Large Interruptions of GAA Repeat Expansion Mutations in Friedreich Ataxia Are Very Rare

Abstract

Friedreich ataxia is a multi-system autosomal recessive inherited disorder primarily caused by homozygous GAA repeat expansion mutations within intron 1 of the frataxin gene. The resulting deficiency of frataxin protein leads to progressive mitochondrial dysfunction, oxidative stress, and cell death, with the main affected sites being the large sensory neurons of the dorsal root ganglia and the dentate nucleus of the cerebellum. The GAA repeat expansions may be pure (GAA)_n in sequence or may be interrupted with regions of non-GAA sequence. To our knowledge, there has been no large-scale study of FRDA patient DNA samples to determine the frequency of large interruptions in GAA repeat expansions. Therefore, we have investigated a panel of 245 Friedreich ataxia patient and carrier DNA samples using GAA repeat PCR amplification and MboII restriction enzyme digestion. We demonstrate that the vast majority (97.8%) of Friedreich ataxia GAA repeat expansion samples do not contain significant sequence changes that would result in abnormal MboII digestion profiles, indicating that they are primarily pure GAA repeats. These results show for the first time that large interruptions in the GAA repeats are very rare.

Keywords: FRDA; Friedreich ataxia; GAA repeat interruptions; GAA repeat mutation; frataxin; trinucleotide repeat expansion disease.

FIGURE 1

MboII digest results. Agarose gel showing MboII digests of GAA PCR products of FRDA samples. The expected 170bp (5′) and 120bp (3′) undigested GAA-flanking fragments from normal pure GAA repeat expansion FRDA samples are shown in lanes 2, 3, and 4. These band sizes can be seen in between the 200 and 100bp fragments of the 1 Kb+ DNA ladder markers, which are loaded into lanes 1 and 11 of the gel. Lane 5 shows a large MboII band of approximately 600bp that was obtained from the positive interrupted GAA repeat sequence from the “NEP” BAC transgenic mouse that contains approximately 500 triplet repeats with the previously determined interrupted sequence of (GAA)₂₁(GGAGAA)₅(GGAGGAGAA)₇₀(GAA)_n (Holloway et al., 2011). In addition for this positive sample, we also identified the expected 5′ flanking band of 170bp, together with a smaller band of less than 100bp that we sequenced and we showed to contain a 27bp deletion in the 3′ flanking region. Lane 6 shows an abnormal band of 200bp representing the 80bp duplication in the 3′ GAA flanking region. Lane 7 shows an abnormal band of approximately 100bp representing the 19bp deletion in the 3′ GAA flanking region. Lanes 8, 9, and 10 contain abnormal bands of approximately 300, 100, and 180bp, respectively, that are likely to contain a region of interrupted GAA repeat sequence within the body of one or other of the large FRDA GAA repeat expansions.

FIGURE 2

Correlation analysis of GAA repeat size with age of onset. A graph representing GAA1 repeat size v age of onset is shown (n = 199). The best fit was for a quadratic model, for which the equations and R²-values are shown (ANOVA P-values are listed in Table 2).

FIGURE 3

MboII digests of GAA repeat expansions from human FRDA somatic tissues and mouse FRDA intergenerational and somatic tissues. Agarose gels showing MboII digests of GAA PCR products of (A) FRDA patient cerebellum tissue samples, (B) YG8sR mouse ear biopsy samples and human FRDA blood samples, and (C) four tissues from one YG8sR mouse. In each case, the expected 170 and 120bp undigested GAA-flanking fragments can be identified in between the 200 and 100bp fragments of the 1 Kb+ DNA ladder marker, which is loaded into the first lane of each gel. (A) Lanes 1–3 show the results from cerebellum tissue samples from three FRDA patients. (B) Lanes 1 and 2 are from FRDA patient blood samples; lanes 3–6 are from ear biopsy samples from 4 GAA repeat expansion-based YG8sR mice of four different generations, and lane 7 is from an ear biopsy sample from the Y47R mouse which has nine GAA repeats. (C) Lanes 1–4 are from brain, cerebellum, heart, and liver tissues of the YG8sR mouse, respectively.