Prenatal fragile X detection using cytoplasmic and nuclear-specific monoclonal antibodies

Qualitative assessment of FMR1 (CGG)n triplet repeat status in normal, intermediate, premutation, full mutation, and mosaic carriers in both sexes: implications for fragile X syndrome carrier and newborn screening

PURPOSE

Fragile X syndrome is caused by expansion and subsequent methylation of a CGG trinucleotide repeat in the FMR1 5'-untranslated region. Southern blot analysis is typically required to determine expansion size for triplet repeat lengths >200. We describe a triplet-primed polymerase chain reaction-based method using automated capillary electrophoresis detection for qualitative assessment of expanded CGG repeats.

METHODS

The assay uses triplet-primed polymerase chain reaction in combination with GC-melting reagents and substitution of 7-deaza-2-deoxyGTP for dGTP. Amplicons are resolved by capillary electrophoresis.

RESULTS

A distinctive pattern of tapering or "stutter" polymerase chain reaction amplification was evident on capillary electrophoresis in male and female patients harboring all expanded allele lengths examined (up to 2000 CGG repeats) and could be used to differentiate normal, intermediate, premutation, and full mutation alleles. Full mutation alleles exhibited an additional late-migrating amplicon on capillary electrophoresis. Mixing experiments demonstrated sensitivity as low as 1% for detection of the full mutation allele. In a 1275-sample concordance study against our existing polymerase chain reaction platform (with Southern blot analysis for repeat lengths ≥55), the triplet-primed polymerase chain reaction method exhibited 100% concordance for normal, intermediate, expanded, and full mutation alleles. This method also detected the full mutation alleles in DNA isolated from blood spots.

CONCLUSION

This assay provides an accurate assessment of FMR1 repeat status and holds promise for use in carrier and newborn screening. The method distinguishes normal homozygous females from full mutation carrying females. Although the method is not useful for accurate sizing, it supplements the classic polymerase chain reaction method and results in significant reduction in the number of Southern blot analyses required to be performed in the laboratory to accurately assess the FMR1 genotype in all individuals.

[FMRP immunodetection on hair roots: application to the diagnosis of fragile X syndrome]

INTRODUCTION

Fragile X syndrome is the most common inherited form of mental retardation. The absence of FMRP protein, codified by the FMR1 gene, results in fragile X phenotype. DNA-based diagnostic methods determine the length of the CGG repeat within the FMR1 gene, the main mutation causing the syndrome. Immunohistochemical diagnostic tests detect all mutations leading to the absence of FMRP expression. Results of the antibody test on hair roots correlate with intellectual quotient in affected men and women.

PATIENTS AND METHODS

Immunohistochemical techniques were used to study FMRP expression in hair roots in a control group to establish the correlation with the length of the CGG repeat. Subsequently, 65 girls and boys with mental retardation attending special schools were screened by using the FMRP test on hair roots.

RESULTS

Males and females molecularly characterized as within the normal and premutated range expressed FMRP in more than 70 % of hair roots. Full mutation carriers expressed FMRP in less than 70 % of hair roots. Immunohistochemical studies in males and females with mental retardation led to the identification of one affected male.

CONCLUSIONS

Fragile X syndrome detection by immunohistochemical testing of hair roots is a valid method of population screening because of the relative noninvasiveness of obtaining samples, and the ease and rapidness of the technique, which can be applied to routine clinical practice.

FMR1 and the fragile X syndrome: human genome epidemiology review

The fragile X syndrome, an X-linked dominant disorder with reduced penetrance, is one of the most common forms of inherited mental retardation. The cognitive, behavioral, and physical phenotype varies by sex, with males being more severely affected because of the X-linked inheritance of the mutation. The disorder-causing mutation is the amplification of a CGG repeat in the 5' untranslated region of FMR1 located at Xq27.3. The fragile X CGG repeat has four forms: common (6-40 repeats), intermediate (41-60 repeats), premutation (61-200 repeats), and full mutation (>200-230 repeats). Population-based studies suggest that the prevalence of the full mutation, the disorder-causing form of the repeat, ranges from 1/3,717 to 1/8,918 Caucasian males in the general population. The full mutation is also found in other racial/ethnic populations; however, few population-based studies exist for these populations. No population-based studies exist for the full mutation in a general female population. In contrast, several large, population-based studies exist for the premutation or carrier form of the disorder, with prevalence estimates ranging from 1/246 to 1/468 Caucasian females in the general population. For Caucasian males, the prevalence of the premutation is approximately 1/1,000. Like the full mutation, little information exists for the premutation in other populations. Although no effective cure or treatment exists for the fragile X syndrome, all persons affected with the syndrome are eligible for early intervention services. The relatively high prevalence of the premutation and full mutation genotypes coupled with technological advances in genetic testing make the fragile X syndrome amenable to screening. The timing as well as benefits and harms associated with the different screening strategies are the subject of current research and discussion.