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Gene mutations that preferentially affect the CNS have been implicated in a number of neurological disorders. This leads to the possibility that a disease-causing mutation present only in CNS tissues could be missed if it were tested in a blood DNA sample only. The commonest mutation in amyotrophic lateral sclerosis (ALS) is an expansion of the hexanucleotide repeats of  C9orf72 . To find out if CNS-specific mutations of this gene could cause some cases of ALS we looked for differences in the size of  C9orf72  repeats between DNA from the CNS and from white blood cells (WBCs) of 38 sporadic ALS patients, using a repeat-primed PCR screening test. We also looked for differences in  C9orf72  repeats in WBC DNA from 6 ALS-discordant and 1 ALS-concordant monozygotic twins. Abnormally expanded  C9orf72  repeats were found in 13% of the ALS CNS samples, as well as in their paired WBC DNA. The 87% of ALS CNS samples with normal-sized  C9orf72  repeats had the same number of repeats in paired WBC samples. All ALS-discordant twins had the same normal numbers of WBC  C9orf72  repeats. Although previous work suggests some tissue mosaicism in  C9orf72  repeat size is probably present, this study indicates that this is not likely to be of sufficient magnitude to result in a normal  C9orf72  repeat length in blood but an abnormally expanded repeat length in the CNS. This suggests that a blood DNA test alone will usually be sufficient to make a diagnosis of  C9orf72  repeat-related ALS.

Can ALS-Associated C9orf72 Repeat Expansions Be Diagnosed on a Blood DNA Test Alone?