Sex-dependent mechanisms for expansions and contractions of the CAG repeat on affected Huntington disease chromosomes.

A total of 254 affected parent-child pairs with Huntington disease (HD) and 440 parent-child pairs with CAG size in the normal range were assessed to determine the nature and frequency of intergenerational CAG changes in the HD gene. Intergenerational CAG changes are extremely rare (3/440 [0.68%]) on normal chromosomes. In contrast, on HD chromosomes, changes in CAG size occur in approximately 70% of meioses on HD chromosomes, with expansions accounting for 73% of these changes. These intergenerational CAG changes make a significant but minor contribution to changes in age at onset (r2 = .19). The size of the CAG repeat influenced larger intergenerational expansions (> 7 CAG repeats), but the likelihood of smaller expansions or contractions was not influenced by CAG size. Large expansions (> 7 CAG repeats) occur almost exclusively through paternal transmission (0.96%; P < 10(-7)), while offspring of affected mothers are more likely to show no change (P = .01) or contractions in CAG size (P = .002). This study demonstrates that sex of the transmitting parent is the major determinant for CAG intergenerational changes in the HD gene. Similar paternal sex effects are seen in the evolution of new mutations for HD from intermediate alleles and for large expansions on affected chromosomes. Affected mothers almost never transmit a significantly expanded CAG repeat, despite the fact that many have similar large-sized alleles, compared with affected fathers. The sex-dependent effects of major expansion and contractions of the CAG repeat in the HD gene implicate different effects of gametogenesis, in males versus females, on intergenerational CAG repeat stability.     

Dentatorubral-pallidoluysian atrophy (DRPLA): Close correlation of CAG repeat expansions with the wide spectrum of clinical presentations and prominent anticipation

Dentatorubral-pallidoluysian atrophy (DRPLA) is a rare autosomal dominant neurodegenerative disease characterized by various combinations of ataxia, choreoathetosis, myoclonus, epilepsy and dementia as well as various ages of onset. We have identified a specific unstable trinucleotide repeat expansion in a gene on the short arm of chromosome 12 as the pathogenic mutation for DRPLA. We investigated how the degree of the expansion of the CAG repeat affects the clinical manifestations of DRPLA. The sizes of the expanded alleles were well correlated with the ages of onset (r = −0.6955, P < 0.001). Patients with progressive myoclonus epilepsy (PME) phenotype had larger expansions (62–79 repeats) and earlier ages of onset (onset before age 20). Furthermore, most of the patients with PME phenotype inherited their expanded alleles from their affected fathers. On the other hand, patients with non-PME phenotype showed later ages of onset (onset after age 20) and smaller expansions (54–67 repeats). When ages of onset of each clinical symptom are compared with sizes of the CAG repeat, there is again a remarkably high correlation of the sizes of CAG repeat with each of the clinical symptoms. Thus the wide variation in clinical manifestations of DRPLA can now be clearly explained based on the degree of CAG repeat expansion, which strongly indicates that the expanded alleles are intimately involved in the neuronal degeneration in dentatofugal and pallidofugal systems.     

The relationship between trinucleotide (GAA) repeat length and clinical features in Friedreich ataxia.

Friedreich ataxia (FA) is associated with the expansion of a GAA trinucleotide repeat in the first intron of the X25 gene. We found both alleles expanded in 67 FA patients from 48 Italian families. Five patients from three families were compound heterozygotes with expansion on one allele and an isoleucine-->phenylalanine change at position 154 on the other one. We found neither expansions nor point mutations in three patients. The length of FA alleles ranged from 201 to 1,186 repeat units, with no overlap with the normal range, and showed a negatively skewed distribution with a peak between 800 and 1,000 repeats. The FA repeat showed meiotic instability with a median variation of 150 repeats. The lengths of both larger and smaller alleles in each patient inversely correlated with age at onset of the disorder. Smaller alleles showed the best correlation, accounting for approximately 50% of the variation of age at onset. Mean allele length was significantly higher in patients with diabetes and in those with cardiomyopathy.     

Correlation between CAG Repeat Length and Clinical Features in Machado-Joseph Disease

Machado-Joseph disease (MJD) is associated with the expansion of a CAG trinucleotide repeat in a novel gene on 14q32.1. We confirmed the presence of this expansion in 156 MJD patients from 33 families of different geographic origins: 15 Portuguese Azorean, 2 Brazilian, and 16 North American of Portuguese Azorean descent. Normal chromosomes contain between 12 and 37 CAG repeats in the MJD gene, whereas MJD gene carriers have alleles within the expanded range of 62–84 CAG units. The distribution of expanded alleles and the gap between normal and expanded allele sizes is either inconsistent with a premutation hypothesis or most (if not all) of the alleles we studied descend from a common ancestor. There is a strong correlation between the expanded repeat size and the age at onset of the disease as well as the clinical presentation. There is mild instability of the CAG tract length with transmission of the expanded alleles; both increase and decrease in size between parents and progeny occur, with larger variations in male than in female transmissions. Together, these effects can partly explain the variability of age at onset and of phenotypic features in MJD; however, other modifying factors must exist.     

Molecular and clinical study of 18 families with ADCA type II: evidence for genetic heterogeneity and de novo mutation.

The SCA7 mutation has been found in 54 patients and 7 at-risk subjects from 17 families who have autosomal dominant cerebellar ataxia (ADCA) II with progressive pigmentary maculopathy. In one isolated case, haplotype reconstruction through three generations confirmed a de novo mutation owing to paternal meiotic instability. Different disease-associated haplotypes segregated among the SCA7-positive kindreds, which indicated a multiple origin of the mutation. One family with the clinical phenotype of ADCA type II did not have the CAG expansion that indicated locus heterogeneity. The distribution of the repeat size in 944 independent normal chromosomes from controls, unaffected at-risk subjects, and one affected individual fell into two ranges. The majority of the alleles were in the first range of 7-19 CAG repeats. A second range could be identified with 28-35 repeats, and we provide evidence that these repeats represent intermediate alleles that are prone to further expansion. The repeat size of the pathological allele, the widest reported for all CAG-repeat disorders, ranged from 37 to approximately 220. The repeat size showed significant negative correlation with both age at onset and age at death. Analysis of the clinical features in the patients with SCA7 confirmed that the most frequently associated features are pigmentary maculopathy, pyramidal tract involvement, and slow saccades. The subjects with <49 repeats tended to have a less complicated neurological phenotype and a longer disease duration, whereas the converse applied to subjects with >/=49 repeats. The degree of instability during meiotic transmission was greater than in all other CAG-repeat disorders and was particularly striking in paternal transmission, in which a median increase in repeat size of 6 and an interquartile range of 12 were observed, versus a median increase of 3 and interquartile range of 3.5 in maternal transmission.     

Anticipation and Instability of IT-15 (CAG)N Repeats in Parent-Offspring Pairs with Huntington Disease

Huntington disease (HD) is an autosomal dominant degenerative disorder caused by an expanded and unstable trinucleotide repeat (CAG)n in a gene (IT-15) on chromosome 4. HD exhibits genetic anticipation—earlier onset in successive generations within a pedigree. From a population-based clinical sample, we ascertained parent-offspring pairs with expanded alleles, to examine the intergenerational behavior of the trinucleotide repeat and its relationship to anticipation. We find that the change in repeat length with paternal transmission is significantly correlated with the change in age at onset between the father and offspring. When expanded triplet repeats of affected parents are separated by median repeat length, we find that the longer paternal and maternal repeats are both more unstable on transmission. However, unlike in paternal transmission, in which longer expanded repeats display greater net expansion than do shorter expanded repeats, in maternal transmission there is no mean change in repeat length for either longer or shorter expanded repeats. We also confirmed the inverse relationship between repeat length and age at onset, the higher frequency of juvenile-onset cases arising from paternal transmission, anticipation as a phenomenon of paternal transmission, and greater expansion of the trinucleotide repeat with paternal transmission. Stepwise multiple regression indicates that, in addition to repeat length of offspring, age at onset of affected parent and sex of affected parent contribute significantly to the variance in age at onset of the offspring. Thus, in addition to triplet repeat length, other factors, which could act as environmental factors, genetic factors, or both, contribute to age at onset. Our data establish that further expansion of paternal repeats within the affected range provides a biological basis of anticipation in HD.     

Methylation mosaicism of 5'-(CGG)(n)-3' repeats in fragile X, premutation and normal individuals

Fragile X syndrome (FRAXA) is characterized at the molecular level by an expansion of a naturally occurring 5′-(CGG)_n-3′ repeat in the promoter and 5′-untranslated region (5′-UTR) of the fragile X mental retardation (FMR1) gene on human chromosome Xq27.3. When expanded, this region is usually hypermethylated. Inactivation of the FMR1 promoter and absence of the FMR1 protein are the likely cause of the syndrome. By using the bisulfite protocol of the genomic sequencing method, we have determined the methylation patterns in this region on single chromosomes of healthy individuals and of selected premutation carriers and FRAXA patients. In control experiments with unmethylated or M-SssI-premethylated DNAs, this protocol has been ascertained to reliably detect all cytidines or 5-methylcytidines as unmethylated or methylated nucleotides, respectively. Analyses of the DNA from FRAXA patients reveal considerable variability in the lengths of the 5′-(CGG)_n-3′ repeats and in the levels of methylation in the repeat and the 5′-UTR. In one patient (OEl) with high repeat length heterogeneity (n = 15 to >200), shorter repeats (n = 20–80) were methylated or unmethylated, longer repeats (n = 100–150) were often completely methylated, but one repeat with n = 160 proved to be completely unmethylated. This type of methylation mosaicism was observed in several FRAXA patients. In healthy females, methylated 5′-CG-3′ sequences were found in some repeats and 5′-UTRs, as expected for the sequences from one of the X chromosomes. The natural FMR1 promoter is methylation sensitive, as demonstrated by the loss of activity in transfection experiments using the unmethylated or M-SssI-premethylated FMR1 promoter fused to the luciferase gene as an activity indicator.     

PolyQ repeat expansions in ATXN2 associated with ALS are CAA interrupted repeats

Amyotrophic lateral sclerosis (ALS) is a devastating, rapidly progressive disease leading to paralysis and death. Recently, intermediate length polyglutamine (polyQ) repeats of 27-33 in ATAXIN-2 (ATXN2), encoding the ATXN2 protein, were found to increase risk for ALS. In ATXN2, polyQ expansions of ≥ 34, which are pure CAG repeat expansions, cause spinocerebellar ataxia type 2. However, similar length expansions that are interrupted with other codons, can present atypically with parkinsonism, suggesting that configuration of the repeat sequence plays an important role in disease manifestation in ATXN2 polyQ expansion diseases. Here we determined whether the expansions in ATXN2 associated with ALS were pure or interrupted CAG repeats, and defined single nucleotide polymorphisms (SNPs) rs695871 and rs695872 in exon 1 of the gene, to assess haplotype association. We found that the expanded repeat alleles of 40 ALS patients and 9 long-repeat length controls were all interrupted, bearing 1-3 CAA codons within the CAG repeat. 21/21 expanded ALS chromosomes with 3CAA interruptions arose from one haplotype (GT), while 18/19 expanded ALS chromosomes with <3CAA interruptions arose from a different haplotype (CC). Moreover, age of disease onset was significantly earlier in patients bearing 3 interruptions vs fewer, and was distinct between haplotypes. These results indicate that CAG repeat expansions in ATXN2 associated with ALS are uniformly interrupted repeats and that the nature of the repeat sequence and haplotype, as well as length of polyQ repeat, may play a role in the neurological effect conferred by expansions in ATXN2.     

Somatic heterogeneity of the CTG repeat in myotonic dystrophy is age and size dependent.

The most common form of adult muscular dystrophy, myotonic dystrophy (DM), is caused by the abnormal expansion of the CTG repeat, located in the 3' UTR of the DM gene. The expanded-CTG allele often presents as a diffused band on Southern blot analysis, suggesting somatic mosaicism. In order to study the somatic instability of the CTG repeat, we have investigated the dynamics of the size heterogeneity of the CTG expansion. Size heterogeneity is shown as a smear on Southern blot and is measured by the midpeak-width ratio of the expanded allele to the normal sized allele. The ratio is also corrected for compression in the higher-molecular-weight region. It is found that the size heterogeneity of the expanded-CTG repeats, of 173 DM patients, correlates well with the age of the patient (r = .81, P << .001). The older patients show larger size variation. This correlation is independent of the sex of either the patient or the transmitting parent. The size heterogeneity of the expansion, based on age groups, is also dependent on the size of the expanded trinucleotide repeat. However, obvious size heterogeneity is not observed in congenital cases, regardless of the size of expansion. Comparison of individual patient samples collected at two different times has confirmed that the degree of size heterogeneity increases with age and has revealed a subtle but definite upward shift in the size of the expanded-CTG allele. The progression of the CTG repeat toward larger expansion with age is further confirmed by small-pool PCR assay that resolved the heterogeneous fragments into discrete bands.(ABSTRACT TRUNCATED AT 250 WORDS)     

FMR1 Premutation Is an Uncommon Explanation for Premature Ovarian Failure in Han Chinese

Background

In premature ovarian failure (POF), cessation of menstruation occurs before the expected age of menopause. Approximately 1% of women are affected. FMR1 premutation was reported to be responsible for up to 3.3%–6.7% of sporadic POF and 13% of familial cases in Caucasians, while the data was absent in Chinese population. Therefore, the impact of FMR1 CGG repeat on ovarian reserve is needed to be investigated in large Chinese cohort.

Methods

The number of FMR1 CGG repeat was determined in 379 Han Chinese women with well-defined 46, XX non-syndromic sporadic POF and 402 controls. The age of menopause onset in respect to CGG repeats was further analyzed.

Results

The frequency of FMR1 premutation in Han Chinese POF was only 0.5% (2/379), although it was higher than that in matched controls (0%, 0/402), it was much lower than that reported in Caucasian with POF (3.3%–6.7%). The prevalence of intermediate FMR1 (41–54) was not increased significantly in sporadic POF than that in controls (2.9% vs. 1.7%, P = 0.343). However, POF patients more often carried a single additional CGG repeat in a single allele than did fertile women (allele-1: 29.7 vs. 28.8, P<0.001; allele-2: 32.6 vs. 31.5, P<0.001). POF patients with both alleles of CGG repeats outside (below or above) the normal range (26–34) showed an earlier age of cessation of menses than those with two alleles within normal range (hom-high/high vs. norm: 20.4±4.8 vs. 24.7±6.4, p<0.01; hom-low/high vs. norm: 18.7±1.7 vs. 24.7±6.4, p<0.01).

Conclusions

FMR1 premutation seems to be an uncommon explanation for POF in Han Chinese. However, having both alleles with CGG repeats outside the normal range might still adversely affect ovarian aging.     

Mutagenic stress modulates the dynamics of CTG repeat instability associated with myotonic dystrophy type 1

The molecular basis of the myotonic dystrophy type 1 is the expansion of a CTG repeat at the DMPK locus. The expanded disease-associated repeats are unstable in both somatic and germ lines, with a high tendency towards expansion. The rate of expansion is directly related to the size of the pathogenic allele, increasing the size heterogeneity with age. It has also been suggested that additional factors, including as yet unidentified environmental factors, might affect the instability of the expanded CTG repeats to account for the observed CTG size dynamics over time. To investigate the effect of environmental factors in the CTG repeat instability, three lymphoblastoid cell lines were established from two myotonic dystrophy patients and one healthy individual, and parallel cultures were concurrently expanded in the presence or absence of the mutagenic chemical mitomycin C for a total of 12 population doublings. The new alleles arising along the passages were analysed by radioactive small pool PCR and sequencing gels. An expansion bias of the stepwise mutation was observed in a (CTG)₁₂₄ allele of a cell line harbouring two modal alleles of 28 and 124 CTG repeats. Interestingly, this expansion bias was clearly enhanced in the presence of mitomycin C. The effect of mitomycin C was also evident in the normal size alleles in two cell lines with alleles of 13/13 and 12/69 repeats, where treated cultures showed new longer alleles. In conclusion, our results indicate that mitomycin C modulates the dynamics of myotonic dystrophy-associated CTG repeats in LBCLs, enhancing the expansion bias of long-pathogenic repeats and promoting the expansion of normal length repeats.     

Fragile X Premutations Are Not a Major Cause of Early Menopause

Fragile X syndrome is an X-linked mental retardation condition that usually is due to a trinucleotide-repeat expansion in the FMR1 gene. Whereas full-mutation alleles (> 230 repeats) lead to fragile X syndrome, premutation alleles (approximately 60-200 repeats) are apparently non-penetrant. However, previous studies have suggested that female premutation carriers may have an increased incidence of premature menopause. To test this possible association, we screened for premutation alleles among 216 women with early menopause (at age < 47 years), 33 of whom had premature menopause (at age < 40 years), as well as among 107 control women, all of whom were ascertained solely on the basis of age at menopause. No full-mutation alleles were found; and only one premutation allele was found, but, it was in a member of the control group. These results are consistent with what would be expected on the basis of chance only. Our sample size was sufficient to rule out a > or = 3-fold increased risk of early menopause and a > or = 9-fold increased risk of premature menopause due to an FMR1 premutation, under a model considering the risk of both sporadic and familial early menopause. Likewise, our results rule out a > or = 4-fold increased risk of familial early menopause and a > or = 26-fold increased risk of familial premature menopause, under a less probable model in which only familial early menopause is considered. These results indicate that the fragile X premutation is not a major risk factor for early menopause and suggest that the risk of premature menopause to fragile X-premutation carriers may not be as great as that reported elsewhere.     

Phenotype correlation and intergenerational dynamics of the Friedreich ataxia GAA trinucleotide repeat.

The Friedreich ataxia (FA) mutation has recently been identified as an unstable trinucleotide GAA repeat present 7-22 times in the normal population but amplified as many as > 1,000 times in FA. Since it is an autosomal recessive disease, FA does not show typical features observed in other dynamic mutation disorders, such as genetic anticipation. We have analyzed the GAA repeat in 104 FA patients and 163 carrier relatives previously defined by linkage analysis. The GAA expansion was detected in all patients, most (94%) of them being homozygous for the mutation. We have demonstrated that clinical variability in FA is related to the size of the expanded alleles: milder forms of the disease-late-onset FA and FA with retained reflexes-are associated with shorter expansions, especially with the smaller of the two expanded alleles. Absence of cardiomyopathy is also associated with shorter alleles. Dynamics of the GAA repeat has been investigated in 212 parent-offspring pairs. Meiotic instability showed a sex bias: paternally transmitted alleles tend to decrease in a linear way that depends on the paternal expansion size, whereas maternal alleles can either increase or decrease. A different pattern of intergenerational variation was also observed, depending on the genetic status of the sib: patients had shorter expansions than were seen in heterozygous carriers. This finding has been interpreted as a postzygotic event. Finally, we have observed that the size of the expansion remains constant in the population through carriers.     

Insights into the mutational history and prevalence of SCA1 in the Indian population through anchored polymorphisms

There is a wide variation in prevalence of spinocerebellar ataxia type 1 (SCA1) in different populations. In the present study, we observed SCA1 in ∼22% (37/167 families) of the autosomal dominant cerebellar ataxias (ADCAs) in the Indian population. We investigated the role of various genetic factors like repeat length, interruption pattern and chromosomal background in predisposing the repeats to instability in these families. We analyzed 12 markers (9 SNPs and 3 microsatellite markers) and found 3 of them, spanning a region of ∼65 kbp to be linked with the disease locus in the Indian population. The haplotype C-4-C defined by rs1476464 (SNP9)-D6S288-rs2075974 (SNP1), which was extremely rare in nonaffected chromosomes (∼3%), was observed to be significantly (P<0.0000) associated with the expanded chromosomes in ∼44% of SCA1 families. This haplotype was found in all nonhuman primates. SNP1 (C/T), which showed a skewed allelic distribution between large (LN > 30 repeats) and small normal (SN ≤ 30 repeats) alleles (P<0.0000) had similar allelic distribution (P=0.3477) in LN and expanded alleles. Our study suggested that LN and expanded chromosomes linked with the ancestral C allele of SNP1 might have originated simultaneously during evolution by the lengthening of repeats. The LN alleles might have accumulated repeat stabilizing non-CAG interruptions during this process. Similar proportions of T allele in SN with single interruptions, LN and expanded chromosomes lend credence to the origin of expanded alleles from singly-interrupted chromosomes. Our analyses using markers linked (anchoring) to SCA1 suggest that prevalence of SCA1 is correlated to both repeat length and number of interruptions in the Indian population. The spectrum of these alleles also points toward the antiquity of SCA1 mutation in the Indian population.Electronic Supplementary Material Supplementary material is available for this article at     

Gonosomal mosaicism in myotonic dystrophy patients: involvement of mitotic events in (CTG)n repeat variation and selection against extreme expansion in sperm.

Myotonic dystrophy (DM) is caused by abnormal expansion of a polymorphic (CTG)n repeat, located in the DM protein kinase gene. We determined the (CTG)n repeat lengths in a broad range of tissue DNAs from patients with mild, classical, or congenital manifestation of DM. Differences in the repeat length were seen in somatic tissues from single DM individuals and twins. Repeats appeared to expand to a similar extent in tissues originating from the same embryonal origin. In most male patients carrying intermediate- or small-sized expansions in blood, the repeat lengths covered a markedly wider range in sperm. In contrast, male patients with large allele expansions in blood (> 700 CTGs) had similar or smaller repeats in sperm, when detectable. Sperm alleles with > 1,000 CTGs were not seen. We conclude that DM patients can be considered gonosomal mosaics, i.e., combined somatic and germ-line tissue mosaics. Most remarkably, we observed multiple cases where the length distributions of intermediate- or small-sized alleles in fathers'' sperm were significantly different from that in their offspring''s blood. Our combined findings indicate that intergenerational length changes in the unstable CTG repeat are most likely to occur during early embryonic mitotic divisions in both somatic and germ-line tissue formation. Both the initial CTG length, the overall number of cell divisions involved in tissue formation, and perhaps a specific selection process in spermatogenesis may influence the dynamics of this process. A model explaining mitotic instability and sex-dependent segregation phenomena in DM manifestation is discussed.     

Relationship of (CAG)nC configuration to repeat instability of the Machado-Joseph disease gene

The mutation responsible for Machado-Joseph disease (MJD) has been identified as an expansion of a CAG trinucleotide repeat in a novel gene on chromosome 14q32.1. The CAG repeat tract is followed by C or G, and alleles are thereby divided into two types on the basis of molecular configuration, (CAG)nC and (CAG)nG. We have studied the relationship between the repeat length and the configuration in 38 patients from 28 Japanese families with MJD, and 31 unrelated normal Japanese subjects. The CAG repeat length in 100 normal alleles ranged from 13 to 37 repeats, while 38 MJD patients had one expanded allele with 64 to 84 repeats. Surprisingly, the expanded alleles had exclusively the (CAG)nC configuration, while both (CAG)nC and (CAG)nG were seen in normal alleles from MJD and control subjects. Furthermore, in normal alleles, the CAG repeat tract was significantly longer in (CAG)nC than in (CAG)nG. These findings suggest that the (CAG)nC configuration is related to repeat instability of the MJD gene. Received: 23 April 1996 / Revised: 24 June 1996     

Hypermethylation of the CpG Island Near the G4C2 Repeat in ALS with a C9orf72 Expansion

The G₄C₂ repeat expansion in C9orf72 is the most common known cause of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). We tested the hypothesis that the repeat expansion causes aberrant CpG methylation near the G₄C₂ repeat, which could be responsible for the downregulation of gene expression. We investigated the CpG methylation profile by two methods using genomic DNA from the blood of individuals with ALS (37 expansion carriers and 64 noncarriers), normal controls (n = 76), and family members of 7 ALS probands with the expansion. We report that hypermethylation of the CpG island 5′ of the G₄C₂ repeat is associated with the presence of the expansion (p < 0.0001). A higher degree of methylation was significantly correlated with a shorter disease duration (p < 0.01), associated with familial ALS (p = 0.009) and segregated with the expansion in 7 investigated families. Notably, we did not detect methylation for either normal or intermediate alleles (up to 43 repeats), bringing to question the current cutoff of 30 repeats for pathological alleles. Our study raises several important questions for the future investigation of large data sets, such as whether the degree of methylation corresponds to clinical presentation (ALS versus FTLD).     

Distribution of CTG repeats at the DMPK gene in myotonic distrophy patients and healthy individuals from the Mexican population

Myotonic dystrophy type 1 (DM1), the most common form of adult muscular dystrophy, is caused by anormal expansion of CTG trinucleotide repeats located in the 3′-untranslated region of the DMPK gene. The clinical features of DM1 are multisystemic and highly variable, and the unstable nature of CTG expansion causes wide genotypic and phenotypic presentations. In this study, we described to our knowledge for the first time the molecular diagnosis of myotonic dystrophy type 1 patients in the Mexican population, applying a fluorescent PCR method in combination with capillary electrophoresis analysis of the amplified products. We identified expanded alleles in 45 out of 50 patients (90%) with clinical features of myotonic disease. Furthermore, genotyping of 400 healthy subjects revealed the presence of 25 different alleles, ranging in size from 5 to 34 repeats. The most frequent allele was 13 CTG repeats (38.87%) and the frequency for alleles over 18 CTG repeats was 6.7%. Molecular test is essential for DM1 diagnosis and distribution of the CTG repeat alleles present in the Mexican population are significantly different from those of other populations.     

C9ORF72 Repeat Expansion in Australian and Spanish Frontotemporal Dementia Patients

A hexanucleotide repeat expansion in C9ORF72 has been established as a common cause of frontotemporal dementia (FTD). However, the minimum repeat number necessary for disease pathogenesis is not known. The aims of our study were to determine the frequency of the C9ORF72 repeat expansion in two FTD patient collections (one Australian and one Spanish, combined n = 190), to examine C9ORF72 expansion allele length in a subset of FTD patients, and to examine C9ORF72 allele length in ‘non-expansion’ patients (those with <30 repeats). The C9ORF72 repeat expansion was detected in 5–17% of patients (21–41% of familial FTD patients). For one family, the expansion was present in the proband but absent in the mother, who was diagnosed with dementia at age 68. No association was found between C9ORF72 non-expanded allele length and age of onset and in the Spanish sample mean allele length was shorter in cases than in controls. Southern blotting analysis revealed that one of the nine ‘expansion-positive’ patients examined, who had neuropathologically confirmed frontotemporal lobar degeneration with TDP-43 pathology, harboured an ‘intermediate’ allele with a mean size of only ∼65 repeats. Our study indicates that the C9ORF72 repeat expansion accounts for a significant proportion of Australian and Spanish FTD cases. However, C9ORF72 allele length does not influence the age at onset of ‘non-expansion’ FTD patients in the series examined. Expansion of the C9ORF72 allele to as little as ∼65 repeats may be sufficient to cause disease.