Duplication of Xq26.2-q27.1, including SOX3, in a mother and daughter with short stature and dyslalia

Duplications of the distal long arm of the X chromosome are rare and carrier females are usually phenotypically normal. We report on a 14-year-old short statured (height and weight <3rd centile) girl with dup(X)(q26.2q27.1) inherited from a short mother. The proband has minor dysmorphic features, lordosis, lack of menarche, late signs of puberty, low prepuberal levels of gonadotrophins and steroids, but borderline low IGF-1 and normal IGF-Bp3 serum levels. Both the proposita and her mother have severe speech problems with stuttering and dyslalia. The 44-year-old mother with a strikingly aged face and a prominent nose, had menarche at 15 years. Both maternal sisters and the grandmother of the proposita are also short. Karyotyping revealed an additional band at Xq26 in all metaphases from the proband, her mother, and two maternal aunts. Molecular cytogenetic investigations revealed an Xq26.2-q27.1 direct duplication of approximately 7.5 Mb that encompasses or disrupts the SOX3 gene, which maps at the distal border of the duplicated segment. A similar chromosomal duplication was reported recently in five families and in each was associated with an abnormal phenotype in males with short stature [Hol et al., 2000; Solomon et al., 2002, 2004]. Using an androgen-receptor (HUMARA) gene methylation assay and FISH, we show that despite preferential inactivation of the dup(Xq) chromosome a significant proportion of lymphocytes in both mother and daughter carry an active duplicated X chromosome. Our findings further suggest that a dosage effect of SOX3 may to be responsible for a speech disorder in addition to short stature secondary to hypopituitarism.

Sry, Sox9 and mammalian sex determination

Sry is the Y-chromosomal gene that acts as a trigger for male development in mammalian embryos. This gene encodes a high mobility group (HMG) box transcription factor that is known to bind to specific target sequences in DNA and to cause a bend in the chromatin. DNA bending appears to be part of the mechanism by which Sry influences transcription of genes downstream in a cascade of gene regulation leading to maleness, but the factors that cooperate with, and the direct targets of, Sry remain to be identified. One gene known to be downstream from Sry in this cascade in Sox9, which encodes a transcription factor related to Sry by the HMG box. Like Sry, mutations in Sox9 disrupt male development, but unlike Sry, the role of Sox9 is not limited to mammals. This review focuses on what is known about the two genes and their likely modes of action, and draws together recent data relating to how they might interconnect with the network of gene activity implicated in testis determination in mammals.

Transcription map of Xq27: candidates for several X-linked diseases

Human Xq27 contains candidate regions for several disorders, yet is predicted to be a gene-poor cytogenetic band. We have developed a transcription map for the entire cytogenetic band to facilitate the identification of the relatively small number of expected candidate genes. Two approaches were taken to identify genes: (1) a group of 64 unique STSs that were generated during the physical mapping of the region were used in RT-PCR with RNA from human adult and fetal brain and (2) ESTs that have been broadly mapped to this region of the chromosome were finely mapped using a high-resolution yeast artificial chromosome contig. This combined approach identified four distinct regions of transcriptional activity within the Xq27 band. Among them is a region at the centromeric boundary that contains candidate regions for several rare developmental disorders (X-linked recessive hypoparathyroidism, thoracoabdominal syndrome, albinism-deafness syndrome, and Borjeson-Forssman-Lehman syndrome). Two transcriptionally active regions were identified in the center of Xq27 and include candidate regions for X-linked mental retardation syndrome 6, X-linked progressive cone dystrophy, X-linked retinitis pigmentosa 24, and a prostate cancer susceptibility locus. The fourth region of transcriptional activity encompasses the FMR1 (FRAXA) and FMR2 (FRAXE) genes. The analysis thus suggests clustered transcription in Xq27 and provides candidates for several heritable disorders for which the causative genes have not yet been found.

Congenital motor nystagmus linked to Xq26-q27

Congenital motor nystagmus (CMN) is a hereditary disorder characterized by bilateral ocular oscillations that begin in the first 6 mo of life. It must be distinguished from those genetic disorders-such as ocular albinism (OA), congenital stationary night blindness (CSNB), and blue-cone monochromatism (BCM)-in which nystagmus accompanies a clinically apparent defect in the visual sensory system. Although CMN is presumed to arise from a neurological abnormality of fixation, it is not known whether the molecular defect is located in the eye or in the brain. It may be inherited in an autosomal dominant, autosomal recessive, or X-linked pattern. Three families with CMN inherited in an X-linked, irregularly dominant pattern were investigated with linkage and candidate gene analysis. The penetrance among obligate female carriers was 54%. Evaluation of markers in the region of the genes for X-linked OA, CSNB, and BCM revealed no evidence of linkage, supporting the hypothesis that CMN represents a distinct entity. The gene was mapped to chromosome Xq26-q27 with the following markers: GATA172D05 (LOD score 3.164; recombination fraction [theta] = 0.156), DXS1047 (LOD score 10.296; theta = 0), DXS1192 (LOD score 8.174; theta = 0.027), DXS1232 (LOD score 6.015; theta = 0.036), DXS984 (LOD score 6.695; theta = 0), and GATA31E08 (LOD score 4.940; theta = 0.083). Assessment of haplotypes and multipoint linkage analysis, which gave a maximum LOD score of 10.790 with the 1-LOD-unit support interval spanning approximately 7 cM, place the gene in a region between GATA172D05 and DXS1192. Evaluation of candidate genes CDR1 and SOX3 did not reveal mutations in affected male subjects.

Identification of CXorf1, a novel intronless gene in Xq27.3, expressed in human hippocampus

We have identified and characterized a novel human gene (Nomenclature Committee of the Genome Database GDB-assigned symbol CXorf1) that maps to the long arm of the X chromosome in Xq27 between loci DXS369 and DXS181, approximately 2.5 Mb centromeric to the FMR1 gene. The CXorf1 gene is conserved in primates, cow, and horse but not in mouse and rat. Northern blot analysis revealed two transcripts, present in the brain and in the G361 melanoma cell line. In situ hybridization experiments performed on sections of human hippocampus showed a clear, uneven localization of the CXorf1 mRNA in specific subfields of this brain area. In particular, CXorf1 was localized in the granular-cell layer of the dentate gyrus and in the CA2-CA3 subfields of Ammon's horn. CXorf1 is one of the first genes from this region to be characterized in detail and, on the basis of its chromosomal location and expression pattern, may have an important function in the brain.

Localisation of X linked recessive idiopathic hypoparathyroidism to a 1.5 Mb region on Xq26-q27

X linked recessive idiopathic hypoparathyroidism (HPT) has been observed in two kindreds from Missouri, USA. Affected subjects, who are males, suffer from infantile onset of epilepsy and hypocalcaemia, which appears to be the result of an isolated congenital defect of parathyroid gland development; females are not affected and are normocalcaemic. The gene causing HPT has been previously mapped to a 7 cM interval, flanked centromerically by F9 and telomerically by DXS98, in Xq26-q27, and an analysis of mitochondrial DNA has established a common ancestry for these two kindreds. In order to define further the map location of HPT and thereby facilitate its isolation, we have undertaken linkage studies using polymorphic loci whose order has been established as Xcen - DXS1001 - DXS294 - DXS102 - F9 - DXS1232 - DXS984 - CDR1 - DXS105 - DXS1205 - DXS1227 - DXS98 - DXS52 - Xqter, within this region. Our results established linkage (lod score > 3) between HPT and eight of these 12 loci and indicated that the most likely location of HPT was within a 1.5 Mb interval flanked centromerically by F9 and telomerically by DXS984. Thus, the results of this study have helped to refine the map location of HPT, and this will facilitate the identification of this putative developmental gene and its role in the embryological formation of the parathyroids.