Molecular genetic analysis of mentally retarded males with features of the fragile-X syndrome.
In males with ID and fragile-X-like faces but normal chromosomes, Southern blot almost never finds the FMR1 expansion, so further gene hunting is low-yield.
01Research in Context
What this study did
The team looked at 20 men with intellectual disability. All had facial features that can signal fragile-X syndrome.
Chromosome tests had already come back normal. The researchers ran Southern blot tests to hunt for the FMR-1 gene expansion anyway.
What they found
No one carried the big CGG repeat that causes fragile-X. The expansion simply was not there.
The result says the gene fault is rare in this exact group: MR males who look fragile-X-like yet have normal chromosomes.
How this fits with other research
Two earlier surveys, Aman et al. (1993) and Dougherty et al. (1994), found fragile-X in about 2 % of institutionalised or clinic-selected men with ID. Those studies used basic chromosome checks, not gene-level tests.
Tassone et al. (2013) widened the net to children with both autism and delay. They still picked up six FMR1 expansions. Their wider catch shows the gene fault hides more often in the ASD-plus-ID mix than in the narrow cytogenetically-normal MR group.
Diemer et al. (2023) screened 235 Filipino children with ASD and found zero full mutations, only intermediate alleles. Together these papers draw the same line: once cytogenetics are clean, the chance of a full FMR1 mutation drops close to zero, no matter the country or label.
Why it matters
You can stop chasing fragile-X DNA after one negative result if the chromosome study is already normal and the client has straight ID without autism. Save the family time, money, and worry. Shift your referral energy toward the ASD-plus-delay group, where the gene expansion still shows up.
Want CEUs on This Topic?
The ABA Clubhouse has 60+ free CEUs — live every Wednesday. Ethics, supervision & clinical topics.
Join Free →Check past lab reports; if chromosome studies were normal and the client has ID without autism, move goals and parent training forward instead of ordering more fragile-X tests.
02At a glance
03Original abstract
The fragile-X[fra(X)] or Martin-Bell syndrome is the most common familial cause of mental retardation and is characterized by the presence of an Xq27.3 chromosome fragile site. Unstable DNA sequences representing large increases in the number of CGG trinucleotide DNA base repeats of the FMR-1 gene are located at the fragile site and responsible for the fra(X) syndrome. In order to identify whether cytogenetically normal yet mentally retarded males without a known cause of their retardation had expansion of the CGG repeat segment of the FMR-I gene, molecular genetic studies using Southern hybridization were performed with two DNA probes (fxa241 and Ox1.9) following digestion of genomic DNA from each patient with restriction enzymes Pstl and EcoRl/Eagl, respectively. DNA studies were performed on 20 (12.3%) out of 162 (122 white and 40 black people) cytogenetically normal mentally retarded males without a known cause of their retardation, but with high anthropometric discriminant values and/or clinical checklist scores identified previously and consistent with the fra(X) syndrome. None of the 20 males showed expansion of the CGG repeat of the FMR-1 gene detectable with the two probes used in this study. While heterogeneous single base pair substitutions, or small deletions or insertions in the FMR-I gene could exist in our patients, aberrations in other X-linked mental retardation genes, not identified to date but whose gene product can produce a phenotype similar to fra(X), either independently or in conjunction with the recently identified FMR-I protein, should be considered and are under investigation. Our study supports the idea that major FMR-I gene expansion detectable with Southern hybridization is rare in cytogenetically normal mentally retarded males, including those with physical and behavioural features seen in the fra(X) syndrome.
Journal of intellectual disability research : JIDR, 1995 · doi:10.1111/j.1365-2788.1995.tb00576.x