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Laboratoire de Génétique Moléculaire, Faculté des Sciences Pharmaceutiques et Biologiques, Université Paris V, 4 Avenue de l'Observatoire, 75006 Paris, France.
a Author for correspondence. Fax 33 1 44 07 17 54; e-mail dvidaud{at}teaser.fr
| Abstract |
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Methods: We developed an assay using TaqManTM real-time quantitative PCR, with p15 as the test sequence and albumin (ALB) as the reference gene. The normalized ratio of p15/ALB is expected to yield a value of ~1 in individuals without the deletion, whereas a ratio of ~0.5, indicating p15 haploinsufficiency, is expected in predisposed individuals.
Results: All patients harboring the previously defined at-risk haplotype were correctly identified using this approach. In six individuals with deletions, the p15/ALB ratios were 0.4720.556 (SD, 0.0130.078). In the five individuals without deletions, the ratios were 0.9191.019 (SD, 0.0060.075).
Conclusions: This is the first report of a high-throughput, automatable gene dosage assay successfully applied to the identification of a germ-line deletion. This approach, not limited by marker informativeness or the need for harvesting live cells, can be applied to any condition with haploinsufficiency and extended to the characterization of most abnormalities of the ploidy.© 1999 American Association for Clinical Chemistry
| Introduction |
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The recent development of real-time quantitative PCR based on the 5'-3' exonuclease activity of the Taq polymerase (3), referred as TaqManTM, has offered the opportunity to set up an original gene dosage assay using the ABI PRISM 7700 Sequence Detection System (Perkin-Elmer Biosystems), in which p15 was selected as the test sequence and albumin (ALB) as the reference disomic gene used to normalize the amounts of input genomic DNAs. Briefly, during the PCR process, a dual-labeled TaqMan probe annealed to the target sequence is cleaved by the 5'-3' exonuclease activity of the Taq polymerase, releasing the reporter dye (FAM) from the quencher dye (TAMRA). Upon excitation of by an argon laser, the release of the FAM produces an increase in the fluorescent emission, which is captured at 518 nm by a charged-couple device camera and analyzed through the algorithms of the ABI PRISM 7700 Sequence Detection System computer software (4)(5).
To develop a direct automatable predictive test in the family with INK4 locus haploinsufficiency, this p15 gene dosage assay was applied to a panel of 11 test, haplotype-positive or -negative, individuals from this large kindred.
| Patients and Methods |
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real-time pcr
In the TaqMan approach, one of the main variables, referred
as Ct, is defined as the fractional cycle number
at which the fluorescence generated by cleavage of the probe (
Rn)
crosses a fixed threshold. The target gene copy number in
unknown samples is inferred by plotting the Ct
value against a calibration curve. To correctly determine the
starting copy number regardless of the precise amounts and qualities of
input genomic DNAs, we also quantified an internal control gene
(ALB) in each single reaction. The normalized gene dose,
N, is given by the following ratio:
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The 7700 Sequence Detection System software automatically
determines the Ct value and infers the starting
copy number in each sample. The real-time PCR and the calibration curve
are presented in Fig. 2
for ALB. Normalized gene doses N
were then determined for each sample in three independent assays, and
the corresponding means were calculated. In this method, a
nondeleted test sequence is expected to yield a ratio of
N = 1, as opposed to N = 0.5 when it is
heterozygously deleted.
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primers and probes
Primers and probes were chosen with assistance of the computer
programs OligoTM, Ver. 4.0 (National Biosciences)
and Primer ExpressTM, Ver. 1.0 (PE Biosystems).
Their nucleotide sequences are shown in Table 1
. Primers were purchased from
Life Technologies, and probes were purchased from PE Biosystems. The
TaqMan PCR Core Reagent Kit, MicroAmp Optical
TubesTM, and MicroAmp Optical
CapsTM were from PE Biosystems.
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pcr amplification
TaqMan amplification reactions were carried out in reaction
volumes of 50 µL, using the components as supplied in the TaqMan PCR
Core Reagent Kit. Each reaction contained 1x TaqMan buffer; 200
nmol/L each primer; 100 nmol/L each corresponding fluorogenic probe; 5
mmol/L MgCl2, 200 µmol/L each dATP, dCTP, and
dGTP; 400 µmol/L dUTP; 1.25 U of AmpliTaqTM
Gold, and 0.5 U of AmpEraseTM uracil
N-glycosylase. Each sample was analyzed in triplicate, using 20 ng of
DNA in each reaction. The reference DNA was serially diluted and run in
parallel to establish the calibration curve and to infer copy numbers
from the cycle thresholds (Cts), assuming a
conversion factor of 6.6 pg of DNA per diploid genome. Thermal cycling
was initiated with a 2-min incubation at 50 °C, followed by a first
denaturation step of 10 min at 95 °C, and then 40 cycles of 95 °C
for 15 s and 65 °C for 1 min.
| Results |
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The six haplotype-sharing affected or healthy individuals showed
INK4 haploinsufficiency, i.e., were heterozygotes for the
INK4 locus deletion [N
0.5
(0.4720.556; SD, 0.0130.078)]. This is in contrast to healthy,
haplotype-negative individuals who were shown to contain two copies of
the p15 gene per cell [N
1
(0.9191.019; SD, 0.0060.075)]. Therefore, all results were
clear-cut with relatively small SDs, rendering the deletional status
unambiguous.
| Discussion |
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In the aim to identify subjects with individual risk in a large French family diagnosed with a CMM-NST syndrome associated with a whole INK4 locus germ-line deletion, a combination of microsatellite markers of the 9p21 region, flanking the deletion and defining the at-risk haplotype was initially studied. However, this indirect test, which was limited by marker informativeness and required the analysis of nuclear families, possibly including both deceased parents and minor children, was not compatible with the present rules of individual consent and secrecy. Thus, a TaqMan PCR-based gene dosage assay based on the quantification of the p15 gene was developed. This direct and accurate test allowed us to rapidly analyze all the family members willing to benefit from predictive testing and to identify those at risk.
After an initial optimization step for adequate primer pairs and probes, the TaqMan gene dosage method can be used efficiently to assay any chromosome, gene, or exon and thus has a wide variety of applications in both clinical and research settings. This is especially the case with autosomal dominant conditions with haploinsufficiency, such as familial breast and ovarian cancer (8), neurofibromatosis type 1 (9), or in carrier detection in X-linked disorders such as Duchenne muscular dystrophy (10). We believe this high-throughput, fast-turnaround time (up to 96 samples/2 h, which corresponds to 12 individuals tested using our approach), and simple method is clearly a cost-effective alternative to fluorescence in situ hybridization in clinical applications. In this respect, TaqMan lends itself particularly well to routine diagnosis of common microdeletion syndromes. Because of its accuracy, this novel approach, more reliable than end-point PCR gene dosage (11) and not limited by short tandem repeat marker informativeness (12), also strengthens the potentialities of DNA diagnosis in the field of human aneuploidies.
| Acknowledgments |
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| Footnotes |
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| References |
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