Table 8.

Methods to Characterize ATXN7 CAG Repeats

Interpretation of CAG Repeat NumberExpected Results by Method
Conventional PCRTriplet-Primed PCR 1Expanded Repeat Analysis 2
Normal, uncertain, mutable normal, reduced penetrance: <19-36 rptsDetected 3See footnote 1.Expansions detected, & repeat size can be approximated. 7, 8
Pathogenic (full penetrance): 37-~460 rptsDetected up to ~100 CAG 4Expansions detected, but repeat size cannot be determined. 5, 6
1.

The design of a triplet-primed PCR (TP-PCR) assay may include conventional PCR primers to size normal repeats and detect expanded repeats in a single assay. The TP-PCR assay itself does not determine repeat size, even alleles in the normal range.

2.

Methods to detect and approximate the size of expanded repeats include long-range PCR sized by gel electrophoresis and Southern blotting. The upper limit of repeat size detected will vary by assay design, laboratory, sample, and/or patient as a result of competition by the normal allele during amplification.

3.

Detection of an apparently homozygous repeat does not rule out the presence of an expanded CAG repeat; thus, testing by TP-PCR or expanded repeat analysis is required to detect a repeat expansion.

4.

The upper limit of repeat size detected will vary by assay design, laboratory, sample, and/or patient as a result of competition from the normal allele during amplification.

5.

TP-PCR for the CAG repeat expansion has been described [Cagnoli et al 2006].

6.

Repeats at the lower end of this range may not show the characteristic stutter pattern that indicates an expanded repeat [Cagnoli et al 2006].

7.

Southern blotting for the CAG repeat expansion has been described [Benton et al 1998].

8.

Precise sizing of repeats is not necessary, as clinical utility for determining the exact repeat number has not been demonstrated.

From: Spinocerebellar Ataxia Type 7

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