Beyond newborn screening: the role of reverse cascade testing in familial disease detection

Over the past 60 years, preventative public health screening programs have evolved since their inception and now include newborn screening (NBS) aimed at identifying infants after birth for a number of rare, congenital, inherited diseases. Most of the conditions detected through NBS are autosomal recessive disorders or exhibit X-linked inheritance, meaning that family members of individuals with these conditions have a higher risk for being either affected or obligate heterozygotes. For example, the X-linked adrenoleukodystrophy (X-ALD) in the screening panel identifies affected newborns and asymptomatic relatives through subsequent testing. Thus, NBS becomes a gateway to family-wide prevention, through the application of reverse cascade testing (RCS). In this paper we examined the scenarios where RCS may be appropriate. Accordingly, we have identified a list of criteria assessing whether a NBS disease would benefit from RCS: (1) autosomal recessive or X-linked inheritance; (2) high carrier rates, (3) variable expressivity, (4) mild or late-onset forms; and (5) association with diagnostic delays and recent addition to the screening panel. More than one criterion usually needs to be met for a disease to benefit from RCS. We have identified a list of diseases and highlighted the potential benefits of RCS: X-ALD, Cystic Fibrosis, Sickle Cell Disease, Spinal Muscular Atrophy and Pompe disease. There are additional scenarios within NBS where disease maternal conditions (3-methylcrotonyl-CoA carboxylase deficiency and carnitine uptake deficiency) or nutritional maternal conditions (vitamin B12 deficiency) may cause a screen-positive NBS result. Whenever a maternal nutritional deficiency is a potential reason for a positive NBS, this is indicative of a non-inherited condition that may require treatment in the newborn owing to possible neurological damage and delay in normal growth in newborns with certain secondary deficiencies. For these cases RCS is recommended, as the mother's status may put her at risk for future adverse events (i.e. cardiovascular and musculoskeletal disorders, hepatic involvement, and neurodegeneration). The RCS-NBS strategy discussed in this paper offers a set of criteria against which diseases can be assessed for the potential need for RCS. Implementation of this strategy requires several considerations including educational needs, ethical issues, uptake of testing, logistics and costs for this expanded screening and counseling, and availability of appropriate specialists for ongoing management.