Homocystinuria via the CBS Gene

  • Summary and Pricing
  • Clinical Features and Genetics
  • Citations
  • Methods
  • Ordering/Specimens
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NGS Sequencing

Test Code Test Copy GenesIndividual Gene PriceCPT Code Copy CPT Codes
5067 CBS$640.00 81406 Add to Order
Pricing Comment

Our most cost-effective testing approach is NextGen sequencing with Sanger sequencing supplemented as needed to ensure sufficient coverage and to confirm NextGen calls that are pathogenic, likely pathogenic or of uncertain significance. If, however, full gene Sanger sequencing only is desired (for purposes of insurance billing or STAT turnaround time for example), please see link below for Test Code, pricing, and turnaround time information.

For Sanger Sequencing click here.
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 26 days.

Clinical Sensitivity

Overall, the sensitivity of this test is expected to be quite high, as most patients reported to date have been found to have two CBS variants detectable via direct sequencing. The majority of studies with larger patient cohorts have reported pathogenic variant detection via direct sequencing in ~95-98% of patient alleles (Gaustadnes et al. 2002; Kruger et al. 2003; Cozar et al. 2011; Karaca et al. 2014).

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Deletion/Duplication Testing via aCGH

Test Code Test Copy GenesIndividual Gene PriceCPT Code Copy CPT Codes
600 CBS$690.00 81479 Add to Order
Pricing Comment

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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 28 days.

Clinical Sensitivity

Large deletions in the CBS gene have been reported, but appear to be a rare cause of disease (Human Gene Mutation Database).

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Clinical Features

The transsulfuration pathway involves the interconversion of methionine to homocysteine, and eventually cysteine. This pathway is the main mechanism for removal of methionine (Mudd, Levy and Kraus 2014). The pyridoxine dependent cystathionine β-synthase (CBS) enzyme catalyzes the conversion of homocysteine to cystathionine (Picker and Levy 2014; Mudd, Levy and Kraus 2014). Defects in this enzyme are the most commonly observed cause of homocystinuria without associated methylmalonic aciduria (Picker and Levy 2014; Mudd, Levy and Kraus 2014). Biochemically, patients with this disorder are found to have greatly increased concentrations of plasma and urine homocysteine, total homocysteine, and marked hypermethioninemia (Picker and Levy 2014). This is in contrast to homocystinura caused by defects in the MTR and MTRR genes, which are typically associated with hypomethioninemia (Carrillo-Carrasco et al. 2013; Watkins and Rosenblatt 2014). Clinically, the most common symptoms are developmental delay or intellectual disability, ectopia lentis and/or severe myopia, skeletal abnormalities which can include thinning and lengthening of the long bones as well as osteoporosis, and vascular disease, including potentially fatal thromboembolisms (Kraus et al. 1999; Picker and Levy 2014; Mudd, Levy and Kraus 2014). Other features observed in CBS deficient patients may include psychiatric disturbances, seizures, and extrapyramidal signs (Picker and Levy 2014; Mudd, Levy and Kraus 2014). Age of onset and expressivity of clinical features displayed by patients varies widely, even within sibships, and can range from affected newborns to adults who were previously asymptomatic first presenting with a thromboembolytic event (Carrillo-Carrasco et al. 2013; Picker and Levy 2014; Mudd, Levy and Kraus 2014). Affected individuals may be identified via newborn screening programs designed to detect hypermethioninemia. However, certain individuals with CBS deficiency may not have increased levels of methionine within the first two to three days of life when newborn screening is performed. Thus, in cases where individuals exhibit clinical symptoms suggestive of CBS deficiency yet had negative newborn screening results, testing for CBS deficiency should still be considered (Picker and Levy 2014). Treatment includes pyridoxine therapy, to which only about half of affected patients are responsive (Kraus et al. 1999), betaine therapy, folate and vitamin B12 supplementation, and limiting dietary intake of protein and methionine (Picker and Levy 2014). Dietary control is most effective at preventing or ameliorating symptoms when introduced early in life (Mudd, Levy and Kraus 2014).


Classic homocystinuria is inherited in an autosomal recessive manner. The CBS gene is the only gene known to be involved. To date, nearly 200 pathogenic variants have been reported in CBS (Human Gene Mutation Database; CBS Mutation Database). The vast majority of documented variants are missense, although small deletions, insertions and splice variants are also observed. Only a handful of nonsense variants have been reported in this gene to date. In addition, gross deletions have been reported, and the CBS gene is thought to be fairly susceptible to large deletions and rearrangements due to the somewhat high number of Alu repeats at this locus (Mudd, Levy and Kraus 2014). The reported pathogenic variants are found predominantly in the conserved active core of the enzyme, encoded by exons two through seven, with approximately a quarter of missense variants identified to date found in exon three. However, pathogenic variants have been reported in all coding exons of the CBS gene (Mudd, Levy and Kraus 2014; Human Gene Mutation Database). Three pathogenic variants in the CBS gene are most commonly reported. The first is the missense variant Ile278Thr, which is considered to be a panethnic variant and is thought to account for approximately half of all homocystinuric alleles (Kraus et al. 1999; Mudd, Levy and Kraus 2014). The second is the missense change Gly307Ser, which is the most commonly reported causative variant in patients of Celtic origin and has been reported to account for greater than 70% of alleles in affected individuals of Irish descent (Kraus et al. 1999; Mudd, Levy and Kraus 2014). Lastly, more recent reports have shown that the Thr191Met missense variant makes up approximately 20 - 75% of alleles in individuals of Iberian descent (Urreizti et al. 2006; Cozar et al. 2011). The CBS protein is part of the transsulfuration pathway in the cell, and is responsible for catalyzing the condensation of serine with homocysteine to form cystathionine, which can then be converted to cysteine. The exact manner in which CBS deficiency leads to the observed clinical symptoms is not fully understood, although a number of possibilities have been proposed (Mudd, Levy and Kraus 2014).

Testing Strategy

For this NextGen test, the full coding regions plus ~10 bp of non-coding DNA flanking each exon are sequenced for the gene listed below. 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 any regions not captured or with insufficient number of sequence reads. All pathogenic, likely pathogenic, or variants of uncertain significance are confirmed by Sanger sequencing.

Indications for Test

Individuals with hypermethioninemia and hyperhomocystinuria and/or hyperhomocystinemia are good candidates for this test, as are individuals with clinical features consistent with CBS deficiency. Family members of patients known to have CBS variants are also good candidates. We will also sequence the CBS gene to determine carrier status.


Official Gene Symbol OMIM ID
CBS 613381
Inheritance Abbreviation
Autosomal Dominant AD
Autosomal Recessive AR
X-Linked XL
Mitochondrial MT


Name Inheritance OMIM ID
Homocystinuria Due To Cbs Deficiency 236200

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Genetic Counselors
  • Carrillo-Carrasco N, Adams D, Venditti CP. 2013. Disorders of Intracellular Cobalamin Metabolism. In: Pagon RA, Adam MP, Ardinger HH, Bird TD, Dolan CR, Fong C-T, Smith RJ, and Stephens K, editors. GeneReviews(®), Seattle (WA): University of Washington, Seattle. PubMed ID: 20301503
  • CBS Mutation Database.
  • Cozar M. et al. 2011. Human Mutation. 32: 835-42.  PubMed ID: 21520339
  • Gaustadnes M. et al. 2002. Human Mutation. 20: 117-26.  PubMed ID: 12124992
  • Human Gene Mutation Database (Bio-base).
  • Karaca M. et al. 2014. Gene. 534: 197-203.  PubMed ID: 24211323
  • Kraus JP. et al. 1999. Human Mutation. 13: 362-75.  PubMed ID: 10338090
  • Kruger W.D. et al. 2003. Human Mutation. 22: 434-41.  PubMed ID: 14635102
  • Mudd HS, Levy HL, Kraus JP. 2014. Disorders of Transsulfuration. In: Valle D, Beaudet AL, Vogelstein B, et al., editors.New York, NY: McGraw-Hill. OMMBID. 
  • Picker JD and Levy HL. 2014. Homocystinuria Caused by Cystathionine Beta-Synthase Deficiency. In: Pagon RA, Adam MP, Ardinger HH, Bird TD, Dolan CR, Fong C-T, Smith RJ, and Stephens K, editors. GeneReviews(®), Seattle (WA): University of Washington, Seattle. PubMed ID: 20301697
  • Urreizti R. et al. 2006. Journal of Human Genetics. 51: 305-13.  PubMed ID: 16479318
  • Watkins D. and Rosenblatt D.S. 2014. Inherited Disorders of Folate and Cobalamin Transport and Metabolism. In: Valle D, Beaudet A.L., Vogelstein B, et al., editors. New York, NY: McGraw-Hill. OMMBID.
Order Kits

NextGen Sequencing using PG-Select Capture Probes

Test Procedure

We use a combination of Next Generation Sequencing (NGS) and Sanger sequencing technologies to cover the full coding regions of the listed genes plus ~20 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 covered by Sanger sequencing.  All pathogenic, likely pathogenic, or variants of uncertain significance are confirmed 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, Common Variants

Human Genome Variation Society (HGVS) recommendations are used to describe sequence variants (  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.

Analytical Validity

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.   

Analytical Limitations

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 ~20 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.

Deletion/Duplication Testing via Array Comparative Genomic Hybridization

Test Procedure

Equal amounts of genomic DNA from the patient and a gender matched reference sample are amplified and labeled with Cy3 and Cy5 dyes, respectively. To prevent any sample cross contamination, a unique sample tracking control is added into each patient sample. Each labeled patient product is then purified, quantified, and combined with the same amount of reference product. The combined sample is loaded onto the designed array and hybridized for at least 22-42 hours at 65°C. Arrays are then washed and scanned immediately with 2.5 µM resolution. Only data for the gene(s) of interest for each patient are extracted and analyzed.

Analytical Validity

PreventionGenetics' high density gene-centric custom designed aCGH enables the detection of relatively small deletions and duplications within a single exon of a given gene or deletions and duplications encompassing the entire gene. PreventionGenetics has established and verified this test's accuracy and precision.

Analytical Limitations

Our dense probe coverage may allow detection of deletions/duplications down to 100 bp; however due to limitations and probe spacing this cannot be guaranteed across all exons of all genes. Therefore, some copy number changes smaller than 100-300 bp within a targeted large exon may not be detected by our array.

This array may not detect deletions and duplications present at low levels of mosaicism or those present in genes that have pseudogene copies or repeats elsewhere in the genome.

aCGH will not detect balanced translocations, inversions, or point mutations that may be responsible for the clinical phenotype.

Breakpoints, if occurring outside the targeted gene, may be hard to define.

The sensitivity of this assay may be reduced when DNA is extracted by an outside laboratory.

Order Kits

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.
  • 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.
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