Final Diagnosis -- Full mutation pattern of the CGG repeat region in the FMR-1 gene consistent with Fragile X Syndrome

FINAL DIAGNOSIS    Full mutation pattern of the CGG repeat region in the FMR-1 gene consistent with Fragile X Syndrome.

DISCUSSION

Fragile X syndrome (also known as Martin-Bell syndrome) is the most common form of inherited mental retardation, with an incidence of ~1:1500 in males and 1:2500 in females. The term refers to the secondary constriction on the long arm of the X chromosome observed when lymphoblasts from affected patients are cultured in folate-deficient media (7). The syndrome is characterized in affected males by mild to severe retardation, autism or behavioral disturbances and may be associated with facial dysmorphic features and macroorchidism(9). Fragile X syndrome is a result of mutations at the fragile X locus of the FMR-1 gene, which encodes an RNA-binding protein that is localized primarily in the cytoplasm (1, 4, 14, 15). A repetitive sequence (CGG)n in the 5' untranslated region of the FMR-1 gene lengthens dramatically in Fragile X patients. Full expansion mutations are correlated with hypermethylation of a CpG island located 250 bp upstream of the CGG repeat region of FMR-1 gene. DNA methylation turns off gene transcription making the gene inactive(11). Repeat expansion accounts for 99% of all FMR-1 mutations associated with Fragile X Syndrome; other rare gene alterations include point mutations and gene deletion(2, 3, 5).

The triplet repeat allele sizes for FMR-1 are categorized as follows (8):

In certain circumstances, PCR is used to amplify across the (CGG)n region in genomic DNA obtained from the patient, followed by size resolution of PCR products by gel or capillary electrophoresis. Standard PCR analysis can accurately determine triplet repeats in the normal, inconclusive, and small premutation ranges, but may not detect large CGG expansions in the full mutation range (>230 repeats) due to limitations of PCR and of capillary electrophoresis. Therefore, Southern blot hybridization with a specific probe is carried out to assess for large permutation and fully expanded alleles.

Fragile X syndrome is inherited in an X-linked manner; however, the gender of the transmitting parent is an important factor in the transmission of Fragile X syndrome. It has long been accepted that individuals with Fragile X syndrome receive the mutated FMR-1 allele from their mothers. Females with premutation (or full mutation) alleles can transmit either premutation or fully expanded alleles to their offspring. Females with 45 to 54 CGG repeats may have an allele that is stable or one that can undergo expansion during oogenesis. Larger premutation alleles are more likely to expand, illustrating the phenomenon of anticipation (amplification of the triplet repeat with successive generations) at the molecular level(4). On the other hand, males with premutation or full mutation alleles can only pass on a premutation allele because expansion is believed to not occur during spermatogenesis. Interestingly, Zeesman et al (16) reported a case of paternal transmission of a full mutation FMR-1 allele; however, argument persists as to whether this can truly occur(13).

Our patient's physical examination is positive for dysmorphic features with large, low set ears which are one of the physical signs often observed in patients with a full mutation pattern in the Fragile X gene. EEG showed a normal awake-and-asleep pattern with a subtle slow background frequency for age. Musumeci et al. studied one hundred and ninety-two fragile X male patients for seizures and EEG findings. Seizures were documented in 35 (18.2%) of 192 patients; seizures began between the ages of 2 and 9 years. They found that seizures were frequently of the complex partial type and less frequently of the partial motor and generalized type(10). Although premutations are not associated with the classic Fragile X phenotype, female premutation carriers have a higher incidence of premature ovarian failure than the general population(12). In addition, some male premutation carriers manifest a cerebellar tremor/ataxia syndrome that is distinct from fragile X syndrome(6).

REFERENCES

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Contributed by Faye F. Gao MD, PhD and Marie C. DeFrances MD, PhD