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5001: CTNS gene - 57 kb common deletion screen

Order this test

Test Code Test Name Turnaround Blood Sample Tissue Sample CPT Codes 2015 CPT Codes 2013
5001 CTNS 3 weeks $350.00 $450.00 81403 81479
57kb deletion screen

Selecting the Appropriate Sample Type

see 5003: CTNS Tiers I & II for more details.

Cystinosis Inheritance and Genetics

The normal CTNS gene has 12 exons and spans over a length of 23kb. The most common mutation by far is the 57 kb deletion which includes exons 1-9 and part of exon 10. Researchers have found that people with cystinosis who are ofNorthern European ancestry have a 46% to 75% probability to be homozygous for the 57 kb deletion. The remaining affected individuals will either be compound heterozygotes for the 57 kb deletion and a point mutation or may have two point mutations. The deleted allele produces no CTNS mRNA whereas most other mutated alleles produce some residual mRNA.

Ordering the CTNS test

Molecular testing for CNTS is performed in two tiers. First the sample is tested for the common 57 kb deletion by multiplex PCR. If deletion testing is negative or heterozygous, then the CTNS coding region including intron/exon boundaries as well as the promoter region are sequenced for point mutations. If test #5003 (57kb del and CTNS sequencing) is ordered, but the patient is found to have a homozygous 57kb deletion, then Mitomed will halt testing and charge only for test 5001. The combined detection rate for clinically affected individuals is estimated at 86% -96%.

Molecular testing of CTNS is performed on blood or buccal swab samples. Genetic testing is appropriate to establish/confirm a diagnosis of affecteds or to assess carrier status of unaffected individuals with a family history of cystinosis. Genetic testing of unrelated individuals interested in carrier testing will be performed if requested. Interpretation of results with respect to residual carrier risks for negative results and variants of unknown significance remains limited.

About Cystinosis

Cystinosis is a rare autosomal recessive lysosomal storage disorder with an incidence of 1 per 100,000 to 200,000 live births. There are three clinical types of cystinosis, listed below. The gene for cystinosis (CTNS) is located at 17p13 and is the only gene associated with the disorder. CTNS encodes the protein cystinosin which transports cystine out of the lysosome and into the cytoplasm. For more information about Cystinosis, please visit GeneTests and OMIM.

Clinical Features:

Classic nephropathic cystinosis (95% of cases):

  • Growth retardation after 6 months
  • Renal tubular Fanconi syndrome before age 1, renal failure by age 10 if untreated
  • Corneal cystine crystals observed through slit lamp examination
  • Other manifestations with later onset include photophobia, hypothyroidism, diabetes mellitus, and hypogonadism in males.

Intermediate cystinosis

  • Same symptoms as those seen in classic cystinosis. However, symptoms may be less severe or subclinical and their onset is later, generally between ages 15 and 25.

Non-nephropathic cystinosis

  • Photophobia secondary to corneal cystine crystals. No extraocular symptoms.

Alternate Diagnostic Testing Methods:

Clinical test

  • Corneal cystine crystals seen on slit lamp examination are diagnostic.

Analyte test

  • Measurement of leukocyte cystine content. Analyte testing is appropriate to establish a diagnosis, monitor cysteamine treatment, or for prenatal testing using amniocentesis or chorionic villus sampling. The test can be done using blood or tissue. Analyte testing is available through the UCSD Cystine Determination Lab.