Spinal muscular atrophy genetic testing experience at an academic medical center

Abstract

Approximately 94% of spinal muscular atrophy (SMA) patients lack both copies of SMN1 exon 7. We report our SMA genetic testing experience (total 1281 cases), using SMA linkage analysis (32 families), SMA diagnostic testing by PCR-RFLP (restriction fragment length polymorphism) to detect the homozygous absence of SMN1 exon 7 (and exon 8) (533 cases), and an assay to determine copy number of SMN1 exon 7 (SMN1 gene dosage analysis) (716 cases). SMN1 gene dosage analysis is used for SMA carrier testing as well as for the confirmation of a heterozygous SMN1 deletion in symptomatic individuals who do not lack both copies of SMN1. We conclude that comprehensive SMA testing, including SMN1 deletion analysis, SMN1 gene dosage analysis, and linkage analysis, offers the most complete evaluation of SMA patients and their families.

Figure 1.

An example of an ABI 373a electropherogram showing a one-base-larger SMN1 peak (actual size 188 bp, shown as 189 bp, arrow) in addition to the expected SMN1 peak (actual size 187 bp, shown as 188 bp, arrowhead), each corresponding to one SMN1 copy by gene dosage analysis. Each peak corresponds to PCR products of an SMN internal standard (SMNis), SMN2 genomic sequence (SMN2), SMN1 genomic sequence (SMN1, with a doublet), a CFTR internal standard (CFTRis), and CFTR genomic sequence (CFTR), from left to right, respectively. Numbers under each peak represent approximate product size in bp (top) and fluorescence intensity measured as an area under curve (bottom).

Figure 2.

A flow chart of the results of SMA diagnostic tests and SMN1 gene dosage analyses for symptomatic and prenatal cases.