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XSense®, Fragile X with Reflex and Chromosome Analysis, Blood

Test code(s) 16326, 16327 (NY)

Question 1. My patient has a developmental disorder. Chromosome analysis and fragile X studies were reported as normal. Are there other cytogenetic studies that can be performed on my patient?

Yes, there are other studies that may be appropriate. There are many causes for developmental disorders, some of which are genetic. In the absence of clinical suspicion for a specific genetic disorder, a microarray analysis may be performed to detect subtle deletions and duplications (Chromosomal Microarray, Postnatal, ClariSure® Oligo-SNP,test code 16478). If clinical suspicion exists for a specific disorder, there may be other genetic testing available. Please contact 866-GENE-INFO to discuss the case with a genetic counselor and for information on adding additional testing.

Question 2. My patient has a family history of mental retardation. Chromosome analysis and fragile X studies were reported as normal. Do these results guarantee my patient won’t have a child with mental retardation?

No. Please call 866-GENE-INFO to discuss this case with a genetic counselor. Documentation of the specific genetic abnormality in the family will be necessary to determine the accuracy of the testing that was performed on your patient.

Question 3. What chromosome disorders does this assay rule out?

This assay rules out:

  1. Trisomies such as Down syndrome (trisomy 21), trisomy 18, and trisomy 13
  2. Sex chromosome abnormalities such as Turner and Klinefelter syndromes
  3. Most rearrangements, including Robertsonian translocations, reciprocal translocations, and inversions
  4. Marker chromosomes
  5. Chromosomal mosaicism

Question 4. What disorders cannot be detected by this assay?

This assay cannot detect:

  1. Submicroscopic gains or losses of genetic material, including microdeletion syndromes such as DiGeorge and Williams
  2. Low level chromosomal mosaicism
  3. Single gene disorders such as cystic fibrosis, Marfan syndrome, neurofibromatosis, etc.

Question 5. What does the term “gray zone allele carrier” mean?

Female gray zone (also known as intermediate) allele carriers have 1 normal size CGG repeat and 1 that is between 45 and 54 CGG repeats in length. The stability of the allele with 45-54 CGG repeats is unknown. It may be stable from generation to generation or it may undergo expansion in future generations. There have been a few reports of a gray zone allele expanding to a full mutation within 2 generations, but the likelihood is low.1

Male gray zone or intermediate allele carriers also have an allele that has between 45 and 54 CGG repeats. The stability of this allele is unknown and may be stable when passed to his daughters, or may expand to a premutation, which could have an impact on his grandchildren.

Question 6. What is the next step if my patient tests positive for a gray zone allele?

Genetic counseling is available to discuss the options available for further testing, if necessary or desired by the patient.

Question 7. The report indicates my female patient has 2 FMR1 alleles in the normal size range (negative). Does this mean my patient is not a fragile X carrier?

The majority of female patients with a negative result (ie, 2 normal size CGG repeats in the FMR1 gene) are not fragile X carriers. This is true even if the patient has an intellectual disability. However, there are extremely rare cases in which fragile X syndrome is caused by an FMR1 gene mutation other than a CGG expansion. If other potential causes of the clinical presentation have been ruled out and the patient has classic fragile X features or a fragile X-positive family history, please call a genetic counselor at 866-GENEINFO to discuss.  

Question 8. The report indicates my male patient has 1 FMR1 allele in the normal size range (negative). Does this mean he does not have fragile X syndrome?

A male patient with 1 FMR1 allele in the normal size range would not be expected to have fragile X syndrome. However, there are extremely rare cases of fragile X syndrome that are due to different types of mutations within the fragile X gene. If other causes for the patient’s clinical presentation have been ruled out and the patient has classic fragile X features, or has a positive family history of fragile X syndrome, please call a genetic counselor at 866-GENEINFO to discuss.

Question 9. My patient has a family history of fragile X and has a normal fragile X CGG repeat test result. Does this result guarantee that my patient is not a carrier of fragile X?

Since most cases of fragile X are due to the expansion of the CGG repeats tested for in this assay, a negative result usually rules out carrier status. However, we recommend a careful examination of fragile X mutation test results in the affected family member. Please call 866-GENEINFO to discuss this case with a genetic counselor if you have further questions.

 

References
 
1.     Fernandez-Carvajal I, Lopez Posadas B, Pan R, et al. Expansion of an FMR1 grey-zone allele to a full mutation in two generations. J Mol Diagn, 2009;11: 306-310.
 
 
 
 Document FAQS.60 Version: 2
 
Version 2 effective 06/04/2014 to present
Version 0 effective 7/27/2012 to 11/05/2012  
This FAQ is provided for informational purposes only and is not intended as medical advice. A physician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.
Document FAQS.60 Version: 2