Cystinuria via the SLC7A9 Gene
- Summary and Pricing
- Clinical Features and Genetics
|Test Code||Test Copy Genes||Price||CPT Code Copy CPT Codes|
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The great majority of tests are completed within 20 days.
86.8% of mutant alleles were identified in a mutation screen for SLC3A1 and SLC7A9 in 164 probands from the International Cystinuria Consortium (Font-Llitjós et al. 2005). Of the fully explained probands, 56 (34.1%) and 68 (41.5%) have two pathogenic variants found in SLC3A1 and SLC7A9, respectively.
Large deletions and duplications are common in both SLC3A1 and SLC7A9 (Schmidt et al. 2003; Barbosa et al. 2012). Large rearrangements were found in 33.3% of mutant alleles (5 in SLC3A1 and 1 in SLC7A9) in a cohort of 12 Portuguese patients affected with cystinuria (Barbosa et al. 2012). In another cohort of 49 patients, 7 and 3 showed copy number variants in SLC3A1 and SLC7A9, respectively (Schmidt et al. 2003).
Cystinuria is characterized by impaired transport of cystine and dibasic amino acids in the proximal renal tubule and gastrointestinal tract (Barbosa et al. 2012). The defective renal reabsorption of cystine leads to the formation of calculi in the urinary tract and consequently, obstructive uropathy, pyelonephritis, and even renal failure. Classification of cystinuria can be based on urine excretion of cystine and dibasic amino acids or genetic profiling (Font-Llitjós et al. 2005; Dello Strologo et al. 2002).
The inheritance of cystinuria is not clear cut (Dello Strologo et al. 2002). In general, cystinuria shows classic autosomal recessive inheritance. However, obligate heterozygote carriers have variable urinary excretion of cystine and dibasic amino acids and may develop nephrolithiasis. Therefore, cystinuria sometimes can be regarded as an autosomal dominant disorder with incomplete penetrance (Barbosa et al. 2012). SLC3A1 and SLC7A9 are the two known cystinuria causative genes.
The SLC7A9 gene (12 coding exons) encodes a member protein of light subunits of amino acid transporters, which functions in the reabsorption of cystine in the kidney tubule. To date, documented genetic defects of SLC7A9 include missense, nonsense, splicing variants, small indels and large deletions/duplications (Human Gene Mutation Database). In particular, large deletions and duplications are common in both SLC3A1 and SLC7A9 (Schmidt et al. 2003; Barbosa et al. 2012).
For this Next Generation Sequencing (NGS) test, sequencing is accomplished by capturing specific regions with an optimized solution-based hybridization kit, followed by massively parallel sequencing of the captured DNA fragments. Additional Sanger sequencing is performed for regions not captured or with insufficient number of sequence reads.
For Sanger sequencing, polymerase chain reaction (PCR) is used to amplify targeted regions. After purification of the PCR products, cycle sequencing is carried out using the ABI Big Dye Terminator v.3.0 kit. PCR products are resolved by electrophoresis on an ABI 3730xl capillary sequencer. In nearly all cases, cycle sequencing is performed separately in both the forward and reverse directions.
This test provides full coverage of all coding exons of the SLC7A9 gene, plus ~10 bases of flanking noncoding DNA. We define full coverage as >20X NGS reads or Sanger sequencing.
Indications for Test
Candidates for this test are patients with cystinuria. Testing is also indicated for family members of patients who have known mutations in the SLC7A9 gene.
|Official Gene Symbol||OMIM ID|
- Genetic Counselor Team - firstname.lastname@example.org
- Wuyan Chen, PhD - email@example.com
- Barbosa M. et al. 2012. Clinical Genetics. 81: 47-55. PubMed ID: 21255007
- Dello Strologo L. et al. 2002. Journal of the American Society of Nephrology : Jasn. 13: 2547-53. PubMed ID: 12239244
- Font-Llitjós M. et al. 2005. Journal of Medical Genetics. 42: 58-68. PubMed ID: 15635077
- Human Gene Mutation Database (HGMD).
- Schmidt C. et al. 2003. Kidney International. 64: 1564-72. PubMed ID: 14531788
NextGen Sequencing using PG-Select Capture Probes
We use a combination of Next Generation Sequencing (NGS) and Sanger sequencing technologies to cover the full coding regions of the listed genes plus ~10 bases of non-coding DNA flanking each exon. As required, genomic DNA is extracted from the patient specimen. For NGS, patient DNA corresponding to these regions is captured using an optimized set of DNA hybridization probes. Captured DNA is sequenced using Illumina’s Reversible Dye Terminator (RDT) platform (Illumina, San Diego, CA, USA). Regions with insufficient coverage by NGS are often covered by Sanger sequencing.
For Sanger sequencing, Polymerase Chain Reaction (PCR) is used to amplify targeted regions. After purification of the PCR products, cycle sequencing is carried out using the ABI Big Dye Terminator v.3.0 kit. PCR products are resolved by electrophoresis on an ABI 3730xl capillary sequencer. In nearly all cases, cycle sequencing is performed separately in both the forward and reverse directions.
Patient DNA sequence is aligned to the genomic reference sequence for the indicated gene region(s). All differences from the reference sequences (sequence variants) are assigned to one of five interpretation categories, listed below, per ACMG Guidelines (Richards et al. 2015).
(1) Pathogenic Variants
(2) Likely Pathogenic Variants
(3) Variants of Uncertain Significance
(4) Likely Benign Variants
(5) Benign Variants
Human Genome Variation Society (HGVS) recommendations are used to describe sequence variants (http://www.hgvs.org). Rare variants and undocumented variants are nearly always classified as likely benign if there is no indication that they alter protein sequence or disrupt splicing.
As of March 2016, 6.36 Mb of sequence (83 genes, 1557 exons) generated in our lab was compared between Sanger and NextGen methodologies. We detected no differences between the two methods. The comparison involved 6400 total sequence variants (differences from the reference sequences). Of these, 6144 were nucleotide substitutions and 256 were insertions or deletions. About 65% of the variants were heterozygous and 35% homozygous. The insertions and deletions ranged in length from 1 to over 100 nucleotides.
In silico validation of insertions and deletions in 20 replicates of 5 genes was also performed. The validation included insertions and deletions of lengths between 1 and 100 nucleotides. Insertions tested in silico: 2200 between 1 and 5 nucleotides, 625 between 6 and 10 nucleotides, 29 between 11 and 20 nucleotides, 25 between 21 and 49 nucleotides, and 23 at or greater than 50 nucleotides, with the largest at 98 nucleotides. All insertions were detected. Deletions tested in silico: 1813 between 1 and 5 nucleotides, 97 between 6 and 10 nucleotides, 32 between 11 and 20 nucleotides, 20 between 21 and 49 nucleotides, and 39 at or greater than 50 nucleotides, with the largest at 96 nucleotides. All deletions less than 50 nucleotides in length were detected, 13 greater than 50 nucleotides in length were missed. Our standard NextGen sequence variant calling algorithms are generally not capable of detecting insertions (duplications) or heterozygous deletions greater than 100 nucleotides. Large homozygous deletions appear to be detectable.
Interpretation of the test results is limited by the information that is currently available. Better interpretation should be possible in the future as more data and knowledge about human genetics and this specific disorder are accumulated.
When Sanger sequencing does not reveal any difference from the reference sequence, or when a sequence variant is homozygous, we cannot be certain that we were able to detect both patient alleles. Occasionally, a patient may carry an allele which does not amplify, due to a large deletion or insertion. In these cases, the report will contain no information about the second allele. Our Sanger and NGS Sequencing tests are generally not capable of detecting Copy Number Variants (CNVs).
We sequence all coding exons for each given transcript, plus ~10 bp of flanking non-coding DNA for each exon. Test reports contain no information about other portions of the gene, such as regulatory domains, deep intronic regions or any currently uncharacterized alternative exons.
In most cases, we are unable to determine the phase of sequence variants. In particular, when we find two likely causative mutations for recessive disorders, we cannot be certain that the mutations are on different alleles.
Our ability to detect minor sequence variants due to somatic mosaicism is limited. Sequence variants that are present in less than 50% of the patient’s nucleated cells may not be detected.
Runs of mononucleotide repeats (eg (A)n or (T)n) with n >8 in the reference sequence are generally not analyzed because of strand slippage during PCR.
Unless otherwise indicated, DNA sequence data is obtained from a specific cell-type (usually leukocytes from whole blood). Test reports contain no information about the DNA sequence in other cell-types.
We cannot be certain that the reference sequences are correct.
Rare, low probability interpretations of sequencing results, such as for example the occurrence of de novo mutations in recessive disorders, are generally not included in the reports.
We have confidence in our ability to track a specimen once it has been received by PreventionGenetics. However, we take no responsibility for any specimen labeling errors that occur before the sample arrives at PreventionGenetics.
myPrevent - Online Ordering
- The test can be added to your online orders in the Summary and Pricing section.
- Once the test has been added log in to myPrevent to fill out an online requisition form.
- A completed requisition form must accompany all specimens.
- Billing information along with specimen and shipping instructions are within the requisition form.
- All testing must be ordered by a qualified healthcare provider.
(Delivery accepted Monday - Saturday)
- Collect 3 ml -5 ml (5 ml preferred) of whole blood in EDTA (purple top tube) or ACD (yellow top tube). For Test #500-DNA Banking only, collect 10 ml -20 ml of whole blood.
- For small babies, we require a minimum of 1 ml of blood.
- Only one blood tube is required for multiple tests.
- Ship blood tubes at room temperature in an insulated container. Do not freeze blood.
- During hot weather, include a frozen ice pack in the shipping container. Place a paper towel or other thin material between the ice pack and the blood tube.
- In cold weather, include an unfrozen ice pack in the shipping container as insulation.
- At room temperature, blood specimen is stable for up to 48 hours.
- If refrigerated, blood specimen is stable for up to one week.
- Label the tube with the patient name, date of birth and/or ID number.
(Delivery accepted Monday - Saturday)
- Send in screw cap tube at least 5 µg -10 µg of purified DNA at a concentration of at least 20 µg/ml for NGS and Sanger tests and at least 5 µg of purified DNA at a concentration of at least 100 µg/ml for gene-centric aCGH, MLPA, and CMA tests, minimum 2 µg for limited specimens.
- For requests requiring more than one test, send an additional 5 µg DNA per test ordered when possible.
- DNA may be shipped at room temperature.
- Label the tube with the composition of the solute, DNA concentration as well as the patient’s name, date of birth, and/or ID number.
- We only accept genomic DNA for testing. We do NOT accept products of whole genome amplification reactions or other amplification reactions.
(Delivery preferred Monday - Thursday)
- PreventionGenetics should be notified in advance of arrival of a cell culture.
- Culture and send at least two T25 flasks of confluent cells.
- Some panels may require additional flasks (dependent on size of genes, amount of Sanger sequencing required, etc.). Multiple test requests may also require additional flasks. Please contact us for details.
- Send specimens in insulated, shatterproof container overnight.
- Cell cultures may be shipped at room temperature or refrigerated.
- Label the flasks with the patient name, date of birth, and/or ID number.
- We strongly recommend maintaining a local back-up culture. We do not culture cells.