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Acute Intermittent Porphyria via the HMBS Gene

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

Sequencing

Test Code TestIndividual Gene PriceCPT Code Copy CPT Codes
776 HMBS$870.00 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

In patients with decreased PBG deaminase activity indicative of AIP, underlying mutations in the HMBS gene were detected in 240 of 252 individuals (Puy et al. 1997). Analytical sensitivity for detection of causative mutations in the HMBS gene is >95% as large deletions are present in only a few cases (Whatley et al. 2009).

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Deletion/Duplication Testing via aCGH

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

# of Genes Ordered

Total Price

1

$690

2

$730

3

$770

4-10

$840

11-30

$1,290

31-100

$1,670

Over 100

Call for quote

Turnaround Time

The great majority of tests are completed within 28 days.

Clinical Features

Acute Intermittent porphyria (AIP) is a metabolic disorder due to impairment of the third enzyme, porphobilinogen (PBG) deaminase, in the heme biosynthetic pathway. AIP is the most common form of acute porphyria affecting about 1 in 10,000 Swedish, 3 in 100,000 Finnish, and 5-10 in 100,000 US individuals. It is characterized by intermittent attacks of neurological dysfunction including severe abdominal pain, neuropsychiatric symptoms, nausea, vomiting, tachycardia and hypertension. Acute, life-threatening, attacks may be provoked by certain drugs, stress, infections, alcohol, caloric restriction, or endocrine factors with bouts typically resolving within two weeks. Symptom onset primarily occurs during the third and fourth decades of life with women being more susceptible to attacks, although some individuals with AIP may remain asymptomatic throughout life (Whatley and Badminton 1993). Pre-symptomatic diagnosis of AIP is helpful in preventing acute attacks through avoidance of certain triggers highlighted above. Biochemical analysis showing elevated PBG and aminolevulinic acid (ALA) levels in urine are primary indicators for AIP but may appear normal during asymptomatic phases. Impaired erythrocyte PBGD activity is also a hallmark of AIP, but is also seen in patients with anemia, liver disease, uremia, chronic polyarthritis, and malignancies. Genetic testing has been shown helpful in diagnosis of AIP during asymptomatic phases and in differential diagnosis (Puy et al. 1997; Kauppinen and Fraunberg 2002).

Genetics

AIP is primarily inherited in an autosomal dominant manner through mutations in the HMBS gene with incomplete penetrance. Individuals homozygous for HMBS mutations have also been reported in a few cases with severe AIP presenting before two years of age (Solis et al. 2004). Missense, splice site alterations, nonsense, and small insertions/deletions represent 45%, 18%, 15%, and 22% of causative variants for AIP with about half occurring within exons 10-14 (Puy et al. 1997; Kauppinen and Fraunberg 2002; Yang et al. 2008). Large deletions have been reported in a few cases (Whatley et al. 2009). The HMBS gene encodes porphobilinogen (PGB) deaminase which is the third enzyme involved in the heme biosynthetic pathway. Causative variants primarily lead to premature protein termination or destabilization leading to decreased PGB deaminase levels and activity (Whatley and Badminton 1993; Bustad et al. 2013).

Testing Strategy

Our DNA sequencing test involves bidirectional Sanger sequencing of the entire HMBS gene plus ~20 bp of flanking non-coding DNA on either side of each exon. 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

Patients with symptoms consistent with AIP and biochemical analysis indicating elevated urinary PBG and ALA levels or decreased erythrocyte PBG deaminase activity are primary indicators for disease. With patients being asymptomatic between attacks, urinary PGB and ALA levels may appear normal.

Gene

Official Gene Symbol OMIM ID
HMBS 609806
Inheritance Abbreviation
Autosomal Dominant AD
Autosomal Recessive AR
X-Linked XL
Mitochondrial MT

Disease

Name Inheritance OMIM ID
Acute Intermittent Porphyria 176000

CONTACTS

Genetic Counselors
Geneticist
Citations
  • Bustad HJ, Vorland M, Rønneseth E, Sandberg S, Martinez A, Toska K. 2013. Conformational stability and activity analysis of two hydroxymethylbilane synthase mutants, K132N and V215E, with different phenotypic association with acute intermittent porphyria. Bioscience Reports 33: 617–626. PubMed ID: 23815679
  • Kauppinen R, Fraunberg M von und zu. 2002. Molecular and biochemical studies of acute intermittent porphyria in 196 patients and their families. Clinical chemistry 48: 1891–1900. PubMed ID: 12406973
  • Puy H, Deybach JC, Lamoril J, Robreau AM, Silva V Da, Gouya L, Grandchamp B, Nordmann Y. 1997. Molecular epidemiology and diagnosis of PBG deaminase gene defects in acute intermittent porphyria. The American Journal of Human Genetics 60: 1373–1383. PubMed ID: 9199558
  • Solis C, Martinez-Bermejo A, Naidich TP, Kaufmann WE, Astrin KH, Bishop DF, Desnick RJ. 2004. Acute intermittent porphyria: studies of the severe homozygous dominant disease provides insights into the neurologic attacks in acute porphyrias. Arch. Neurol. 61: 1764-1770. PubMed ID: 15534187
  • Whatley SD, Badminton MN. 1993. Acute Intermittent Porphyria. 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: 20301372
  • Whatley SD, Mason NG, Woolf JR, Newcombe RG, Elder GH, Badminton MN. 2009. Diagnostic Strategies for Autosomal Dominant Acute Porphyrias: Retrospective Analysis of 467 Unrelated Patients Referred for Mutational Analysis of the HMBS, CPOX, or PPOX Gene. Clinical Chemistry 55: 1406–1414. PubMed ID: 19460837
  • Yang C-C, Kuo H-C, You H-L, Wang J, Huang C-C, Liu C-Y, Lan M-Y, Stephenson DA, Lee M-J. 2008. HMBS Mutations in Chinese Patients with Acute Intermittent Porphyria. Annals of Human Genetics 72: 683–686. PubMed ID: 18627369
Order Kits
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.

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.
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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