FMR2 CCG Repeat Detection Using Junction-Specific PCR Primers
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Solution Overview
Problem
Current methods are inadequate for accurately detecting expansions of tandem repeats, such as CCG repeats in the FMR2 gene, which are associated with fragile X-related mental retardation, and there is a need for a reliable method to determine the presence or absence of these expansions in individuals.
Innovation Solution
A reaction mixture is provided that includes a primer pair with specific upstream and downstream primers designed to hybridize to the junctions of the CCG repeat tract in the FMR2 gene, allowing for the detection of CCG repeat expansions through PCR amplification.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional PCR methods are used to detect tandem repeat expansions, then the detection process is simple, but the measurement precision and reliability are inadequate for accurately distinguishing between normal, intermediate, and expanded alleles
Solution Approach 1:
The patent applies local quality by designing primers with specific local characteristics: the downstream primer contains a CGG repeat segment (less than four consecutive CGG triplet repeats) that hybridizes to the junction between the CCG repeat tract and downstream sequence, while the upstream primer contains a CCG repeat segment that hybridizes to the junction between the CCG repeat tract and upstream sequence. These localized repeat segments in specific primer regions enable accurate detection of CCG repeat expansions by creating characteristic amplification patterns that distinguish normal from expanded alleles.
Solution Approach 2:
The patent employs parameter changes by varying the repeat segment composition and length in the primers. The downstream primer contains fewer CGG repeats (less than four consecutive triplet repeats) compared to traditional primers, and the upstream primer contains multiple CCG repeats. These parameter changes in primer composition create differential amplification efficiency and product size based on the template's CCG repeat status, enabling precise detection of expansion states.
2Reliability
If standard primers are used for amplification, then the ease of manufacture is high, but the ability to distinguish between different repeat allele categories is insufficient
Solution Approach 1:
The patent applies preliminary action by incorporating repeat segments directly into the primer sequences before amplification. The downstream primer is pre-designed with a CGG repeat segment and the upstream primer with a CCG repeat segment, positioned to hybridize at the junctions of the CCG repeat tract. This preliminary incorporation of repeat-specific sequences in the primers ensures that the PCR reaction itself will generate products with sizes and patterns that directly reflect the template's repeat expansion status, enabling reliable allele classification without additional post-PCR analysis steps.
3Measurement precision
If conventional detection methods are applied, then the time required for diagnosis is reduced, but the measurement precision for detecting small repeat expansions is inadequate
Solution Approach 1:
The patent replaces complex mechanical or biochemical detection systems with a simplified PCR-based approach using specially designed primers. Instead of using complex Southern blotting, restriction enzyme digestion, or sequencing methods to detect CCG repeat expansions, the invention uses primers with embedded repeat segments that create characteristic amplification patterns. This substitution maintains rapid detection capability while significantly improving measurement precision for distinguishing normal, intermediate, and expanded alleles through the inherent size and pattern differences in the PCR products.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method enables precise identification of CCG repeat expansions, distinguishing between normal, intermediate, premutation, and full mutation alleles, thereby facilitating early detection and management of fragile X-related disorders.
Implementation Method 1
the downstream primer comprises a CGG repeat segment which contains less than four consecutive CGG triplet repeats and hybridizes to the junction between the 3' end of the CCG repeat tract in the FMR2 gene and sequence directly 3' thereto
Implementation Method 2
the upstream primer comprises a CCG repeat segment comprising multiple repeats, the repeat segment comprising less than four consecutive CCG triplet repeats and hybridizes to the junction between the 5' end of the CCG repeat tract in the FMR2 gene and sequence directly 5' thereto
Data Source
Figure 1
Figure 2A~2F
Figure 2G~2K
AI summary
Methods for determining the presence or absence of expansion of CGG repeat sequence in the FMR1 gene presence or absence of expansion of CCG repeat sequence in the FMR2 gene are provided. The methods are useful in identifying an individual with normal/intermediate, versus premutation or full mutation allele of FMR1 gene and FMR2 gene due to the expansion of CGG repeats and CCG repeats in the S'-untranslated region respectively. The methods are also useful for screening newborns for fragile X syndrome or for screening women to determine heterozygosity status with full premutation of the CCG repeat tract. The methods are also useful in estimating the premutation and full mutation carrier frequency and estimating the prevalence of FXTAS AND FXPOI in a population. The methods are simple, rapid and require small amount of sample.