Читать книгу Protocols for High-Risk Pregnancies - Группа авторов - Страница 110

Screening for spinal muscular atrophy should be offered to all women who are considering pregnancy or are currently pregnant. Screening entails evaluation of the number of copies of SMN1, which is two in nonaffected noncarriers of spinal muscular atrophy. It is important to note that some individuals will have a chromosome with two copies and a chromosome with zero copies, which will be detected as a normal overall SMN1 copy number of two; these individuals are unaffected but can transmit a chromosome with zero copies of SMN1 to their offspring, who will thus be affected. Therefore, individuals should be counseled regarding the residual risk of carrier status despite a normal screening result. There is also a relatively high 2% de novo mutation rate of SMN1 that limits the ability to completely determine risk of spinal muscular atrophy. Copy number of SMN2 is generally not tested as part of carrier screening (and therefore, carrier screening cannot predict phenotype) but is used for prognostic purposes in the setting of diagnostic testing.

For individuals with a family history of spinal muscular atrophy, reports of genetic testing of the affected individual and carrier testing of the parent should be reviewed to determine residual risk in the setting of a negative screen. If reports are unavailable, testing should instead be offered to the low‐risk partner.