Haplotype analysis of the internationally distributed BRCA1 c.3331_3334delCAAG founder mutation reveals a common ancestral origin in Iberia.

BACKGROUND: The BRCA1 c.3331_3334delCAAG founder mutation has been reported in hereditary breast and ovarian cancer families from multiple Hispanic groups. We aimed to evaluate BRCA1 c.3331_3334delCAAG haplotype diversity in cases of European, African, and Latin American ancestry. METHODS: BC mutation carrier cases from Colombia (n = 32), Spain (n = 13), Portugal (n = 2), Chile (n = 10), Africa (n = 1), and Brazil (n = 2) were genotyped with the genome-wide single nucleotide polymorphism (SNP) arrays to evaluate haplotype diversity around BRCA1 c.3331_3334delCAAG. Additional Portuguese (n = 13) and Brazilian (n = 18) BC mutation carriers were genotyped for 15 informative SNPs surrounding BRCA1. Data were phased using SHAPEIT2, and identical by descent regions were determined using BEAGLE and GERMLINE. DMLE+ was used to date the mutation in Colombia and Iberia. RESULTS: The haplotype reconstruction revealed a shared 264.4-kb region among carriers from all six countries. The estimated mutation age was ~ 100 generations in Iberia and that it was introduced to South America early during the European colonization period. CONCLUSIONS: Our results suggest that this mutation originated in Iberia and later introduced to Colombia and South America at the time of Spanish colonization during the early 1500s. We also found that the Colombian mutation carriers had higher European ancestry, at the BRCA1 gene harboring chromosome 17, than controls, which further supported the European origin of the mutation. Understanding founder mutations in diverse populations has implications in implementing cost-effective, ancestry-informed screening.

DNA glycosylases involved in base excision repair may be associated with cancer risk in BRCA1 and BRCA2 mutation carriers.

Single Nucleotide Polymorphisms (SNPs) in genes involved in the DNA Base Excision Repair (BER) pathway could be associated with cancer risk in carriers of mutations in the high-penetrance susceptibility genes BRCA1 and BRCA2, given the relation of synthetic lethality that exists between one of the components of the BER pathway, PARP1 (poly ADP ribose polymerase), and both BRCA1 and BRCA2. In the present study, we have performed a comprehensive analysis of 18 genes involved in BER using a tagging SNP approach in a large series of BRCA1 and BRCA2 mutation carriers. 144 SNPs were analyzed in a two stage study involving 23,463 carriers from the CIMBA consortium (the Consortium of Investigators of Modifiers of BRCA1 and BRCA2). Eleven SNPs showed evidence of association with breast and/or ovarian cancer at p<0.05 in the combined analysis. Four of the five genes for which strongest evidence of association was observed were DNA glycosylases. The strongest evidence was for rs1466785 in the NEIL2 (endonuclease VIII-like 2) gene (HR: 1.09, 95% CI (1.03-1.16), p = 2.7 × 10(-3)) for association with breast cancer risk in BRCA2 mutation carriers, and rs2304277 in the OGG1 (8-guanine DNA glycosylase) gene, with ovarian cancer risk in BRCA1 mutation carriers (HR: 1.12 95%CI: 1.03-1.21, p = 4.8 × 10(-3)). DNA glycosylases involved in the first steps of the BER pathway may be associated with cancer risk in BRCA1/2 mutation carriers and should be more comprehensively studied.

Analysis of Lynch Syndrome Mismatch Repair Genes in Women with Endometrial Cancer.

OBJECTIVE: Endometrial cancer is the second most frequent neoplasm in women with Lynch syndrome (LS). We sought to assess whether analyzing women with endometrial cancer would identify families with LS not identified with current clinical criteria. METHODS: We included women diagnosed with endometrial cancer younger than 50 years and also older if they had a family cancer history associated with LS. In blood samples obtained, we analyzed mutations in mismatch repair (MMR) genes, as well as protein expression by immunohistochemistry and microsatellite instability (MSI) in tumour tissue. RESULTS: A total of 103 patients were enrolled. We detected 14 pathogenic mutations and 4 genetic variants of unknown clinical significance in MMR genes. We found MSI in 41.66% of the women with a pathogenic mutation. In this group, 76.92% showed loss of at least one MMR protein. Women with mutations were younger at diagnosis, but all of them had a family history compatible with LS. CONCLUSIONS: Analysis of the MMR genes, in particular MSH6, seems to be appropriate in women with endometrial cancer and a family history of tumours associated with LS.

The highly prevalent BRCA2 mutation c.2808_2811del (3036delACAA) is located in a mutational hotspot and has multiple origins.

BRCA2-c.2808_2811del (3036delACAA) is one of the most reported germ line mutations in non-Ashkenazi breast cancer patients. We investigated its genetic origin in 51 Spanish carrier families that were genotyped with 11 13q polymorphic markers. Three independent associated haplotypes were clearly distinguished accounting for 23 [west Castilla y León (WCL)], 20 [east Castilla y León (ECL)] and 6 (South of Spain) families. Mutation age was estimated with the Disequilibrium Mapping using Likelihood Estimation software in a range of 45-68 and 45-71 generations for WCL and ECL haplotypes, respectively. The most prevalent variants, c.2808_2811del and c.2803G > A, were located in a double-hairpin loop structure (c.2794-c.2825) predicted by Quikfold that was proposed as a mutational hotspot. To check this hypothesis, random mutagenesis was performed over a 923 bp fragment of BRCA2, and 86 DNA variants were characterized. Interestingly, three mutations reported in the mutation databases (c.2680G > A, c.2944del and c.2957dup) were replicated and 20 affected the same position with different nucleotide changes. Moreover, five variants were placed in the same hairpin loop of c.2808_2811del, and one affected the same position (c.2808A > G). In conclusion, our results support that at least three different mutational events occurred to generate c.2808_2811del. Other highly prevalent DNA variants, such as BRCA1-c.68_69delAG, BRCA2-c.5946delT and c.8537delAG, are concentrated in hairpin loops, suggesting that these structures may represent mutational hotspots.

Trisomy 18p caused by a supernumerary marker with a chromosome 13/21 centromere: a possible recurrent chromosome aberration.

We present a clinical and molecular cytogenetic characterization of two new patients with a complex supernumerary marker consisting of the entire short arm of chromosome 18 with a chromosome 13/21 centromere. One patient is a girl with a nonsyndromic intellectual disability and the second is a prenatally diagnosed fetus. To our knowledge, these are the fourth and fifth such cases to be described in the literature, suggesting the existence of a possible recurring constitutional structural chromosome abnormality.

CDKL5 gene status in female patients with epilepsy and Rett-like features: two new mutations in the catalytic domain.

BACKGROUND: Mutations in the cyclin-dependent kinase-like 5 gene (CDKL5) located in the Xp22 region have been shown to cause a subset of atypical Rett syndrome with infantile spasms or early seizures starting in the first postnatal months. METHODS: We performed mutation screening of CDKL5 in 60 female patients who had been identified as negative for the methyl CpG-binding protein 2 gene (MECP2) mutations, but who had current or past epilepsy, regardless of the age of onset, type, and severity. All the exons in the CDKL5 gene and their neighbouring sequences were examined, and CDKL5 rearrangements were studied by multiplex ligation-dependent probe amplification (MLPA). RESULTS: Six previously unidentified DNA changes were detected, two of which were disease-causing mutations in the catalytic domain: a frameshift mutation (c.509_510insGT; p.Glu170GlyfsX36) and a complete deletion of exon 10. Both were found in patients with seizures that started in the first month of life. CONCLUSIONS: This study demonstrated the importance of CDKL5 mutations as etiological factors in neurodevelopmental disorders, and indicated that a thorough analysis of the CDKL5 gene sequence and its rearrangements should be considered in females with Rett syndrome-like phenotypes, severe encephalopathy and epilepsy with onset before 5 months of age. This study also confirmed the usefulness of MLPA as a diagnostic screening method for use in clinical practice.

Risk of cognitive impairment in female premutation carriers of fragile X premutation: analysis by means of robust segmented linear regression models.

This report describes a study focused on the relationship between CGG repeat length, FMRP, mRNA levels and cognitive functioning in premutation carriers (PM) carriers of Fragile X Syndrome (FXS). We studied 60 females-43 with PM and 17 with normal (N) alleles-from 25 FXS Spanish families. The Wechsler scales were administered to all subjects and new blood samples and hair roots were taken to study mRNA and FMRP levels. Using lowess curves together with segmented models we showed that within the premutation range, IQ scores tend to decrease when the number of CGG repeats increases and the FMRP values decrease. Furthermore, we discovered cut-off points in the molecular variables that seem to change the probability of having some cognitive impairment. Specifically, for those PM females in the upper premutation range (CGG > or = 100) and with FMRP expression < 60% in hair roots, a 10% decrement of FMRP expression represents a significant decrease in IQ scores of about six points, which is more evident for Full-Scale IQ (P-value = 0.035) and Performance IQ (P-value = 0.045) than for Verbal IQ (P-value = 0.074). On the contrary, we did not find any significant correlation between FMR1 mRNA levels and the IQ scores, probably due to the fact that mRNA levels were measured in blood. In conclusion, our findings suggest that the PM can have some effect on cognitive ability in female carriers, although these effects may be subtle. In these cases, it would be advisable to carry out a hair root analysis of FMRP.

Molecular and Clinical Characterization of a Novel Nonsense Variant in Exon 1 of the UPF3B Gene Found in a Large Spanish Basque Family (MRX82).

X-linked intellectual disability (XLID) is known to explain up to 10% of the intellectual disability in males. A large number of families in which intellectual disability is the only clinically consistent manifestation have been described. While linkage analysis and candidate gene testing were the initial approaches to find genes and variants, next generation sequencing (NGS) has accelerated the discovery of more and more XLID genes. Using NGS, we resolved the genetic cause of MRX82 (OMIM number 300518), a large Spanish Basque family with five affected males with intellectual disability and a wide phenotypic variability among them despite having the same pathogenic variant. Although the previous linkage study had mapped the locus to an interval of 7.6Mb in Xq24-Xq25 of the X chromosome, this region contained too many candidate genes to be analysed using conventional approaches. NGS revealed a novel nonsense variant: c.118C > T; p.Gln40* in UPF3B, a gene previously implicated in XLID that encodes a protein involved in nonsense-mediated mRNA decay (NMD). Further molecular studies showed that the mRNA transcript was not completely degraded by NMD. However, UPF3B protein was not detected by conventional Western Blot analysis at least downstream of the 40 residue demonstrating that the phenotype could be due to the loss of functional protein. This is the first report of a premature termination codon before the three functional domains of the UPF3B protein and these results directly implicate the absence of these domains with XLID, autism and some dysmorphic features.

High proportion of large genomic rearrangements in hMSH2 in hereditary nonpolyposis colorectal cancer (HNPCC) families of the Basque Country.

Hereditary nonpolyposis colorectal cancer (HNPCC), which represents the most common form of inherited colorectal cancer, results from germline alterations of the mismatch repair genes MSH2, MLH1 and MSH6. Rearrangements of MSH2 and MLH1 are involved in at least 10% and 4.3%, respectively, of the HNPCC families fulfilling the Amsterdam (AMS) criteria. We applied a recently developed method, multiplex ligation-dependent probe amplification (MLPA), to study MLH1/MSH2 copy number changes in 29 unrelated Basque Country HNPCC families. We detected six different genomic rearrangements in total (6/29=20.69%), four in MSH2 gene (13.79%), and two in MLH1 gene. All of the MSH2 rearrangements were genomic deletions involving several exons. The MLH1 rearrangements were initially detected as one deletion of exon 18 and one deletion of exon 19, but after sequencing analysis, these deletions were not confirmed and corresponded to base pair mutations. We conclude that MLPA is an excellent tool for detecting exon copy number changes in MLH1 and MSH2 in the DNA from HNPCC patients, although all detected rearrangements should be confirmed by an independent molecular methodology. Furthermore, our results in the Basque Country show higher percentages of rearrangements than previously published by other authors.

Candidate genetic modifiers for breast and ovarian cancer risk in BRCA1 and BRCA2 mutation carriers.

BACKGROUND: BRCA1 and BRCA2 mutation carriers are at substantially increased risk for developing breast and ovarian cancer. The incomplete penetrance coupled with the variable age at diagnosis in carriers of the same mutation suggests the existence of genetic and nongenetic modifying factors. In this study, we evaluated the putative role of variants in many candidate modifier genes. METHODS: Genotyping data from 15,252 BRCA1 and 8,211 BRCA2 mutation carriers, for known variants (n = 3,248) located within or around 445 candidate genes, were available through the iCOGS custom-designed array. Breast and ovarian cancer association analysis was performed within a retrospective cohort approach. RESULTS: The observed P values of association ranged between 0.005 and 1.000. None of the variants was significantly associated with breast or ovarian cancer risk in either BRCA1 or BRCA2 mutation carriers, after multiple testing adjustments. CONCLUSION: There is little evidence that any of the evaluated candidate variants act as modifiers of breast and/or ovarian cancer risk in BRCA1 or BRCA2 mutation carriers. IMPACT: Genome-wide association studies have been more successful at identifying genetic modifiers of BRCA1/2 penetrance than candidate gene studies.

Population genetic data of eight tetrameric short tandem repeats (STRs) in Casablanca resident population to use in forensic casework.

The allele frequencies for eight short tandem repeat (STR) loci HUMvWA, HUMFES/FPS, HUMF13A, HUMF13B, HUMTHO1, HUMTPOX, HUMCSF1P0, HUMLPL included in Geneprint STR kits were obtained from 234 unrelated individuals in Casablanca.

MECP2 gene study in a large cohort: testing of 240 female patients and 861 healthy controls (519 females and 342 males).

The MECP2 gene located on Xq28 is one of the most important genes contributing to the spectrum of neurodevelopmental disorders. Therefore, we present our experience in the molecular study of this gene. MECP2 was thoroughly tested for the presence of mutations (sequencing of four exons and rearrangements) in 120 female patients: 28 with classic Rett syndrome, five with atypical Rett syndrome, and 87 with heterogeneous phenotypes with some Rett-like features. Another 120 female patients with intellectual disability of unknown origin were also studied, but in these cases we only tested exons 3 and 4. Finally, 861 healthy controls (519 females and 342 males) were also studied for exon 3 and 4. Eighteen different pathological mutations were found, five of them previously undescribed, and four large deletions detected by multiplex ligation-dependent probe amplification. All were de novo mutations not present in the parents. In conclusion, i) MECP2 is one of the most important genes in the diagnosis of genetic intellectual disability in females; ii) MECP2 must be studied not only in patients with classical/atypical Rett syndrome but also in patients with other phenotypes related to Rett syndrome; and iii) for the new variants, it is important to perform complementary studies, including the analysis of large populations of healthy individuals and the use of in silico programs.

Analysis of PALB2 gene in BRCA1/BRCA2 negative Spanish hereditary breast/ovarian cancer families with pancreatic cancer cases.

BACKGROUND: The PALB2 gene, also known as FANCN, forms a bond and co-localizes with BRCA2 in DNA repair. Germline mutations in PALB2 have been identified in approximately 1% of familial breast cancer and 3-4% of familial pancreatic cancer. The goal of this study was to determine the prevalence of PALB2 mutations in a population of BRCA1/BRCA2 negative breast cancer patients selected from either a personal or family history of pancreatic cancer. METHODS: 132 non-BRCA1/BRCA2 breast/ovarian cancer families with at least one pancreatic cancer case were included in the study. PALB2 mutational analysis was performed by direct sequencing of all coding exons and intron/exon boundaries, as well as multiplex ligation-dependent probe amplification. RESULTS: Two PALB2 truncating mutations, the c.1653T>A (p.Tyr551Stop) previously reported, and c.3362del (p.Gly1121ValfsX3) which is a novel frameshift mutation, were identified. Moreover, several PALB2 variants were detected; some of them were predicted as pathological by bioinformatic analysis. Considering truncating mutations, the prevalence rate of our population of BRCA1/2-negative breast cancer patients with pancreatic cancer is 1.5%. CONCLUSIONS: The prevalence rate of PALB2 mutations in non-BRCA1/BRCA2 breast/ovarian cancer families, selected from either a personal or family pancreatic cancer history, is similar to that previously described for unselected breast/ovarian cancer families. Future research directed towards identifying other gene(s) involved in the development of breast/pancreatic cancer families is required.

A child with mild X-linked intellectual disability and a microduplication at Xp22.12 including RPS6KA3.

Multiplex ligation-dependent probe amplification (MLPA) and array- comparative genomic hybridization analysis have been proven to be useful in the identification of submicroscopic copy-number imbalances in families with nonsyndromic X-linked intellectual disability (NS-XLID). Here we report the first description of a child with mild intellectual disability and a submicroscopic duplication at Xp22.12 identified by MLPA with a P106 MRX kit (MRC-Holland, Amsterdam, Netherlands) and further confirmed and characterized with a custom 244-k oligo-array, fluorescence in situ hybridization, quantitative polymerase chain reaction (qPCR), and immunoblotting. This 1.05-megabase duplication encompasses 7 genes, RPS6KA3 being the only of these genes known to be related to ID. The proband was an 8-year-old boy referred to the genetics unit for psychomotor retardation and learning disabilities. Both maternal brothers also showed learning difficulties and delayed language during childhood in a similar way to the proband. These boys also carried the duplication, as did the healthy mother and grandmother of the proband. The same duplication was also observed in the 5-year-old younger brother who presented with features of developmental delay and learning disabilities during the previous year. Increased RPS6KA3/RSK2 levels were demonstrated in the proband by qPCR and immunoblotting. To our knowledge, this is the first family identified with a submicroscopic duplication including the entire RPS6KA3/RSK2 gene, and our findings suggest that an increased dose of this gene is responsible for a mild form of NS-XLID.

Breast and ovarian cancer risk evaluation in families with a disease-causing mutation in BRCA1/2.

Germline mutations in BRCA1 and BRCA2 confer high risks of breast and ovarian cancer, and their identification allows genetic testing of at-risk relatives. However, estimates of these risks illustrate controversies, depending on the published series. The penetrance, the earlier onset of the disease and the effect of mutations on the risk of developing breast and ovarian cancer were evaluated in 344 females belonging to 34 families from the Basque Country in Spain, in which BRCA1 or BRCA2 mutations were transmitted. Kaplan-Meier survival curves were used to derive cumulative probability curves for breast and ovarian cancer by mutation status, birth cohort and mutation position, and significance of the differences was assessed using the log-rank test. The estimated probability for breast cancer by age 70 is about 64% in BRCA1 and 69% in BRCA2, whereas the probability of developing ovarian cancer is about 37% and 25% for BRCA1 and BRCA2, respectively. There is a marginally significant higher risk of developing ovarian cancer in BRCA1 families than in BRCA2 families. The effect of birth cohort on breast cancer cumulative incidence presents an increased risk for females born after 1966 and a decreased risk for those born before 1940. There is no association between mutation position and breast cancer; however, ovarian cancer is associated to BRCA1, presenting exon 11 as an ovarian cluster. These results are important for the breast and ovarian cancer diagnosis and prevention in at-risk families.

MLH1 founder mutations with moderate penetrance in Spanish Lynch syndrome families.

The variants c.306+5G>A and c.1865T>A (p.Leu622His) of the DNA repair gene MLH1 occur frequently in Spanish Lynch syndrome families. To understand their ancestral history and clinical effect, we performed functional assays and a penetrance analysis and studied their genetic and geographic origins. Detailed family histories were taken from 29 carrier families. Functional analysis included in silico and in vitro assays at the RNA and protein levels. Penetrance was calculated using a modified segregation analysis adjusted for ascertainment. Founder effects were evaluated by haplotype analysis. The identified MLH1 c.306+5G>A and c.1865T>A (p.Leu622His) variants are absent in control populations and segregate with the disease. Tumors from carriers of both variants show microsatellite instability and loss of expression of the MLH1 protein. The c.306+5G>A variant is a pathogenic mutation affecting mRNA processing. The c.1865T>A (p.Leu622His) variant causes defects in MLH1 expression and stability. For both mutations, the estimated penetrance is moderate (age-cumulative colorectal cancer risk by age 70 of 20.1% and 14.1% for c.306+5G>A and of 6.8% and 7.3% for c.1865T>A in men and women carriers, respectively) in the lower range of variability estimated for other pathogenic Spanish MLH1 mutations. A common haplotype was associated with each of the identified mutations, confirming their founder origin. The ages of c.306+5G>A and c.1865T>A mutations were estimated to be 53 to 122 and 12 to 22 generations, respectively. Our results confirm the pathogenicity, moderate penetrance, and founder origin of the MLH1 c.306+5G>A and c.1865T>A mutations. These findings have important implications for genetic counseling and molecular diagnosis of Lynch syndrome.

A study on MSH2 and MLH1 mutations in hereditary nonpolyposis colorectal cancer families from the Basque Country, describing four new germline mutations.

Hereditary non-polyposis colorectal cancer (HNPCC) or Lynch syndrome underlies between 2 and 5% of all colorectal cancer. It is inherited as an autosomal dominant condition due to mutations in the mismatch repair genes. Fifty-four non-related index cases, 21 of them fulfilling Amsterdam criteria I or II, were studied. Ten (10/21 = 47.6%) different pathological mutations were found in this group, two of which had not previously been reported--one in MLH1 and the other in MSH2-. In the remaining patients, we also found another family with one of these new mutations, and four additional changes, two of which were also new--a pathological change in MSH2 and a second change of uncertain significance in MLH1-, while the other two changes had already been reported. Of all mutations, eight were found in MSH2 (8/15 = 53.3%) and seven in MLH1 (7/15 = 46.6%), suggesting a slightly greater involvement of MSH2 in HNPCC than MLH1 in our population, in contrast to the results reported by other authors.

Two cases of deletion 2q37 associated with segregation of an unbalanced translocation 2;21: choanal atresia leading to misdiagnosis of CHARGE syndrome.

CONTEXT: The phenotypic variability of patients with syndromes presenting with dysmorphism makes clinical diagnosis difficult, leading to an exhaustive genetic study to determine the underlying mechanism so that a proper diagnosis could be established. OBJECTIVE: To genetically characterize siblings, the older sister diagnosed with Albright hereditary osteodystrophy and the younger one with CHARGE syndrome. DESIGN: Clinical case report. METHODS: Clinical, biochemical, and radiological studies were performed on the family. In addition, molecular genetic studies including sequencing of GNAS, typing of microsatellites on 2q and 21q, and multiplex ligation-dependent probe amplification of subtelomeric regions were performed, as well as confirmatory fluorescent in situ hybridization analysis. RESULTS: The genetic analysis revealed that both sisters presented a 2q37 deletion due to the maternal unbalanced segregation of a 2;21 translocation. CONCLUSIONS: This is the first report of a 2q37 deletion where differential diagnosis of CHARGE syndrome is needed due to the appearance of choanal atresia.

Analysis of the molecular parameters that could predict the risk of manifesting premature ovarian failure in female premutation carriers of fragile X syndrome.

OBJECTIVE: To study three molecular parameters (number of CGG repeats, X-inactivation ratio, and expression of FMR1 mRNA) in premutation carriers of fragile X syndrome with and without premature ovarian failure (POF) to find differences between these two groups that could be useful in reproductive counseling. DESIGN: A retrospective clinical and molecular genetic study of 42 known premutation carriers of fragile X syndrome aged 40 years or older, 25 with POF and 17 without. A blood sample to obtain mRNA was taken from all of them. They all lived in five autonomous communities in northern Spain. RESULTS: Although the relationship among mRNA levels, X-inactivation ratio, and CGG repeats seems to be similar both in women with POF and in those without: in women with POF, the effect of the CGG repeats on the mRNA levels was statistically significant (P = 0.0437), but in women without POF, it was not (P = 0.0724). Moreover, we confirmed previous results on the nonlinear association between CGG repeat number and the manifestation of POF, showing that the likelihood of having POF was significantly higher with fewer than 100 CGG repeats compared with 100 or more CGG repeats (odds ratio = 13.09, P = 0.0240). CONCLUSIONS: Our present work suggests that mRNA and X-inactivation studies in blood are not relevant in predicting POF in female premutation carriers of fragile X syndrome. However, having a permutation of fewer than 100 repeats could represent a significant risk of POF.

Analysis of FMR1 gene expression in female premutation carriers using robust segmented linear regression models.

Fragile X syndrome is caused by the absence or reduction of the fragile X mental retardation protein (FMRP) because FMR1 gene expression is reduced. Alleles with repeat sizes of 55-200 are classified as premutations, and it has been demonstrated that FMR1 expression is elevated in the premutation range. However, the majority of the studies reported were performed in males. We studied FMR1 expression in 100 female fragile X family members from the northern region of Spain using quantitative (fluorescence) real-time polymerase chain reaction. Of these 100 women, 19 had normal alleles, 19 were full mutation carriers, and 62 were premutation carriers. After confirming differences between the three groups of females, and increased levels of the FMR1 transcript among premutation carriers, we found that the relationship between mRNA levels and repeat size is nonlinear. These results were obtained using a novel methodology that, based on the size of the CGG repeats, allows us to find out the most probable threshold from which the relationship between CGG repeat number and mRNA levels changes. Using this approach, a significant positive correlation between CGG repeats and total mRNA levels has been found in the premutation range <100 CGG, but this correlation diminishes from 100 onward. However, when correcting by the X inactivation ratio, mRNA levels increase as the number of CGG repeats increases, and this increase is highly significant over 100 CGG. We suggest that due to skewed X inactivation, mRNA levels tend to normalize in females when the number of CGG repeats increases.