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Cataract 23 (CTRCT23) via the CRYBA4 Gene

  • Summary and Pricing
  • Clinical Features and Genetics
  • Citations
  • Methods
  • Ordering/Specimens
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TEST METHODS

Sequencing

Test Code Test Copy GenesIndividual Gene PriceCPT Code Copy CPT Codes
2547 CRYBA4$580.00 81479 Add to Order
Targeted Testing

For ordering targeted known variants, please proceed to our Targeted Variants landing page.

Turnaround Time

The great majority of tests are completed within 18 days.

Clinical Sensitivity

The clinical sensitivity of this test may range up to 8%. In three independent studies conducted in India, none of the 60 pediatric cataract cases (Devi et al. 2008), 3.1% (1/32) of patients with lamellar cataract and microphthalmia (Billingsley et al. 2006), and 8% (8/100) of congenital cataract cases (Kumar et al. 2013) showed causative CRYBA4 sequence variants. 

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Clinical Features

Cataract 23 (CTRCT23) is a common congenital, bilateral, lamellar vision disorder that causes blindness in infants (Zhou et al. 2010). It is characterized by the development of blurred and dimmed vision resulting from clouding of the lens (opacification) due to changes in its microarchitecture (Kumar et al. 2013). This particular damage to the lens induces light to scatter as well as proteins to aggregate, thereby resulting in loss of transparency (Hejtmancik 2008). Intrafamilial variability in the morphology and location within the lens commonly occurs in CTRCT23 (Billingsley et al. 2006). The incidence of congenital cataract has been estimated to be roughly 2.5 per 10,000 live births (Wirth et al. 2002; Yi et al. 2011). Perinatal ocular examination in newborns via red reflex examination is generally conducted using an ophthalmoscope (American Academy of Pediatrics 2002), whereas young children are assessed by slit-lamp microscopy (Li et al. 2013). Congenital cataract is usually treated by surgery and early primary intraocular lens implantation during the first year of life (Ventura et al. 2013).

Genetics

CTRCT23 is an autosomal dominant vision disorder that is caused by pathogenic sequence variants in the crystallin, beta-A4 (CRYBA4) gene, which is located on chromosome 22q12.1 (Billingsley et al. 2006). The CRYBA4 gene consists of five coding exons that encode a 196-amino acid structural protein called beta-crystallin A4, which plays an important role in the maintenance of lens transparency and refractive index. Pathogenic sequence variants in the CRYBA4 gene significantly reduces the stability of the beta-crystallin A4 monomer, thus disrupting protein folding. These disease-causing variants also interfere with interactions with other cyrstallin proteins (i.e., CRYBA3 and CRYBB3) that are critical for lens transparency. The CRYBA4 protein is 92%-94% similar to the rat and bovine beta-A4 crystallin proteins (Lampi et al. 1997). The developing human lens consists of approximately 11 major soluble proteins, of which 5% comprises the CRYBA4 protein. To date, a total of about 5 pathogenic CRYBA4 sequence variants have been reported, all of which are missense variants (Human Gene Mutation Database).

Testing Strategy

This test involves bidirectional DNA Sanger sequencing of all coding exons of the CRYBA4 gene. The full coding region of each exon plus ~20 bp of flanking non-coding DNA on either side are sequenced. We will also sequence any single exon (Test #100) in family members of patients with a known mutation or to confirm research results.

Indications for Test

The ideal CRYBA4 test candidates are individuals who present with congenital, autosomal dominant cataract.

Gene

Official Gene Symbol OMIM ID
CRYBA4 123631
Inheritance Abbreviation
Autosomal Dominant AD
Autosomal Recessive AR
X-Linked XL
Mitochondrial MT

Disease

Name Inheritance OMIM ID
Cataract 23 AD 610425

Related Tests

Name
Anophthalmia / Microphthalmia Sequencing Panel
Congenital Cataracts Sequencing Panel

CONTACTS

Genetic Counselors
Geneticist
Citations
  • American Academy of Pediatrics. 2002. Pediatrics. 109: 980-1. PubMed ID: 11986467
  • Billingsley G. et al. 2006. American Journal of Human Genetics. 79: 702-9. PubMed ID: 16960806
  • Devi RR. et al. 2008. Molecular Vision. 14: 1157-70. PubMed ID: 18587492
  • Hejtmancik J.F. 2008. Seminars in cell & developmental biology. 19: 134-49. PubMed ID: 18035564
  • Human Gene Mutation Database (Bio-base).
  • Kumar M. et al. 2013. Molecular Vision. 19: 2436-50. PubMed ID: 24319337
  • Lampi KJ. et al. 1997. The Journal of Biological Chemistry. 272: 2268-75. PubMed ID: 8999933
  • Li LH. et al. 2013. The British Journal of Ophthalmology. 97: 588-91. PubMed ID: 23426739
  • Ventura MC. et al. 2013. Arquivos Brasileiros De Oftalmologia. 76: 240-3. PubMed ID: 24061837
  • Wirth MG. et al. 2002. The British Journal of Ophthalmology. 86: 782-6. PubMed ID: 12084750
  • Yi J. et al. 2011. International Journal of Ophthalmology. 4: 422-32. PubMed ID: 22553694
  • Zhou Guangkai et al. 2010. Molecular Vision. 16: N/A. PubMed ID: 20577656
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TEST METHODS

Bi-Directional Sanger Sequencing

Test Procedure

Nomenclature for sequence variants was from the Human Genome Variation Society (http://www.hgvs.org).  As required, DNA is extracted from the patient specimen.  PCR is used to amplify the indicated exons plus additional flanking non-coding sequence.  After cleaning of the PCR products, cycle sequencing is carried out using the ABI Big Dye Terminator v.3.0 kit.  Products are resolved by electrophoresis on an ABI 3730xl capillary sequencer.  In most cases, sequencing is performed in both forward and reverse directions; in some cases, sequencing is performed twice in either the forward or reverse directions.  In nearly all cases, the full coding region of each exon as well as 20 bases of non-coding DNA flanking the exon are sequenced.

Analytical Validity

As of March 2016, we compared 17.37 Mb of Sanger DNA sequence generated at PreventionGenetics to NextGen sequence generated in other labs. We detected only 4 errors in our Sanger sequences, and these were all due to allele dropout during PCR. For Proficiency Testing, both external and internal, in the 12 years of our lab operation we have Sanger sequenced roughly 8,800 PCR amplicons. Only one error has been identified, and this was due to sequence analysis error.

Our Sanger sequencing is capable of detecting virtually all nucleotide substitutions within the PCR amplicons. Similarly, we detect essentially all heterozygous or homozygous deletions within the amplicons. Homozygous deletions which overlap one or more PCR primer annealing sites are detectable as PCR failure. Heterozygous deletions which overlap one or more PCR primer annealing sites are usually not detected (see Analytical Limitations). All heterozygous insertions within the amplicons up to about 100 nucleotides in length appear to be detectable. Larger heterozygous insertions may not be detected. All homozygous insertions within the amplicons up to about 300 nucleotides in length appear to be detectable. Larger homozygous insertions may masquerade as homozygous deletions (PCR failure).

Analytical Limitations

In exons where our sequencing did not reveal any variation between the two alleles, we cannot be certain that we were able to PCR amplify both of the patient’s alleles. Occasionally, a patient may carry an allele which does not amplify, due for example to a deletion or a large insertion. In these cases, the report contains no information about the second allele.

Similarly, our sequencing tests have almost no power to detect duplications, triplications, etc. of the gene sequences.

In most cases, only the indicated exons and roughly 20 bp of flanking non-coding sequence on each side are analyzed. Test reports contain little or no information about other portions of the gene, including many regulatory regions.

In nearly all 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 for example 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 and cycle sequencing.

Unless otherwise indicated, the sequence data that we report are based on DNA isolated from a specific tissue (usually leukocytes). Test reports contain no information about gene sequences in other tissues.

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Ordering Options


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.
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.

SPECIMEN TYPES
WHOLE BLOOD

(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.

DNA

(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.

CELL CULTURE

(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.
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