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Test ID: FXYM    
Known 45,X, Mosaicism Reflex Analysis, FISH

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Useful For Suggests clinical disorders or settings where the test may be helpful

This test provides a sensitive method for identifying sex chromosome mosaicism (ie, low level detection)

Genetics Test Information Provides information that may help with selection of the correct test or proper submission of the test request

Only appropriate to detect low levels of sex chromosome mosaicism when a nonmosaic 45,X karyotype has been observed.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Ullrich-Turner syndrome (UTS), also called Turner syndrome, is a genetic disorder associated with the apparent loss of a sex chromosome. Routine cytogenetic methods have identified 3 types of chromosomal abnormalities in UTS patients: loss of an entire X chromosome (45,X), structural X chromosome abnormalities, and mosaicism with an X or Y abnormality. In mosaicism, 2 or more populations of cells with different karyotypes are present (eg, 45,X/47,XXX).

 

The incidence of UTS is approximately 1/3,000 newborn girls. Many of these patients demonstrate the 45,X karyotype. About 30% to 50% are mosaic, with either a 45,X/46,XX karyotype or a structurally abnormal X chromosome. Fewer than 15% of patients with UTS appear to have mosaicism with a 46,XY cell population or a Y chromosome rearrangement.

 

Identifying the mosaic status of patients with UTS is of clinical importance because phenotypic expression and clinical management are dependent upon the karyotype result. Patients with a Y chromosome have a 15% to 25% increased risk of gonadoblastoma.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

An interpretive report will be provided.

Interpretation Provides information to assist in interpretation of the test results

An XX clone is confirmed when > or =1.0% cells display with 2 X chromosome signals. An XY clone is confirmed when > or =0.6% cells display a 1 X and 1 Y signal pattern.

 

Females with a 45,X/46,XX karyotype have no increased risk of gonadoblastoma and generally have a more moderate expression of Turner syndrome features than females with a nonmosaic 45,X karyotype. The presence of a Y chromosome confers increased risk of gonadoblastoma.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Failure to identify an XY signal pattern does not rule out the possibility of <0.6% Y chromosome mosaicism.

Supportive Data

In a group of 22 patients with a nonmosaic 45,X karyotype by conventional cytogenetics, we have identified 3 (14%) patients with a mosaic X/XX result using FISH analysis. These results were confirmed by analysis of additional metaphase cells. Of the 3 patients, 2 have an iso(X) chromosome and 1 has what appears to be a structurally normal X chromosome.

 

The potential of maternal cells in the peripheral circulation of newborn females with a 45,X/46,XX karyotype result was investigated. Thirty specimens from newborn males (<2 days old) were tested, and based on the analysis of 500 interphase nuclei from each specimen, no XX cells were identified in any specimen, suggesting a low likelihood of maternal cells in newborn specimens.

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

1. Canto P, Kofman-Alfaro S, Jiminez AL, et al: Gonadoblastoma in Turner syndrome patients with nonmosaic 45,X karyotype and Y chromosome sequences. Cancer Genet Cytogenet 2004;150:70-72

2. Gravholt CH, Fedder J, Naeraa RW, Muller J: Occurrence of gonadoblastoma in females with Turner syndrome and Y chromosome material: a population study. J Clin Endocrinol Metab 2000;85:3199-3202

3. Sybert VP, McCauley E: Turner syndrome. N Engl J Med 2004;351:1227-1238

Special Instructions and Forms Describes specimen collection and preparation information, test algorithms, and other information pertinent to test. Also includes pertinent information and consent forms to be used when requesting a particular test