Functional disomies of the X chromosome influence the cell selection and hence the X inactivation pattern in females with balanced X-autosome translocations: a review of 122 cases

Late diagnosis of the X-linked MCT8 deficiency (Allan-Herndon-Dudley syndrome) in a teenage girl with primary ovarian insufficiency

OBJECTIVES

To report an unusual case of MCT8 deficiency (Allan-Herndon-Dudley syndrome), an X-linked condition caused by pathogenic variants in the SLC16A2 gene. Defective transport of thyroid hormones (THs) in this condition leads to severe neurodevelopmental impairment in males, while heterozygous females are usually asymptomatic or have mild TH abnormalities.

CASE PRESENTATION

A girl with profound developmental delay, epilepsy, primary amenorrhea, elevated T3, low T4 and free T4 levels was diagnosed with MCT8-deficiency at age 17 years, during evaluation for primary ovarian insufficiency (POI). Cytogenetic analysis demonstrated balanced t(X;16)(q13.2;q12.1) translocation with a breakpoint disrupting SLC16A2. X-chromosome inactivation studies revealed a skewed inactivation of the normal X chromosome.

CONCLUSIONS

MCT8-deficiency can manifest clinically and phenotypically in women with SLC16A2 aberrations when nonrandom X inactivation occurs, while lack of X chromosome integrity due to translocation can cause POI.

Trisomy silencing by XIST: translational prospects and challenges

XIST RNA is heavily studied for its role in fundamental epigenetics and X-chromosome inactivation; however, the translational potential of this singular RNA has been much less explored. This article combines elements of a review on XIST biology with our perspective on the translational prospects and challenges of XIST transgenics. We first briefly review aspects of XIST RNA basic biology that are key to its translational relevance, and then discuss recent efforts to develop translational utility of XIST for chromosome dosage disorders, particularly Down syndrome (DS). Remarkably, it was shown in vitro that expression of an XIST transgene inserted into one chromosome 21 can comprehensively silence that chromosome and "dosage compensate" Trisomy 21, the cause of DS. Here we summarize recent findings and discuss potential paths whereby ability to induce "trisomy silencing" can advance translational research for new therapeutic strategies. Despite its common nature, the underlying biology for various aspects of DS, including cell types and pathways impacted (and when), is poorly understood. Recent studies show that an inducible iPSC system to dosage-correct chromosome 21 can provide a powerful approach to unravel the cells and pathways directly impacted, and the developmental timing, information key to design pharmacotherapeutics. In addition, we discuss prospects of a more far-reaching and challenging possibility that XIST itself could be developed into a therapeutic agent, for targeted cellular "chromosome therapy". A few rare case studies of imbalanced X;autosome translocations indicate that natural XIST can rescue an otherwise lethal trisomy. The potential efficacy of XIST transgenes later in development faces substantial biological and technical challenges, although recent findings are encouraging, and technology is rapidly evolving. Hence, it is compelling to consider the transformative possibility that XIST-mediated chromosome therapy may ultimately be developed, for specific pathologies seen in DS, or other duplication disorders.

Population variability in X-chromosome inactivation across 9 mammalian species

One of the two X chromosomes in female mammals is epigenetically silenced in embryonic stem cells by X chromosome inactivation (XCI). This creates a mosaic of cells expressing either the maternal or the paternal X allele. The XCI ratio, the proportion of inactivated parental alleles, varies widely among individuals, representing the largest instance of epigenetic variability within mammalian populations. While various contributing factors to XCI variability are recognized, namely stochastic and/or genetic effects, their relative contributions are poorly understood. This is due in part to limited cross-species analysis, making it difficult to distinguish between generalizable or species-specific mechanisms for XCI ratio variability. To address this gap, we measured XCI ratios in nine mammalian species (9,143 individual samples), ranging from rodents to primates, and compared the strength of stochastic models or genetic factors for explaining XCI variability. Our results demonstrate the embryonic stochasticity of XCI is a general explanatory model for population XCI variability in mammals, while genetic factors play a minor role.

Variability of cross-tissue X-chromosome inactivation characterizes timing of human embryonic lineage specification events

X-chromosome inactivation (XCI) is a random, permanent, and developmentally early epigenetic event that occurs during mammalian embryogenesis. We harness these features to investigate characteristics of early lineage specification events during human development. We initially assess the consistency of X-inactivation and establish a robust set of XCI-escape genes. By analyzing variance in XCI ratios across tissues and individuals, we find that XCI is shared across all tissues, suggesting that XCI is completed in the epiblast (in at least 6-16 cells) prior to specification of the germ layers. Additionally, we exploit tissue-specific variability to characterize the number of cells present during tissue-lineage commitment, ranging from approximately 20 cells in liver and whole blood tissues to 80 cells in brain tissues. By investigating the variability of XCI ratios using adult tissue, we characterize embryonic features of human XCI and lineage specification that are otherwise difficult to ascertain experimentally.

Allelic and dosage effects of NHS in X-linked cataract and Nance-Horan syndrome: a family study and literature review

Nance–Horan syndrome (NHS) is a rare X-linked dominant disorder caused by mutation in the NHS gene on chromosome Xp22.13. (OMIM 302350). Classic NHS manifested in males is characterized by congenital cataracts, dental anomalies, dysmorphic facial features and occasionally intellectual disability. Females typically have a milder presentation. The majority of reported cases of NHS are the result of nonsense mutations and small deletions. Isolated X-linked congenital cataract is caused by non-recurrent rearrangement-associated aberrant NHS transcription. Classic NHS in females associated with gene disruption by balanced X-autosome translocation has been infrequently reported. We present a familial NHS associated with translocation t(X;19) (Xp22.13;q13.1). The proband, a 28-year-old female, presented with intellectual disability, dysmorphic features, short stature, primary amenorrhea, cleft palate, and horseshoe kidney, but no NHS phenotype. A karyotype and chromosome microarray analysis (CMA) revealed partial monosomy Xp/partial trisomy 19q with the breakpoint at Xp22.13 disrupting the NHS gene. Family history revealed congenital cataracts and glaucoma in the patient’s mother, and congenital cataracts in maternal half-sister and maternal grandmother. The same balanced translocation t(X;19) was subsequently identified in both the mother and maternal half-sister, and further clinical evaluation of the maternal half-sister made a diagnosis of NHS. This study describes the clinical implication of NHS gene disruption due to balanced X-autosome translocations as a unique mechanism causing Nance–Horan syndrome, refines dose effects of NHS on disease presentation and phenotype expressivity, and justifies consideration of karyotype and fluorescence in situ hybridization (FISH) analysis for female patients with familial NHS if single-gene analysis of NHS is negative.

Detection of a novel unbalanced X;21 translocation in a girl with Turner syndrome phenotype

OBJECTIVE

To describe a novel unbalanced X;21 translocation resulting in a derivative pseudodicentric chromosome X;21 lacking the critical region for ovarian development and function, in a 16-year-old girl referred for cytogenetic analysis due to primary amenorrhea and Turner-like features.

METHODS

Cytogenetic analysis of the proband and her parents was performed on peripheral blood lymphocytes by GTG banding. Molecular cytogenetic FISH analysis was performed on metaphase preparations, using X chromosome centromeric probe and telomeric and pancentromeric peptide nucleic acid (PNA) analog probes. The HUMARA assay as well as methylation studies for PCSK1N and FMR-1 loci were performed.

RESULTS

Cytogenetic analysis revealed a de novo unbalanced X;21 translocation, described as 45,X,der(X)t(X;21)(q22.2;p11.2),-21. FISH analysis showed that the derivative X chromosome carried both the X and 21 centromeres, as well as, the Xp and 21q telomeres. The karyotype was thus reevaluated as 45,X,psu dic(21;X)(21qter→21p13::Xq22.2→Xpter),-21. X inactivation studies revealed that the derivative chromosome was of paternal origin and confirmed the selective inactivation of the derivative X segment of the pseudodicentric chromosome.

CONCLUSIONS

Primary amenorrhea and other Turner-like characteristics of the proband are apparently due to the loss of the Xq22.2→Xqter critical region which contains critical genes for the ovarian development and function. The chromosome X segment of the derivative pseudodicentric chromosome is selectively inactivated, but inactivation does not seem to spread onto the translocated chromosome 21, accounting probably for the lack of severe clinical consequences which would result from monosomy 21.

Reproductive risks and preimplantation genetic testing intervention for X-autosome translocation carriers

RESEARCH QUESTION

What is the genetic cause of multiple congenital disabilities in a girl with a maternal balanced X-autosome translocation [t(X-A)]? Is preimplantation genetic testing (PGT), to distinguish non-carrier from euploid/balanced embryos and prioritize transfer, an effective and applicable strategy for couples with t(X-A)?

DESIGN

Karyotype analysis, whole-exome sequencing and X inactivation analysis were performed for a girl with congenital cardiac anomalies, language impairment and mild neurodevelopmental delay. PGT based on next-generation sequencing after microdissecting junction region (MicroSeq) to distinguish non-carrier and carrier embryos was used in three couples with a female t(X-A) carrier (cases 1-3).

RESULTS

The girl carried a maternal balanced translocation 46,X,t(X;1)(q28;p31.1). Whole-exome sequencing revealed no monogenic mutation related to her phenotype, but she carried a rare skewed inactivation of the translocated X chromosome that spread to the adjacent interstitial 1p segment, contrary to her mother. All translocation breakpoints in cases 1-3 were successfully identified and each couple underwent one PGT cycle. Thirty oocytes were retrieved, and 13 blastocysts were eligible for biopsy, of which six embryos had a balanced translocation and only four were non-carriers. Three cryopreserved embryo transfers with non-carrier status embryos resulted in the birth of two healthy children (one girl and one boy), who were subsequently confirmed to have normal karyotypes.

CONCLUSIONS

This study reported a girl with multiple congenital disabilities associated with a maternal balanced t(X-A) and verified that the distinction between non-carrier and carrier embryos is an effective and applicable strategy to avoid transferring genetic and reproductive risks to the offspring of t(X-A) carriers.

Novel Complex Unbalanced Dicentric X-Autosome Rearrangement in a Thoroughbred Mare with a Mild Effect on the Phenotype

Complex structural X chromosome abnormalities are rare in humans and animals, and not recurrent. Yet, each case provides a fascinating opportunity to evaluate X chromosome content and functional status in relation to the effect on the phenotype. Here, we report the first equine case of a complex unbalanced X-autosome rearrangement in a healthy but short in stature Thoroughbred mare. Studies of about 200 cells by chromosome banding and FISH revealed an abnormal 2n = 63,X,der(X;16) karyotype with a large dicentric derivative chromosome (der). The der was comprised of normal Xp material, a palindromic duplication of Xq12q21, and a translocation of chromosome 16 to the inverted Xq12q21 segment by the centromere, whereas the distal Xq22q29 was deleted from the der. Microsatellite genotyping determined a paternal origin of the der. While there was no option to experimentally investigate the status of X chromosome inactivation (XCI), the observed mild phenotype of this case suggested the following scenario to retain an almost normal genetic balance: active normal X, inactivated X-portion of the der, but without XCI spreading into the translocated chromosome 16. Cases like this present unique resources to acquire information about species-specific features of X regulation and the role of X-linked genes in development, health, and disease.

Coffin-Lowry syndrome in a girl with 46,XX,t(X;11)(p22;p15)dn: Identification of RPS6KA3 disruption by whole genome sequencing

We report a Japanese girl with Coffin-Lowry syndrome phenotype such as hypertelorism, hypodontia, and tapering fingers and 46,XX,t(X;11)(p22;p15)dn. Whole genome sequencing revealed RPS6KA3 disruption by the translocation, and X-inactivation analysis indicated preferential inactivation of the normal X chromosome. The results explain the development of an X-linked disease in this girl.

Breakpoint mapping at nucleotide resolution in X-autosome balanced translocations associated with clinical phenotypes

Precise breakpoint mapping of balanced chromosomal rearrangements is crucial to identify disease etiology. Ten female patients with X-autosome balanced translocations associated with phenotypic alterations were evaluated, by mapping and sequencing their breakpoints. The rearrangements' impact on the expression of disrupted genes, and inferred mechanisms of formation in each case were assessed. For four patients that presented one of the chromosomal breaks in heterochromatic and highly repetitive segments, we combined cytogenomic methods and short-read sequencing to characterize, at nucleotide resolution, breakpoints that occurred in reference genome gaps. Most of rearrangements were possibly formed by non-homologous end joining and have breakpoints at repeat elements. Seven genes were found to be disrupted in six patients. Six of the affected genes showed altered expression, and the functional impairment of three of them were considered pathogenic. One gene disruption was considered potentially pathogenic, and three had uncertain clinical significance. Four patients presented no gene disruptions, suggesting other pathogenic mechanisms. Four genes were considered potentially affected by position effect and the expression abrogation of one of them was confirmed. This study emphasizes the importance of breakpoint-junction characterization at nucleotide resolution in balanced rearrangements to reveal genetic mechanisms associated with the patients' phenotypes, mechanisms of formation that originated the rearrangements, and genomic nature of disrupted DNA sequences.

The decision on the embryo to transfer after Preimplantation Genetic Diagnosis for X-autosome reciprocal translocation in male carrier

Background

The aim of Preimplantation Genetic Diagnosis (PGD) on embryos produced in vitro is to identify the embryos without genetic or chromosomal defect from those embryos that will develop the genetic disease or are chromosomally abnormal. In case of PGD for structural chromosome indication (PGR-SR), the normal/balanced embryos are transferred in the maternal uterus. This protocol is valid and widely applied for autosomal chromosome translocation. But which embryo should be transferred after preimplantation genetic diagnosis (PGD-SR) for X-3 reciprocal translocation in male patient?

Case presentation

The female patient was 26 years old with normal 46,XX karyotype. The male patient had a karyotype with balanced translocation 46,Y,t(X;3)(p11.2;p14)mat, inherited from the mother. The female patient underwent two cycles of ovarian stimulation. In the first cycle, the metaphase II oocytes were vitrified, while in the second cycle they were used as fresh. ICSI was performed on vitrified/warmed and fresh oocytes. Embryos were biopsied at blastocyst stage. Chromosomal analysis was performed by Next Generation Sequencing.

Eleven blastocysts were biopsied from 23 vitrified/warmed and fresh metaphase II oocytes. Two embryos were diagnosed 46,XY; two embryos were diagnosed 46,XX; four embryos were diagnosed with unbalanced translocations and three embryos were diagnosed aneuploid. We knew that the two embryos diagnosed as 46,XX inherited the balanced translocation from the father and the two embryos diagnosed as 46,XY had a normal karyotype. It was explain to the couple that the phenotype of balanced translocated female embryos cannot be predicted because of the random inactivation of X chromosome and that could also occur on the der(X). The couple asked to have a 46,XY embryo transferred. Clinical pregnancy was obtained and non invasive prenatal test confirmed PGD-SR result.

Conclusions

Proposing PGD-SR for gonosome-autosome reciprocal translocation implies the risk to exclude balanced translocated female embryos with a normal phenotype for transfer because the early and late normal development at post-natal stage cannot be predicted based on the only chromosomal analysis.

Mild phenotypes associated with an unbalanced X-autosome translocation, 46,X,der(X)t(X;8)(q28;q13)

Unbalanced X-autosome translocation can result in various phenotypic manifestations. We present the first case of 46,X,der(X)t(X;8)(q28;q13) in a 34-year-old female with relatively mild manifestations, including congenital heart disease, epicanthal fold, mild intellectual disability, and menstrual irregularity. Our findings expand the known spectrum of unbalanced X-autosome translocations, for improved clinical management.

A foetus with 18p11.32-q21.2 duplication and Xp22.33-p11.1 deletion derived from a maternal reciprocal translocation t(X;18)(q13;q21.3)

BACKGROUND

Non-invasive prenatal testing (NIPT) evaluates circulating cell-free DNA (cfDNA) and has been widely applied, with highly accurate results for detecting foetal trisomies 21, 18 and 13. Recently, increasing attention has been paid to the clinical application of the non-invasive detection of foetal sub-chromosomal duplications and deletions beyond common aneuploidies.

CASE PRESENTATION

A 32-year-old healthy pregnant woman was referred to the Medical Genetic Centre of Ganzhou Maternal and Child Health Care Hospital. As routine practice, ultrasound examination at a gestational age of 16 weeks showed that the foetus is normal. To avoid invasive prenatal diagnosis procedures, an NIPT was offered to further screen for common foetal chromosomal abnormalities. The result showed that there was an approximately 50.94 Mb duplication in p11.32-q21.2 of chromosome 18 and an approximately 58.46 Mb deletion in p22.33-p11.1 of chromosome X. In addition, the chromosome karyotypes of the parents and foetus were also analysed. Chromosome karyotype analysis results showed that foetal karyotype was 46,X,der(18), the maternal karyotype was 46,XX,t(X;18)(q13;q21.3), and the paternal karyotype revealed no obvious abnormality.

CONCLUSION

In this case, we successfully detected a healthy pregnant woman with balanced translocation X;18(q13;q21.3) and described the foetal karyotype as 46,X,der(18)t(X;18)(q11;q21.1)mat. Our report illustrated these cases which present complex X;autosome balance translocation and X;autosome unbalance translocation which may contribute to severe clinical phenotypes. In addition, our report also proved that the interruption of genes in the Xq critical region is not only reason of primary infertility. Finally, we prompted that NIPT might play a role in the first trimester screening of sub-chromosomal rearrangement.

Human X chromosome inactivation and reactivation: implications for cell reprogramming and disease

X-chromosome inactivation (XCI) is an exemplar of epigenetic regulation that is set up as pluripotent cells differentiate. Once established, XCI is stably propagated, but can be reversed in vivo or by pluripotent reprogramming in vitro Although reprogramming provides a useful model for inactive X (Xi) reactivation in mouse, the relative instability and heterogeneity of human embryonic stem (ES) cells and induced pluripotent stem cells hampers comparable progress in human. Here we review studies aimed at reactivating the human Xi using different reprogramming strategies. We outline our recent results using mouse ES cells to reprogramme female human fibroblasts by cell-cell fusion. We show that pluripotent reprogramming induces widespread and rapid chromatin remodelling in which the human Xi loses XIST and H3K27m3 enrichment and selected Xi genes become reactivated, ahead of mitotic division. Using RNA sequencing to map the extent of human Xi reactivation, and chromatin-modifying drugs to potentiate reactivation, we outline how this approach could be used to better design strategies to re-express human X-linked loci. As cell fusion induces the expression of human pluripotency genes that represent both the 'primed' and 'naive' states, this approach may also offer a fresh opportunity to segregate human pluripotent states with distinct Xi expression profiles, using single-cell-based approaches.This article is part of the themed issue 'X-chromosome inactivation: a tribute to Mary Lyon'.

Functional monosomy of 6q27-qter and functional disomy of Xpter-p22.11 due to X;6 translocation with an atypical X-inactivation pattern

Pattern of X chromosome inactivation (XCI) is typically random in females. However, chromosomal rearrangements affecting the X chromosome can result in XCI skewing due to cell growth disadvantage. In case of an X;autosome translocation, this usually leads to an XCI pattern of 100:0 with the derivative X being the active one in the majority of females. A de novo balanced X;6 translocation [46,X,t(X;6)(p22.1;q27)] and a completely skewed XCI pattern (100:0) were detected in a female patient with microcephaly, cerebellar vermis hypoplasia, heart defect, and severe developmental delay. We mapped the breakpoint regions using fluorescence in situ hybridization and found the X-linked gene POLA1 to be disrupted. POLA1 codes for the catalytic subunit of the polymerase α-primase complex which is responsible for initiation of the DNA replication process; absence of POLA1 is probably incompatible with life. Consequently, by RBA banding we determined which of the X chromosomes was the active one in the patient. In all examined lymphocytes the wild-type X chromosome was active. We propose that completely skewed XCI favoring the normal X chromosome resulted from death of cells with an active derivative X that was caused by a non-functional POLA1 gene. In summary, we conclude that functional monosomy of 6q27-qter and functional disomy of Xpter-p22.11 are responsible for the clinical phenotype of the patient. This case demonstrates the importance of determining which one of the X chromosomes underwent inactivation to correlate clinical features of a female with an X;autosome translocation with the nature of the genetic alteration.

Determining the role of skewed X-chromosome inactivation in developing muscle symptoms in carriers of Duchenne muscular dystrophy

Duchenne and Becker dystrophinopathies (DMD and BMD) are X-linked recessive disorders caused by mutations in the dystrophin gene that lead to absent or reduced expression of dystrophin in both skeletal and heart muscles. DMD/BMD female carriers are usually asymptomatic, although about 8 % may exhibit muscle or cardiac symptoms. Several mechanisms leading to a reduced dystrophin have been hypothesized to explain the clinical manifestations and, in particular, the role of the skewed XCI is questioned. In this review, the mechanism of XCI and its involvement in the phenotype of BMD/DMD carriers with both a normal karyotype or with X;autosome translocations with breakpoints at Xp21 (locus of the DMD gene) will be analyzed. We have previously observed that DMD carriers with moderate/severe muscle involvement, exhibit a moderate or extremely skewed XCI, in particular if presenting with an early onset of symptoms, while DMD carriers with mild muscle involvement present a random XCI. Moreover, we found that among 87.1 % of the carriers with X;autosome translocations involving the locus Xp21 who developed signs and symptoms of dystrophinopathy such as proximal muscle weakness, difficulty to run, jump and climb stairs, 95.2 % had a skewed XCI pattern in lymphocytes. These data support the hypothesis that skewed XCI is involved in the onset of phenotype in DMD carriers, the X chromosome carrying the normal DMD gene being preferentially inactivated and leading to a moderate-severe muscle involvement.

Is the resulting phenotype of an embryo with balanced X-autosome translocation, obtained by means of preimplantation genetic diagnosis, linked to the X inactivation pattern?

OBJECTIVE

To examine if a balanced female embryo with X-autosome translocation could, during its subsequent development, express an abnormal phenotype.

DESIGN

Preimplantation genetic diagnosis (PGD) analysis on two female carriers with maternal inherited X-autosome translocations.

SETTING

Infertility center and genetic laboratory in a public hospital.

PATIENT(S)

Two female patients carriers undergoing PGD for a balanced X-autosome translocations: patient 1 with 46,X,t(X;2)(q27;p15) and patient 2 with 46,X,t(X;22)(q28;q12.3).

INTERVENTION(S)

PGD for balanced X-autosome translocations.

MAIN OUTCOME MEASURE(S)

PGD outcomes, fluorescence in situ hybridization in biopsied embryos and meiotic segregation patterns analysis of embryos providing from X-autosome translocation carriers.

RESULT(S)

Controlled ovarian stimulation facilitated retrieval of a correct number of oocytes. One balanced embryo per patient was transferred and one developed, but the patient miscarried after 6 weeks of amenorrhea. In X-autosome translocation carriers, balanced Y-bearing embryos are most often phenotypically normal and viable. An ambiguous phenotype exists in balanced X-bearing embryos owing to the X inactivation mechanism. In 46,XX embryos issued from an alternate segregation, der(X) may be inactivated and partially spread transcriptional silencing into a translocated autosomal segment. Thus, the structural unbalanced genotype could be turned into a viable functional balanced one. It is relevant that a discontinuous silencing is observed with a partial and unpredictable inactivation of autosomal regions. Consequently, the resulting phenotype remains a mystery and is considered to be at risk of being an abnormal phenotype in the field of PGD.

CONCLUSION(S)

It is necessary to be cautious regarding to PGD management for this type of translocation, particularly in transferred female embryos.

Genetic mechanisms leading to primary amenorrhea in balanced X-autosome translocations

OBJECTIVE

To map the X-chromosome and autosome breakpoints in women with balanced X-autosome translocations and primary amenorrhea, searching candidate genomic loci for female infertility.

DESIGN

Retrospective and case-control study.

SETTING

University-based research laboratory.

PATIENT(S)

Three women with balanced X-autosome translocation and primary amenorrhea.

INTERVENTION(S)

Conventional cytogenetic methods, genomic array, array painting, fluorescence in situ hybridization, and quantitative reverse transcription-polymerase chain reaction.

MAIN OUTCOME MEASURE(S)

Karyotype, copy number variation, breakpoint mapping, and gene expression levels.

RESULT(S)

All patients presented with breakpoints in the Xq13q21 region. In two patients, the X-chromosome breakpoint disrupted coding sequences (KIAA2022 and ZDHHC15 genes). Although both gene disruptions caused absence of transcription in peripheral blood, there is no evidence that supports the involvement of these genes with ovarian function. The ZDHHC15 gene belongs to a conserved syntenic region that encompasses the FGF16 gene, which plays a role in female germ line development. The break in the FGF16 syntenic block may have disrupted the interaction between the FGF16 promoter and its cis-regulatory element. In the third patient, although both breakpoints are intergenic, a gene that plays a role in the DAX1 pathway (FHL2 gene) flanks distally the autosome breakpoint. The FHL2 gene may be subject to position effect due to the attachment of an autosome segment in Xq21 region.

CONCLUSION(S)

The etiology of primary amenorrhea in balanced X-autosome translocation patients may underlie more complex mechanisms than interruption of specific X-linked candidate genes, such as position effect. The fine mapping of the rearrangement breakpoints may be a tool for identifying genetic pathogenic mechanisms for primary amenorrhea.

X-chromosomal inactivation directly influences the phenotypic manifestation of X-linked protoporphyria

X-linked protoporphyria (XLP), a rare erythropoietic porphyria, results from terminal exon gain-of-function mutations in the ALAS2 gene causing increased ALAS2 activity and markedly increased erythrocyte protoporphyrin levels. Patients present with severe cutaneous photosensitivity and may develop liver dysfunction. XLP was originally reported as X-linked dominant with 100% penetrance in males and females. We characterized 11 heterozygous females from six unrelated XLP families and show markedly varying phenotypic and biochemical heterogeneity, reflecting the degree of X-chromosomal inactivation of the mutant gene. ALAS2 sequencing identified the specific mutation and confirmed heterozygosity among the females. Clinical history, plasma and erythrocyte protoporphyrin levels were determined. Methylation assays of the androgen receptor and zinc-finger MYM type 3 short tandem repeat polymorphisms estimated each heterozygotes X-chromosomal inactivation pattern. Heterozygotes with equal or increased skewing, favoring expression of the wild-type allele had no clinical symptoms and only slightly increased erythrocyte protoporphyrin concentrations and/or frequency of protoporphyrin-containing peripheral blood fluorocytes. When the wild-type allele was preferentially inactivated, heterozygous females manifested the disease phenotype and had both higher erythrocyte protoporphyrin levels and circulating fluorocytes. These findings confirm that the previous dominant classification of XLP is inappropriate and genetically misleading, as the disorder is more appropriately designated XLP.

Premature ovarian failure caused by a heterozygous missense mutation in POF1B and a reciprocal translocation 46,X,t(X;3)(q21.1;q21.3)

In a patient affected by premature ovarian failure, a reciprocal translocation between chromosomes X and 3 and an additional heterozygous missense mutation in the X-linked gene POF1B were detected. Homozygosity for POF1B mutations is well-known to be associated with premature ovarian failure. In this case, the rare combination of skewed X inactivation due to the reciprocal translocation involving one X chromosome and heterozygosity for a known POF1B mutation explains the phenotype.

The incidence and type of chromosomal translocations from prenatal diagnosis of 3800 patients in the republic of macedonia

Robertsonian and reciprocal chromosomal translocations are the most frequent type of structural chromosomal aberrations in the human population. We report the frequency and type of detected translocations in 10 years of prenatal diagnosis of 3800 prenatal samples. The materials came from amniocentesis and chorionic villus samples (CVS). We detected seven Robertsonian translocations (0.18%), eight autosomal reciprocal translocations (0.21%) and one sex chromosome translocation (0.03%). The overall frequency of all translocations was 0.42%. Balanced state translocations were 0.29% and the frequency of translocations in an unbalanced state was 0.13%. There was one balanced de novo X-autosome translocation [46,X,t(X;10)(p11.23;q22.3)] and one balanced double translocation [46,XX,t(1;21);t(7;16)(1p21; 21q11) (7q31;16q23)] inherited from the mother. Most of the detected translocations were the result of unknown familial translocations, but some of them had been previously detected in one of the parents. In order to detect the recurrence risk for future pregnancies, we proposed genetic counseling in each of the cases and we established whether the parents were heterozygous for the same translocation. Histopatological findings for some unbalanced translocations correlated with phenotypes of detected unbalanced karyotypes.

Disruption of HDX gene in premature ovarian failure

We present a case of a 19-year-old phenotypically normal girl with premature ovarian failure. Cytogenetic analysis using G banding and fluorescence in situ hybridization (FISH) from cultured peripheral blood lymphocytes of the patient and the family revealed a de novo X;15 translocation and the imbalance to be 46,X,t(X;15)(Xpter → Xq21::15q11 → 15qter;15pter → 15q11::Xq21 → Xqter). ish (CEPX+, wep15+, ISNRPN+, PML+, D15S10+, wcp15-, SNRRN-, PML-)[20]. The X chromosome inactivation (XCI) assay revealed a completely skewed XCI pattern in which selective pressure favors an active maternal allele. The Affymetrix 2.7 M cytogenetics whole-Genome array confirmed the chromosomal imbalance and identified disruption of the HDX gene at Xq21, the translocation breakpoint.

Translocation t(X;11)(q22;q25) in a woman with premature ovarian failure

Genetic, autoimmune, environmental, iatrogenic, and idiopathic factors are known to cause premature ovarian failure (POF). This report describes an X;11 translocation, t(X;11)(q22;q25), in a woman diagnosed with POF. The FSH level was found to be elevated. Menstrual cycle was regular initially, and she had a spontaneous abortion at the 5th month of gestation at 16 years of age. Her mother was karyotypically normal while her father was not investigated. Male carriers of X;autosome translocations are mostly infertile, and hence the translocation is presumed to be of de novo origin. Fluorescence in situ hybridization using whole chromosome paint probes confirmed the rearrangement.

Cytogenetic abnormalities in 222 infertile men with azoospermia and oligospermia in Iran: Report and review

BACKGROUND:

Infertility affects approximately 10%-15% of couples in reproductive age. In half of the couples, causes are male-related, associated with impaired spermatogenesis. There is a complex correlation between genetics and infertility. Several factors affect on gametogenesis, from which factors that lead to chromosomal abnormalities are one of the best known. The aim of this study was to determine type and rate of chromosomal abnormalities in infertile azoospermic and oligospermic males in Iranian population.

MATERIALS AND METHODS:

The records of a total of 222 participants were evaluated retrospectively.

RESULTS:

As a whole we observed 13.96% chromosomal abnormality, from which 12.15% showed numerical and 1.8% showed structural abnormalities.

CONCLUSION:

Comparison of our results with the review of the literature shows a higher incidence (4- fold) of gonosomal, in particular, numerical gonosomal, chromosomal anomalies. Cytogenetic analysis is strongly suggested for infertile men, particularly in those who suffer from azoospermia.

Clinical correlation between premature ovarian failure and a chromosomal anomaly in a 22-year-old Caucasian woman: a case report

Introduction

Premature ovarian failure is defined as the cessation of ovarian activity before the age of 40 years. It is biochemically characterized by low levels of gonadal hormones (estrogens and inhibins) and high levels of gonadotropins (luteinizing hormone and follicle-stimulating hormone).

Case presentation

Our patient, a 22-year-old Caucasian woman under evaluation for infertility, had experienced secondary amenorrhea from the age of 18. No positive family history was noted regarding premature menopause. An examination of our patient’s karyotype showed the presence of a reciprocal translocation, apparently balanced, which had the X chromosome long arm (q13) and the 14 chromosome short arm (p12) with consequent karyotype: 46, X, t(X; 14)(q13;p12).

Conclusions

Our study has underlined that karyotyping is one of the fundamental investigations in the evaluation of amenorrhea. It highlighted a genetic etiology, in the form of a chromosomal abnormality, as the causal factor in amenorrhea.

Autism spectrum disorder in a girl with a de novo x;19 balanced translocation

Balanced X-autosome translocations are rare, and female carriers are a clinically heterogeneous group of patients, with phenotypically normal women, history of recurrent miscarriage, gonadal dysfunction, X-linked disorders or congenital abnormalities, and/or developmental delay. We investigated a patient with a de novo X;19 translocation. The six-year-old girl has been evaluated due to hyperactivity, social interaction impairment, stereotypic and repetitive use of language with echolalia, failure to follow parents/caretakers orders, inconsolable outbursts, and persistent preoccupation with parts of objects. The girl has normal cognitive function. Her measurements are within normal range, and no other abnormalities were found during physical, neurological, or dysmorphological examinations. Conventional cytogenetic analysis showed a de novo balanced translocation, with the karyotype 46,X,t(X;19)(p21.2;q13.4). Replication banding showed a clear preference for inactivation of the normal X chromosome. The translocation was confirmed by FISH and Spectral Karyotyping (SKY). Although abnormal phenotypes associated with de novo balanced chromosomal rearrangements may be the result of disruption of a gene at one of the breakpoints, submicroscopic deletion or duplication, or a position effect, X; autosomal translocations are associated with additional unique risk factors including X-linked disorders, functional autosomal monosomy, or functional X chromosome disomy resulting from the complex X-inactivation process.

Spread of X-chromosome inactivation into chromosome 15 is associated with Prader-Willi syndrome phenotype in a boy with a t(X;15)(p21.1;q11.2) translocation

X-chromosome inactivation (XCI) is an essential mechanism in females that compensates for the genome imbalance between females and males. It is known that XCI can spread into an autosome of patients with X;autosome translocations. The subject was a 5-year-old boy with Prader-Willi syndrome (PWS)-like features including hypotonia, hypo-genitalism, hypo-pigmentation, and developmental delay. G-banding, fluorescent in situ hybridization, BrdU-incorporated replication, human androgen receptor gene locus assay, SNP microarrays, ChIP-on-chip assay, bisulfite sequencing, and real-time RT-PCR were performed. Cytogenetic analyses revealed that the karyotype was 46,XY,der(X)t(X;15)(p21.1;q11.2),-15. In the derivative chromosome, the X and half of the chromosome 15 segments showed late replication. The X segment was maternal, and the chromosome 15 region was paternal, indicating its post-zygotic origin. The two chromosome 15s had a biparental origin. The DNA methylation level was relatively high in the region proximal from the breakpoint, and the level decreased toward the middle of the chromosome 15 region; however, scattered areas of hypermethylation were found in the distal region. The promoter regions of the imprinted SNRPN and the non-imprinted OCA2 genes were completely and half methylated, respectively. However, no methylation was found in the adjacent imprinted gene UBE3A, which contained a lower density of LINE1 repeats. Our findings suggest that XCI spread into the paternal chromosome 15 led to the aberrant hypermethylation of SNRPN and OCA2 and their decreased expression, which contributes to the PWS-like features and hypo-pigmentation of the patient. To our knowledge, this is the first chromosome-wide methylation study in which the DNA methylation level is demonstrated in an autosome subject to XCI.

X-chromosome inactivation: molecular mechanisms from the human perspective

X-chromosome inactivation is an epigenetic process whereby one X chromosome is silenced in mammalian female cells. Since it was first proposed by Lyon in 1961, mouse models have been valuable tools to uncover the molecular mechanisms underlying X inactivation. However, there are also inherent differences between mouse and human X inactivation, ranging from sequence content of the X inactivation center to the phenotypic outcomes of X-chromosome abnormalities. X-linked gene dosage in males, females, and individuals with X aneuploidies and X/autosome translocations has demonstrated that many human genes escape X inactivation, implicating cis-regulatory elements in the spread of silencing. We discuss the potential nature of these elements and also review the elements in the X inactivation center involved in the early events in X-chromosome inactivation.

Severe Alport syndrome in a young woman caused by a t(X;1)(q22.3;p36.32) balanced translocation

The course of renal involvement and hearing loss is much milder in most female X-linked Alport syndromes than in male patients. We examined the molecular mechanism of development of the disease in a female patient with severe Alport syndrome. The patient showed heavy proteinuria, hematuria, neurosensory hearing loss and primary amenorrhea. Renal biopsy findings of electron microscopy and immunostaining of the alpha5 chain of type IV collagen indicated a female X-linked Alport syndrome. G-banding chromosomal analysis showed a t(X;1)(q22.3;p36.32) balanced translocation. Analysis of the collagen type IV (COL4A5) gene by genomic DNA sequencing, complementary DNA (cDNA) sequencing and multiplex ligation-dependent probe amplification assay showed no mutations or deletions/duplications of the gene. However, fluorescence in situ hybridization using the probes for exon 1 and exon 51 of the COL4A5 gene showed disruption of one copy of the gene. Replication R-banding chromosomal analysis indicated preferential inactivation of the normal X chromosome. This is the first report of severe Alport syndrome in a female patient carrying a balanced translocation between the chromosome X and 1 producing the disruption of one copy of COL4A5 gene and silencing of the other copy because of preferential inactivation of the normal X chromosome. Chromosomal abnormalities should be considered in female patients with severe forms of Alport syndrome.

X chromosome inactivation in clinical practice

X chromosome inactivation (XCI) is the transcriptional silencing of the majority of genes on one of the two X chromosomes in mammalian females. Females are, therefore, mosaics for two cell lines, one with the maternal X and one with the paternal X as the active chromosome. The relative proportion of the two cell lines, the X inactivation pattern, may be analyzed by simple assays in DNA from available tissues. This review focuses on medical issues related to XCI in X-linked disorders, and on the value of X inactivation analysis in clinical practice.

Atypical X-chromosome inactivation in an X;1 translocation patient demonstrating Xq28 functional disomy

X-chromosome inactivation (XCI) is an epigenetic process used to regulate gene dosage in mammalian females by silencing genes on one X-chromosome. While the pattern of XCI is typically random in normal females, abnormalities of the X-chromosome may result in skewing due to disadvantaged cell growth. We describe a female patient with an X;1 translocation [46,X,t(X;1)(q28;q21)] and unusual pattern of XCI who demonstrates functional disomy of the Xq28 region distal to the translocation breakpoint. There was complete skewing of XCI in the patient, along with the atypical findings of an active normal X chromosome and an inactive derivative X. Characterization of the translocation revealed that the patient's Xq28 breakpoint interrupts the DKC1 gene. Molecular analysis of the breakpoint region revealed functional disomy of Xq28 genes distal to DKC1. We propose that atypical XCI occurred in the patient due to a post-inactivation cell selection mechanism likely initiated by disruption of DKC1. As a result, the pattern of XCI is opposite that of the expected for an X;autosome translocation. Therefore, we suggest the phenotypic abnormalities found in the patient are a result of functional disomy in the Xq28 region.

Lyonization effects of the t(X;16) translocation on the phenotypic expression in a rare female with Menkes disease

Menkes disease (MD) is a rare and severe X-linked recessive disorder of copper metabolism. The MD gene, ATP7A (ATPase Cu++ transporting alpha polypeptide), encodes an ATP-dependent copper-binding membrane protein. In this report, we describe a girl with typical clinical features of MD, carrying a balanced translocation between the chromosomes X and 16 producing the disruption of one copy of ATP7A gene and the silencing of the other copy because of the chromosome X inactivation. Fluorescence in situ hybridization experiments with bacterial derived artificial chromosome probes revealed that the breakpoints were located within Xq13.3 and 16p11.2. Replication pattern analysis demonstrated that the normal X chromosome was late replicating and consequently inactivated, whereas the der(X)t(X;16), bearing the disrupted ATP7A gene, was active. An innovative approach, based on FMR1 (fragile X mental retardation 1) gene polymorphism, has been used to disclose the paternal origin of the rearrangement providing a new diagnostic tool for determining the parental origin of defects involving the X chromosome and clarifying the mechanism leading to the cytogenetic rearrangement that occurred in our patient.

Distal Xq duplication and functional Xq disomy

Distal Xq duplications refer to chromosomal disorders resulting from involvement of the long arm of the X chromosome (Xq). Clinical manifestations widely vary depending on the gender of the patient and on the gene content of the duplicated segment. Prevalence of Xq duplications remains unknown. About 40 cases of Xq28 functional disomy due to cytogenetically visible rearrangements, and about 50 cases of cryptic duplications encompassing the MECP2 gene have been reported. The most frequently reported distal duplications involve the Xq28 segment and yield a recognisable phenotype including distinctive facial features (premature closure of the fontanels or ridged metopic suture, broad face with full cheeks, epicanthal folds, large ears, small and open mouth, ear anomalies, pointed nose, abnormal palate and facial hypotonia), major axial hypotonia, severe developmental delay, severe feeding difficulties, abnormal genitalia and proneness to infections. Xq duplications may be caused either by an intrachromosomal duplication or an unbalanced X/Y or X/autosome translocation. In XY males, structural X disomy always results in functional disomy. In females, failure of X chromosome dosage compensation could result from a variety of mechanisms, including an unfavourable pattern of inactivation, a breakpoint separating an X segment from the X-inactivation centre in cis, or a small ring chromosome. The MECP2 gene in Xq28 is the most important dosage-sensitive gene responsible for the abnormal phenotype in duplications of distal Xq. Diagnosis is based on clinical features and is confirmed by CGH array techniques. Differential diagnoses include Prader-Willi syndrome and Alpha thalassaemia-mental retardation, X linked (ATR-X). The recurrence risk is significant if a structural rearrangement is present in one of the parent, the most frequent situation being that of an intrachromosomal duplication inherited from the mother. Prenatal diagnosis is performed by cytogenetic testing including FISH and/or DNA quantification methods. Management is multi-specialist and only symptomatic, with special attention to prevention of malnutrition and recurrent infections. Educational and rehabilitation support should be offered to all patients.

Skewed X inactivation is associated with phenotype in a female with adrenal hypoplasia congenita

Adrenal hypoplasia congenita (AHC) can occur due to deletions or mutations in the DAX 1 (NR0B1) gene on the X chromosome (OMIM 300200). This form of AHC is therefore predominantly seen in boys. Deletion of the DAX 1 gene can also be part of a larger contiguous deletion including the centromeric dystrophin and glycerol kinase (GK) genes. We report a girl with a de novo deletion at Xp21.2 on the maternal chromosome, including DAX1, the GK gene and 3′ end of the dystrophin gene, who presented with salt losing adrenal insufficiency and moderate developmental delay, but relatively mild features of muscular dystrophy. Investigation using the androgen receptor as a marker gene identified skewed inactivation of the X chromosome. In the patient’s leucocytes, the paternal X chromosome was completely inactive, but in muscle 20% of the active chromosomes were of paternal origin. Thus skewed X inactivation (deletion on the active maternal X chromosome with an inactive paternal X chromosome) is associated with AHC in a female. Variability in X inactivation between tissues may account for the pronounced salt loss and adrenal insufficiency but mild muscular dystrophy.

West syndrome in a patient with balanced translocation t(X;18)(p22;p11.2)

We describe a case of West syndrome with the balanced translocation t(X;18)(p22;p11.2). Treatment with high-dose vitamin B6, adrenocorticotropic hormone, thyrotropin-releasing hormone, and antiepileptic compounds was not effective, and the patient exhibited persistent refractory seizures and severe developmental delays. Although no mutation analysis and X chromosome inactivation were performed, we suggest that the chromosomal abnormality in the present patient is the main etiologic factor responsible for the infantile spasms and severe developmental delay.

Use of somatic cell nuclear transfer to study meiosis in female cattle carrying a sex-dependent fertility-impairing X-chromosome abnormality

Animal models have played an important part in establishing our knowledge base on reproduction, development, and the occurrence and impact of chromosome abnormalities. Translocations involving the X chromosome and an autosome are unique in that they elicit sex-dependent infertility, with male carriers rendered sterile by synaptic anomalies during meiosis, whereas female carriers conceive but repeatedly abort. Until now the limited access to relevant fetal oocytes has precluded direct study of meiotic events in female carriers. Because somatic cell nuclear transfer (SCNT) circumvents meiotic problems associated with fertility disturbances in translocation carriers, we used SCNT to generate embryos, fetuses, and calves from a cell line derived from a deceased subfertile X-autosome translocation carrier cow to study the meiotic configurations in carrier oocytes. Data from 33 replicates involving 2470 oocyte-donor-cell complexes were assessed for blastocyst development and of these, 42 blastocysts were transferred to 21 recipients. Fourteen pregnancies were detected on day 35 of gestation. One of these was sacrificed for ovary retrieval on day 94 and three went to term. Features of oocytes from the fetal ovary and from the newborn ovaries were examined. Of the pachytene spreads analyzed, 16%, 82%, and 1.5% exhibited quadrivalent, trivalent/univalent, and bivalent/univalent/univalent structures, respectively, whereas among the diakinesis/metaphase I spreads, 16% ring, 75% chain, and 8.3% bivalent/bivalent configurations were noted, suggesting that the low fertility among female carriers may be related to synaptic errors in a predominant proportion of oocytes. Our results indicate that fibroblasts carrying the X-autosome translocation can be used for SCNT to produce embryos, fetuses, and newborn clones to study such basic aspects of development as meiosis and to generate carriers that cannot easily be reproduced by conventional breeding.

A male infant with a 9.6 Mb terminal Xp deletion including theOA1 locus: Limit of viability of Xp deletions in males

Males with deletions of or within Xp22.3-pter display variable contiguous gene syndromes including manifestations of Léri-Weill syndrome, chondrodysplasia punctata, mental retardation, ichthyosis, Kallmann syndrome, and ocular albinism. Herein, we report on a male infant with a large, cytogenetically visible, terminal Xp deletion defined by extensive FISH and STS marker analysis to encompass 9.6 Mb, and findings of all of the disorders mentioned above. His deletion approximates the largest Xp terminal deletion ever reported in a male individual. Since the extent of terminal Xp deletions viable in males is limited by the position of male lethal genes in Xp22.2 at about 10-11 Mb from the telomere, this patient falls into the category of the most severe male terminal Xp deletion phenotype.

X chromosome-inactivation patterns of 1,005 phenotypically unaffected females

X-chromosome inactivation is widely believed to be random in early female development and to result in a mosaic distribution of cells, approximately half with the paternally derived X chromosome inactive and half with the maternally derived X chromosome inactive. Significant departures from such a random pattern are hallmarks of a variety of clinical states, including being carriers for severe X-linked diseases or X-chromosome cytogenetic abnormalities. To evaluate the significance of skewed patterns of X inactivation, we examined patterns of X inactivation in a population of >1,000 phenotypically unaffected females. The data demonstrate that only a very small proportion of unaffected females show significantly skewed inactivation, especially during the neonatal period. By comparison with this data set, the degree of skewed inactivation in a given individual can now be quantified and evaluated for its potential clinical significance.

Heterotaxy and cardiac defect in a girl with chromosome translocation t(X;1)(q26;p13.1) and involvement of ZIC3

We report on a 2-year-old girl with situs ambiguus comprising right-sided stomach and spleen, left-sided liver and complex cardiac defect. Psychomotor development of this patient was normal, and no other major abnormalities were present. Chromosome analysis revealed a de novo balanced chromosome translocation t(X;1)(q26;p13.1). Molecular cytogenetic investigations identified a breakpoint spanning BAC clone on the X-chromosome containing the ZIC3 gene. Mutations in ZIC3 are associated with situs ambiguus and cardiac defects predominantly in males. This is the first report of a live born girl with an X-autosome translocation involving the ZIC3 region.

Somatic chromosomal abnormalities in infertile men and women

Infertility--the inability to achieve conception or sustain a pregnancy through to live birth--is very common and affects about 15% of couples. While chromosomal or genetic abnormalities associated with azoospermia, severe oligozoospermia or primary ovarian failure were of no importance for reproduction prior to the era of in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI), advances in assisted reproductive techniques (ART) now enable many infertile couples to have children. These developments have raised the question of the genetic consequences of ICSI: concerns of the potential harm of the invasive procedure and concerns about the genetic risk. The infertile male and female definitely have an increased risk to carry a chromosomal abnormality. Detection of such an abnormality is of fundamental importance for the diagnosis of infertility, the following treatment, the evaluation of the risk for the future child and the appropriate management of the pregnancy to be obtained. Therefore, cytogenetic screening of both partners is mandatory prior to any type of ART. The present review is based on several surveys on male and female infertility and analyzes the types and frequencies of the different reported chromosome abnormalities according to the type of impairment of spermatogenesis and the type of treatment planned or performed. With regard to assisted reproductive techniques (especially ICSI) the main types of chromosomal abnormalities are discussed and their potential risks for ICSI. If available, reported cases of performed ICSI and its outcome are presented. The detection of an abnormal karyotype should lead to comprehensive genetic counselling, which should include all well-known information about the individual type of anomaly, its clinical relevance, its possible inheritance, the genetic risk of unbalanced offspring, and the possibilities of prenatal diagnosis. Only this proceeding allows at-risk couples to make an informed decision regarding whether or not to proceed with ART. These decisions can be made only when both partners have clearly understood the genetic risks and possible consequences when ART is used.

Loss of ZDHHC15 expression in a woman with a balanced translocation t(X;15)(q13.3;cen) and severe mental retardation

X-linked mental retardation (XLMR) affects one in 600 males and is highly heterogeneous. We describe here a 29-year-old woman with severe nonsyndromic mental retardation and a balanced reciprocal translocation between chromosomes X and 15 [46,XX,t(X;15)(q13.3;cen)]. Methylation studies showed a 100% skewed X-inactivation in patient-derived lymphocytes indicating that the normal chromosome X is retained inactive. Physical mapping of the breakpoints localised the Xq13.3 breakpoint to within 3.9 kb of the first exon of the ZDHHC15 gene encoding a zinc-finger and a DHHC domain containing product. Expression analysis revealed that different transcript variants of the gene are expressed in brain. ZDHHC15-specific RT-PCR analysis on lymphocytes from the patient revealed an absence of ZDHHC15 transcript variants, detected in control samples. We suggest that the absence of the ZDHHC15 transcripts in this patient contributes to her phenotype, and that the gene is a strong candidate for nonsyndromic XLMR.

Disruptions of the novel KIAA1202 gene are associated with X-linked mental retardation

The extensive heterogeneity underlying the genetic component of mental retardation (MR) is the main cause for our limited understanding of the aetiology of this highly prevalent condition. Hence we set out to identify genes involved in MR. We investigated the breakpoints of two balanced X;autosome translocations in two unrelated female patients with mild/moderate MR and found that the Xp11.2 breakpoints disrupt the novel human KIAA1202 (hKIAA1202) gene in both cases. We also identified a missense exchange in this gene, segregating with the Stocco dos Santos XLMR syndrome in a large four-generation pedigree but absent in >1,000 control X-chromosomes. Among other phenotypic characteristics, the affected males in this family present with severe MR, delayed or no speech, seizures and hyperactivity. Molecular studies of hKIAA1202 determined its genomic organisation, its expression throughout the brain and the regulation of expression of its mouse homologue during development. Transient expression of the wild-type KIAA1202 protein in HeLa cells showed partial colocalisation with the F-actin based cytoskeleton. On the basis of its domain structure, we argue that hKIAA1202 is a new member of the APX/Shroom protein family. Members of this family contain a PDZ and two ASD domains of unknown function and have been shown to localise at the cytoskeleton, and play a role in neurulation, cellular architecture, actin remodelling and ion channel function. Our results suggest that hKIAA1202 may be important in cognitive function and/or development.

Functional disomy of the Xq28 chromosome region

We report on two patients, a boy and a girl, with an additional Xq28 chromosome segment translocated onto the long arm of an autosome. The karyotypes were 46,XY,der(10)t(X;10)(q28;qter) and 46,XX,der(4)t(X;4)(q28;q34), respectively. In both cases, the de novo cryptic unbalanced X-autosome translocation resulted in a Xq28 chromosome functional disomy. To our knowledge, at least 17 patients with a distal Xq chromosome functional disomy have been described in the literature. This is the third report of a girl with an unbalanced translocation yielding such a disomy. When the clinical features of both patients are compared to those observed in patients reported in the literature, a distinct phenotype emerges including severe mental retardation, facial dysmorphic features with a wide face, a small mouth and a thin pointed nose, major axial hypotonia, severe feeding problems and proneness to infections. A clinically oriented FISH study using subtelomeric probes is necessary to detect such a cryptic rearrangement.