This is the first report of international screening and cascade testing utilizing the new fragile X PCR-based screening of blood spots, a technique developed by Tassone et al
). Of the 104 samples successfully processed, 15 were positive and confirmed by Southern blot 14.42%. All of the confirmed positives were members of families with a known proband, including the three brothers with ID who had never been tested for fragile X DNA, although their pediatrician thought that they clinically had FXS. They were detected with our screening and confirmed by Southern blot analysis.
Although no cases of FXS were found in any of the nonfragile X-related screening groups, these findings are within the range of expected results, given the relatively small sample size in each group. Based on the literature, it is expected that roughly 2–6% of individuals with autism will have FXS as the genetic etiology. Given a sample size of 34 children, it is expected that 0–2 individuals should test positive for fragile X. Similarly, it is expected that between 1% and 3% of individuals with developmental delay or intellectual deficiency has FXS, which, given a sample size of 24 people, is likely to yield no positive results. For the other two groups (those with Parkinson's-like presentation and those with a family history of ID) the expected percentages are much smaller, as was the tested sample size, so again a lack of any positive results is within the expected range. Therefore, though high-risk blood spot screening would be beneficial to these nonfragile X-related clinical groups, its utility would be better demonstrated with larger sample sizes involving hundreds of individuals.
For the group of individuals with a known family history of FXS, blood spot screening effectively identified fragile X full and premutation alleles. In doing so, the screening provided these individuals with a presumptive diagnosis that was made definitive with Southern blot/PCR follow-up testing. No DNA testing for the fragile X gene is currently available in Guatemala. Follow-up efforts have provided these families with genetic counseling via phone from the M.I.N.D. Institute and have placed them in contact with local fragile X parent support groups.
High-risk blood spot screening in Guatemala yielded success on two fronts. On the one hand, 36 individuals with a known family history of FXS (and thus a known risk of inheriting an expanded allele) were given a diagnosis. Fifteen of these individuals were correctly diagnosed with an expanded allele; a diagnosis they would likely not have received if they had to rely on more expensive, logistically difficult testing. Therefore, blood spot screening is an effective tool to use for cascade screening, particularly in populations where the screening cohort may be small and genetic testing resources may be scarce.
Further, although the screening cohorts of the other clinical groups (autism, DD/ID, Parkinson's-like, family history of ID with unconfirmed etiology) were small in number and did not result in any expanded alleles, being screened allowed each of these individuals to rule out FXS as the cause of their disabilities. None of these individuals would have been screened had it not been for the low cost and feasibility of the blood spot PCR screening technique. Therefore, although larger screening cohorts are necessary to identify cases of FXS in these groups, high-risk blood spot screening is a cost-efficient and an easy way to rule out a FXS mutation.