Clinical spectrum of SIX3-associated mutations in holoprosencephaly: correlation between genotype, phenotype and function.

BACKGROUND: Holoprosencephaly (HPE) is the most common structural malformation of the human forebrain. There are several important HPE mutational target genes, including the transcription factor SIX3, which encodes an early regulator of Shh, Wnt, Bmp and Nodal signalling expressed in the developing forebrain and eyes of all vertebrates. OBJECTIVE: To characterise genetic and clinical findings in patients with SIX3 mutations. METHODS: Patients with HPE and their family members were tested for mutations in HPE-associated genes and the genetic and clinical findings, including those for additional cases found in the literature, were analysed. The results were correlated with a mutation-specific functional assay in zebrafish. RESULTS: In a cohort of patients (n = 800) with HPE, SIX3 mutations were found in 4.7% of probands and additional cases were found through testing of relatives. In total, 138 cases of HPE were identified, 59 of whom had not previously been clinically presented. Mutations in SIX3 result in more severe HPE than in other cases of non-chromosomal, non-syndromic HPE. An over-representation of severe HPE was found in patients whose mutations confer greater loss of function, as measured by the functional zebrafish assay. The gender ratio in this combined set of patients was 1.5:1 (F:M) and maternal inheritance was almost twice as common as paternal. About 14% of SIX3 mutations in probands occur de novo. There is a wide intrafamilial clinical range of features and classical penetrance is estimated to be at least 62%. CONCLUSIONS: Our data suggest that SIX3 mutations result in relatively severe HPE and that there is a genotype-phenotype correlation, as shown by functional studies using animal models.

Distribution of sex chromosomes in dysgenetic gonads of mixed type.

In a four-week-old child with female external and internal genitalia but with clitoris hypertrophy chromosome analysis from blood lymphocytes revealed a 46,XY karyotype. No deletion of Y chromosomal sequences was detected by PCR analysis of genomic DNA isolated from peripheral blood leucocytes. Because of the increased risk for gonadal tumours, gonadectomy was performed. Conventional cytogenetic analysis of the left dysgenetic gonad revealed a gonosomal mosaicism with a 45,X cell line in 27 of 50 metaphases. The dysgenetic left gonad demonstrated a significantly higher proportion (P = 0.005) of cells carrying a Y chromosome (46.3%) than the streak gonad from the right side (33.9%). Histomorphological examination of the left gonad revealed immature testicular tissue and rete-like structures as well as irregular ovarian type areas with cystic follicular structures. Interphase FISH analysis of the different tissues of this dysgenetic gonad demonstrated variable proportions of cells with an X and a Y chromosome. Whereas Sertoli cells and rete-like structures revealed a significantly higher proportion of XY cells in relation to the whole section of the dysgenetic gonad (P < 0.0001), almost all granulose-like cells carried no Y chromosome. The proportion of XY/X cells in theca-like cells and Leydig cells was similar to that of the whole dysgenetic gonad. In contrast to these findings, spermatogonia exclusively contained an XY constellation.

PLAG1 activation in lipoblastoma coinciding with low-level amplification of a derivative chromosome 8 with a deletion del(8)(q13q21.2).

Lipoblastoma is a benign uncommon soft-tissue-tumor resembling fetal adipose tissue affecting mainly children under three years of age. In lipoblastoma, the typical cytogenetic changes are clonal rearrangements involving chromosomal region 8q11-->q13. The oncogene PLAG1 (pleomorphic adenoma gene 1) is located within this chromosomal region on band 8q12. Recent reports have demonstrated that in lipoblastoma, the PLAG1 gene is activated by 'promoter-swapping'. Herein, we demonstrate that in lipoblastoma, the PLAG1 gene may also be activated by low-level amplification. We report on a lipoblastoma with the karyotype 48 approximately 50,XX,del(8)(q13q21.2),+del(8)(q13q21.2)x4[cp12]. Subsequent FISH analysis on uncultured tumor cells confirmed this result and demonstrated a low-level amplification of the chromosomal region 8pter-->8q13 and 8q21.2-->8qter. A partial monosomy was seen for the chromosomal region 8q13-->8q21.2. No other gains or losses were observed by CGH analysis. RT-PCR analysis showed that the PLAG1 gene is activated in the tumor sample of the lipoblastoma analyzed, in contrast to normal fatty tissue without PLAG1 expression. In conclusion, our results demonstrate that low-level amplification is a further mechanism of PLAG1 activation in lipoblastomas.

Molecular characterization of the DICE1 (DDX26) tumor suppressor gene in lung carcinoma cells.

We have determined the genomic structure of the candidate tumor suppressor gene DICE1 (DDX26). The DICE1 gene colocalizes with microsatellite marker D13S284 telomeric to the RB1 gene in chromosomal region 13q14.3. The DICE1 gene encodes 18 exons that are preceded by a GC-rich promoter region. CpG sites flanking a predicted TATA box were found to be hypermethylated in tumor cells that exhibited decreased DICE1 expression. This suggests tumor-specific transcriptional silencing of the DICE1 gene may occur. Aberrantly spliced products were detected in two of three DICE1 expressing cell lines. The predicted DICE1 amino acid sequence is evolutionarily conserved in mouse, fruit fly (D. melanogaster), and nematode (C. elegans). A DEAD box characteristic of ATP-dependent helicases is the predominant motif found in DICE1 and its mouse and fruit fly homologues. Motifs other than the DEAD box are reminiscent of members of the helicase superfamily II but there is considerable variation from the typical DEAD box helicases. Expression of DICE1 green fluorescent fusion protein showed a preferential localization of DICE1 in the nucleus. This suggests that DICE1 is involved in nuclear processes such as DNA repair, transcription, or RNA splicing.

Clinical and molecular aspects of androgen receptor defects.

The androgen receptor (AR) is a ligand-dependent transcription factor involved in various biological processes such as sex differentiation, sexual maturation and spermatogenesis. Disorders of AR function cause a wide spectrum of androgen insensitivity syndromes. The phenotypes vary from women with female external genitalia through patients with genital ambiguity to men with normal male genitalia but infertile. The CAG repeat in exon A is important for transactivation function of the AR and consequently for many androgen-dependent processes. Expansion of this repeat is the cause of the X-linked spinal and bulbar muscular atrophy (SBMA, Kennedy's disease). Mutations of the AR gene occur commonly in prostate cancers and are significant for prognosis of the disease.

Molecular cytogenetic techniques for the diagnosis of chromosomal abnormalities in childhood disease.

Fluorescence in situ hybridisation and comparative genomic hybridisation are technologies firmly established in cytogenetics. These methods complement conventional banding techniques and offer additional clinical and research applications. The present paper has two purposes: (a) to introduce to these molecular cytogenetic techniques and (b) to give some examples of childhood diseases due to chromosomal aberrations.

Chimerism in a fertile woman with 46,XY karyotype and female phenotype.

We report on the unexpected finding of a 46,XY karyotype in a 30 year-old woman with normal ovarian function and a former pregnancy at 17 years of age. Chromosome analysis was performed prior to intracyoplasmic sperm injection (ICSI), due to infertility of her husband. Repeated chromosome analysis in lymphocytes of the female resulted in a normal male karyotype. Fluorescence in-situ hybridization (FISH) analysis of cultured lymphocyte interphase nuclei detected in 99% of the cells one X and one Y chromosome-specific signal respectively, whereas two X chromosome-specific signals were observed in only 1% of the nuclei. Chromosome analysis of fibroblasts of ovarian and muscular tissues as well as of skin revealed a normal female karyotype (46,XX). Chimerism could be proven by variable number of tandem repeats (VNTR) analysis. Since the case history of the patient revealed that her twin brother died shortly after birth, it can be assumed that chimerism is caused by feto-fetal transfusion during pregnancy and delivery of the proposita.

Mosaic tetrasomy 9p in a girl with multiple congenital anomalies: cytogenetic and molecular-cytogenetic studies.

We report on a 16-year-old girl with tetrasomy 9p mosaicism. Clinical investigations disclosed a malformation syndrome with craniofacial abnormalities, dysplasia of the right clavicle, short neck with cervical ribs, patella dislocation, Dandy-Walker malformation, mental retardation and blindness. Karyotype analysis of blood lymphocytes indicated an additional marker in the size of a C-group chromosome with a large heterochromatic block in 88% of the investigated metaphases. The origin and structure of this additional marker could not be determined by chromosome banding. Application of fluorescence in situ hybridisation and comparative genomic hybridisation identified the origin of the marker chromosome, demonstrating the effectiveness of molecular-cytogenetic investigations in the diagnosis of structural and numerical chromosome abnormalities.

The size of the CAG repeat in exon 1 of the androgen receptor gene shows no significant relationship to impaired spermatogenesis in an infertile Caucasoid sample of German origin.

The androgen receptor (AR) gene, located on the X-chromosome at Xq11-12, contains in exon 1 a polymorphic CAG repeat which codes for a polyglutamine tract. Contractions of the CAG repeat are said to be related to prostate cancer. In contrast, sizeable expansion of the CAG repeat can cause spinal and bulbar muscular atrophy (SBMA). In infertile patients of Chinese origin and in a Melbourne multinational population impaired sperm production has been postulated to be related to moderate expansions of the polyglutamine tract. In a study of a Swedish population of infertile patients these findings could not be corroborated. The aim of our investigation was to examine the correlation between the length of the CAG repeat and impaired sperm production in an infertile Caucasoid patient sample of German ethnic origin. We found no statistically significant relationship between the size of the CAG repeat or polyglutamine tract and idiopathic impaired sperm production in the population studied. The variability of the results by various investigators may be attributed to different ethnic origins and hence different genetic modifiers of the populations studied and/or to the high probability that these infertile males may represent a heterogeneous group with respect to the causes of defective spermatogenesis.

X-linked dominant Charcot-Marie-Tooth disease: suggestion of linkage with a cloned DNA sequence from the proximal Xq.

A large kindred with the X-linked dominant form of peroneal muscular atrophy (Charcot-Marie-Tooth disease) was analyzed for individual variation in the length of DNA fragments after restriction endonuclease digestion. A systematic search was performed for linkage with a series of cloned single-copy DNA sequences of known regional assignment to the human X chromosome. Close linkage was found with the pDP34 probe (DXYS1 locus, Xq13-q21), suggesting that the gene responsible for the disease is located on the proximal long arm of the X chromosome.

Incidental prenatal detection of an Xp deletion using an anonymous primer pair for fetal sexing.

We report on the incidental prenatal detection of an interstitial X-chromosomal deletion in a male fetus and his mother by fetal sexing with a primer pair recognizing an X-Y homologous locus (DXYS19), formerly unassigned on the X chromosome. The proband asked for prenatal diagnosis because of her elevated age and risk of Duchenne muscular dystrophy (DMD). Prior to molecular genetic testing for DMD, fetal sexing was carried out on DNA prepared from cultured amniocytes. PCR analysis revealed the expected Y-chromosomal product, but did not show the constitutive X-chromosomal fragment. The absence of the X-chromosomal fragment in the fetus and on one X chromosome of the mother was confirmed by Southern hybridization of HindIII restricted DNA with probe pJA1165 (DXYS19). DXYS19X was mapped to Xp22.3 by combining several approaches, including: (1) analysis of somatic cell hybrid lines containing different fragments of the human X chromosome; (2) Southern hybridization of a yeast artificial chromosome (YAC)-filter panel provided by the Resource Center/Primary Database (RZPD); (3) FISH analysis; and (4) re-evaluation of two patients with interstitial deletions in Xp22.3. The extent of the deletion in the fetus was estimated by further markers from Xp22.3 and found to include the STS gene. Mental retardation could not be excluded since some mentally retarded patients exhibit overlapping deletions.

Short stature homeobox-containing gene deletion screening by fluorescence in situ hybridisation in patients with short stature.

UNLABELLED: The short stature homeobox-containing gene (SHOX) on the short arm of the X and Y chromosomes is an important determining factor of stature phenotype. Absence of the SHOX gene is a main cause for short stature in patients with Turner syndrome. Mutations of the SHOX gene can also be responsible for Léri-Weill syndrome (dyschondrosteosis). The aim of this study was to determine the frequency of SHOX deletions in short stature children and to delineate indications for SHOX deletion screening. Out of 50 probands, 35 had idiopathic short stature, 12 cases showed additional anomalies of the forearms (in particular Madelung deformity) and three patients were affected by a congenital heart defect. Chromosomal investigations with fluoresence in situ hybridisation did not reveal a SHOX deletion in any patient with idiopathic short stature. In five of the 12 patients (41.7%) with anomalies of the forearms, a SHOX deletion on one sex chromosome could be detected. No deletion was observed in the three cases with additional heart defects. CONCLUSION: The frequency of short stature homeobox-containing gene deletions in patients with idiopathic short stature appears to be very low and does not require a fluorescence in situ hybridisation analysis. Short stature in association with anomalies of the forearms such as Madelung deformity makes a deletion more probable and therefore screening for such deletions is recommended in these cases.