Non-Syndromic Monogenic Obesity via KSR2 Gene Sequencing with CNV Detection
- Summary and Pricing
- Clinical Features and Genetics
Sequencing and Del/Dup
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This test is also offered via our exome backbone with CNV detection (click here). The exome-based test may be higher priced, but permits reflex to the entire exome or to any other set of clinically relevant genes.
For ordering sequencing of targeted known variants, please proceed to our Targeted Variants landing page.
The great majority of tests are completed within 20 days.
In a study of 2,101 severely obese individuals of mixed European ancestry, 2.1% were found to have rare variants in the KSR2 gene in contrast with 1.0% of matched controls (Pearce et al. 2013. PubMed ID: 24209692). These data provide a rough estimate of clinical sensitivity at approximately 1%. Analytical sensitivity is expected to be high because all reported causative variants are detectable by sequencing.
Non-syndromic monogenic obesity is a group of single-gene disorders with obesity as an isolated or predominant feature. Obesity is defined as an increase in fat mass that is sufficient to adversely affect health and reduce longevity (Fontaine et al. 2003. PubMed ID: 12517229). Clinically, body-mass index (BMI) is the accepted measure of adipose mass. For adults, a BMI greater than 30 is typically used to diagnose a person as obese. BMI is a multifactorial trait that is frequently influenced by multiple genes as well as environmental and lifestyle factors (Gusev et al. 2014. PubMed ID: 25439723). However, to date, eleven genes have been specifically implicated in non-syndromic monogenic obesity: LEP, LEPR, SH2B1, POMC, PCSK1, MC4R, NTRK2, BDNF, SIM1, KSR2, and TUB (Pigeyre et al. 2016. PubMed ID: 27154742).
KSR2-related obesity is characterized by severe early-onset obesity (<10 years of age), hyperphagia in childhood, low heart rate, reduced basal metabolic rate, and severe insulin resistance (Pearce et al. 2013. PubMed ID: 24209692). Importantly, the Pearce et al. study provided anecdotal evidence that carriers of suspected pathogenic KSR2 variants respond well to the common anti-diabetes medication Metformin. Therefore, this molecular diagnosis may provide information that would guide treatment decisions for patients.
Pathogenic variants in KSR2 cause autosomal dominant non-syndromic obesity. The KSR2 gene encodes the kinase suppressor of Ras2 (KSR2) protein which regulates energy consumption, metabolic expenditure, and substrate utilization via the Ras/Raf/MEK/ERK signaling pathway. KSR2 is an intracellular scaffolding protein. In response to growth factor signaling, it translocates to the plasma membrane where it catalyzes MEK phosphorylation by scaffolding Ras, Raf, and MEK in a complex (Michaud et al. 1997. PubMed ID: 9371754; Roy et al. 2002. PubMed ID: 11850406). Through this process KSR2 activates gene expression by coordinating the ERK signaling cascade in both time and space.
Functional studies in cell culture have determined that pathogenic variants in the KSR2 gene disrupt Raf-MEK-ERK signaling causing decreases in fatty acid oxidation and glucose oxidation. These defects are attenuated by the application of the diabetes drug metformin (Pearce et al. 2013. PubMed ID: 24209692).
The predominant inheritance of KSR2-related obesity is autosomal dominant; however, one severely obese individual has been described with compound heterozygous rare variants, and recessive inheritance has not been ruled out (Pearce et al. 2013. PubMed ID: 24209692). Incomplete penetrance and variable expressivity was also noted in this study, so some carriers may be unaffected or mildly affected. The causative variants include missense, nonsense, and frameshift alterations, the majority of which cluster to the C-terminal kinase domain. To date, no founder variants, large gene rearrangements, or de novo variants have been described.
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.
Copy number variants (CNVs) are also detected from NGS data. We utilize a CNV calling algorithm that compares mean read depth and distribution for each target in the test sample against multiple matched controls. Neighboring target read depth, distribution, and zygosity of any variants within each target region are used to reinforce CNV calls. All CNVs are confirmed using another technology such as aCGH, MLPA, or PCR before they are reported.
This test provides full coverage of all coding exons of the KSR2 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 isolated obesity and other features which may include childhood hyperphagia, low heart rate, and insulin resistance.
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- Genetic Counselor Team - firstname.lastname@example.org
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Sequencing and Deletion/Duplication Testing via 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 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.
Deletion and Duplication Testing via NGS
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.