but only one copy of the STS gene. This data corroborated that most sporadic cases do not represent de novo mutations, and that FISH must be included in the analysis of mothers of sporadic cases when they present with normal STS activity, in order to correctly diagnose the XLI carrier state.

Deletion of exons 1-5 of the STS gene causing X-linked ichthyosis

X-linked ichthyosis is an inherited disorder due to steroid sulfatase deficiency. It is clinically characterized by dark, adhesive, and regular scales of the skin. Most X-linked ichthyosis patients present large deletions of the STS gene and flanking markers; a minority show a point mutation or partial deletion of the STS gene. In this study we analyzed the STS gene in a family with simultaneous occurrence of X-linked ichthyosis and ichthyosis vulgaris. X-linked ichthyosis diagnosis was confirmed through steroid sulfatase assay in leukocytes using 7-[3H]-dehydroepiandrosterone sulfate as a substrate. Exons 1, 2, 5, and 6-10, and the 5' flanking markers DXS1130, DXS1139, and DXS996 of the STS gene were analyzed by polymerase chain reaction. X-linked ichthyosis patients of the family (n = 4 males) had undetectable levels of STS activity (0.00 pmol per mg protein per h). The DNA analysis showed that only exons 6-10 and the 5' flanking markers of the STS gene were present. We report the first partial deletion of the STS gene spanning exons 1-5 in X-linked ichthyosis patients.

Novel point mutations in the steroid sulfatase gene in patients with X-linked ichthyosis: transfection analysis using the mutated genes

X-linked ichthyosis is caused by steroid sulfatase deficiency which results from abnormalities in its coding gene. The majority of X-linked ichthyosis patients ( approximately 90%) have complete or partial deletions of the steroid sulfatase gene. In this study, we examined the mutations of the steroid sulfatase gene in two unrelated X-linked ichthyosis patients without complete deletion of the gene. Polymerase chain reaction-single-strand conformation polymorphism and direct sequencing analyses showed that each patient has a different single base pair substitution within exon 8 encoding the C-terminal half of the steroid sulfatase polypeptide. Both mutations resulted in the transversion of functional amino acids: a G-->C substitution at nucleotide 1344, causing a predicted change of a glycine to an arginine, and a C-->T substitution at nucleotide 1371, causing a change from a glutamine to a stop codon. In vitro steroid sulfatase cDNA expression using site-directed mutagenesis revealed that these mutations are in fact pathogenic and reflect the levels of steroid sulfatase enzyme activities in each of the X-linked ichthyosis patients.

Deletion patterns of the STS gene and flanking sequences in Israeli X-linked ichthyosis patients and carriers: analysis by polymerase chain reaction and fluorescence in situ hybridization techniques

BACKGROUND

Deletion of the entire steroid sulfatase (STS) gene is the most common molecular defect in X-linked ichthyosis (XLI) patients. Usually, additional flanking sequences are also missing. The aim of this study was to estimate the extent of deletions in an ethnically heterogeneous population of Israeli XLI patients.

METHODS

Multiplex polymerase chain reaction (PCR) and fluorescence in situ hybridization (FISH) techniques were applied in the analysis of blood samples of 24 patients and amniotic cells of seven affected fetuses from 22 unrelated families.

RESULTS

In 19 families, a large deletion of the 2-3 megabase was found. It included the whole STS gene and spanned adjacent areas up- and downstream between the loci DXS 1139 and DXS 1132. Two unrelated families of Iraqi ancestry had a partial deletion of the gene and its centromeric adjacent sequence. In another family, the telomeric end of the extragenic segment was only partially missing. Application of FISH on metaphase blood cells and interphase amniotic cells confirmed the diagnosis of XLI in all patients, except the three with partial intragenic deletion. In those cases, the remaining fraction of the gene was sufficient to provide a false negative result. Diagnosis of carriers and prenatal diagnosis in uncultured cells was applicable only by FISH.

CONCLUSIONS

Our study revealed a remarkable heterogeneity in the deletion pattern among Israeli patients with XLI. This heterogeneity could not be attributed to specific ethnic groups because of the small size of the study group. More studies involving patients of various ancestries should be carried out. In addition, this study demonstrated the usefulness of the FISH technique in the prenatal diagnosis of fetuses with suspected XLI.

Mutation report: a novel partial deletion of exons 2-10 of the STS gene in recessive X-linked ichthyosis

X-linked ichthyosis is an inherited disease due to steroid sulfatase deficiency. Onset is at birth or early after birth with dark, regular, and adherent scales of skin. Approximately 85%-90% of X-linked ichthyosis patients have large deletions of the STS gene and flanking sequences. Three patients have been identified with partial deletions of the gene. Two deletions have been found at the 3' extreme and the other one implicating exons 2-5. This study describes a novel partial deletion of the STS gene in an X-linked ichthyosis patient. The subject was classified through steroid sulfatase assay in leukocytes using 7-[3H]-dehydroepiandrosterone sulfate as a substrate. Exons 1, 2, 5, and 7-10, and 3' flanking sequences DXS1131, DXS1133, DXS237, DXS1132, DXF22S1, and DXS278 of the STS gene were analyzed through polymerase chain reaction. The DNA analysis showed that exon 1 and 3' flanking sequences from DXS237 to DXS278 were present. In this study we report the fourth partial deletion of the STS gene and the first spanning exons 2-10 in X-linked ichthyosis patients.

X-linked ichthyosis: an update

X-linked ichthyosis is a genetic disorder of keratinization characterized by a generalized desquamation of large, adherent, dark brown scales. Extracutaneous manifestations include corneal opacity and cryptorchidism. Since 1978 it has been known that a deficit in steroid sulphatase enzyme (STS) is responsible for the abnormal cutaneous scaling, although the exact physiological mechanism remains uncertain. The STS gene has been mapped to the distal part of the short arm of the X chromosome. Interestingly, this region escapes X chromosome inactivation and has the highest ratio of chromosomal deletions among all genetic disorders, complete deletions having been found in up to 90% of patients. Diagnosis of patients with X-linked ichthyosis and female carriers is based on biochemical and genetic analysis. The latter currently seems to be the most accurate method in the majority of cases.

Inherited disorders of epidermal keratinization

There have been a number of major discoveries recently in the field of dermatological science which have enabled us to determine the causes of inherited skin diseases of previously unknown etiology. In this paper we will review some important aspects of the biology of epidermal differentiation and the recent advances in understanding of the molecular mechanism underlying genetic diseases of keratinization.