X chromosome instability associated with familial Turner syndrome

Single-nucleotide polymorphism array genotyping is equivalent to metaphase cytogenetics for diagnosis of Turner syndrome

PURPOSE

Turner syndrome is a developmental disorder caused by partial or complete monosomy for the X chromosome in 1 in 2,500 females. We hypothesized that single-nucleotide polymorphism (SNP) array genotyping could provide superior resolution in comparison to metaphase karyotype analysis to facilitate genotype-phenotype correlations.

METHODS

We genotyped 187 Turner syndrome patients with 733,000 SNP marker arrays. All cases met diagnostic criteria for Turner syndrome based on karyotypes (60%) or characteristic physical features. The SNP array results confirmed the diagnosis of Turner syndrome in 100% of cases.

RESULTS

We identified a single X chromosome (45,X) in 113 cases. In 58 additional cases (31%), other mosaic cell lines were present, including isochromosomes (16%), rings (5%), and Xp deletions (8%). The remaining cases were mosaic for monosomy X and normal male or female cell lines. Array-based models of X chromosome structure were compatible with karyotypes in 104 of 116 comparable cases (90%). We found that the SNP array data did not detect X-autosome translocations (three cases) but did identify two derivative Y chromosomes and 13 large copy-number variants that were not detected by karyotyping.

CONCLUSION

Our study is the first systematic comparison between the two methods and supports the utility of SNP array genotyping to address clinical and research questions in Turner syndrome.

A Turner syndrome woman with a ring X chromosome [45,X/46,X,r(X)(p22.3q27)] whose child also had a ring X chromosome

OBJECTIVE

To describe a woman with Turner syndrome with ring X chromosome mosaicism who had a child who possessed the same ring X chromosome.

DESIGN

Polymorphisms of genes located on the X chromosome from genomic DNA of the mother, father, and the child were evaluated.

PATIENT(S)

The mother's karyotype was 45,X [48]/46,X,r(X)(p22.3q27) [2], and the child's karyotype was 45,X[33]/46,X,r(X)(p22.3q27) [17].

INTERVENTION(S)

Polymerase chain reaction was used to amplify short tandem repeats from the loci of the hypoxanthine phosphoribosyltransferase gene and the androgen receptor gene.

RESULT(S)

Alleles for both genes in the child originated from both parents in a heterozygous fashion. The alleles originating from the mother originated from the ring X chromosome. However, the amount of amplified DNA was less than that of a normal X chromosome.

CONCLUSION(S)

The ring X chromosome of the mother was most likely transmitted to the newborn. Thus, an ovum with the ring X chromosome can be fertile and can produce a viable zygote.

Molecular diagnosis of Turner's syndrome

Turner's syndrome is a common disorder which occurs in around 1/3000 live births in girls. Diagnostic use of polymorphic DNA markers for the X chromosome could help to reduce the number of time consuming karyotype analyses needed. The M27 beta probe maps on the X chromosome to Xcen-Xp11-22 and in 83% of female subjects detects heterozygosity with multiallelic polymorphism. In Southern blotting, a single X chromosome yields a single hybridisation band. In this study, genomic DNA was extracted from leucocytes of 49 patients with Turner's syndrome (karyotypes: 45,XO, n = 29; 45,XO/46,XX, n = 4; 46,Xi(Xq), n = 1; 45,XO/46,Xi(Xq), n = 4; 45,XO/46,Xr(X), n = 4; 45,XO/46,XY, n = 4; 46,XXp-, n = 3), digested with EcoRI or HindIII, and analysed by Southern blotting. The molecular data for each patient were compared with DNA controls (homozygous 46,XX, heterozygous 46,XX and 46,XY DNA). A single band of reduced intensity compared to homozygous 46,XX control DNA was seen in 41 cases. Two hybridisation bands of different intensities were seen in four patients, in one of whom mosaicism was suspected on the basis of molecular analysis, despite a 45,XO karyotype. In four cases, Turner's syndrome failed to be detected: one 45,XO/46,XX mosaicism with only 4% of 45,XO cells and three distal Xp deletions. DNA analysis appears to be a useful and rapid tool in screening for Turner's syndrome and could be an alternative to cytogenetic analysis in diagnosing the disorder when severe growth retardation or delayed puberty are not accompanied by a Turner phenotype.