A multistep mutation mechanism drives the evolution of the CAG repeat at MJD/SCA3 locus

Spinocerebellar ataxia type 2 has multiple ancestral origins

INTRODUCTION

Spinocerebellar ataxia type 2 (SCA2) is a dominant neurodegenerative disorder due to expansions of a CAG repeat tract (CAGexp) at the ATXN2 gene. Previous studies found only one ancestral haplotype worldwide, with a C allele at rs695871. This homogeneity was unexpected, given the severe anticipations related to SCA2. We aimed to describe informative ancestral haplotypes found in South American SCA2 families.

METHODS

Seventy-seven SCA2 index cases were recruited from Brazil, Peru, and Uruguay; 263 normal chromosomes were used as controls. The SNPs rs9300319, rs3809274, rs695871, rs1236900 and rs593226, and the STRs D12S1329, D12S1333, D12S1672 and D12S1332, were used to reconstruct haplotypes.

RESULTS

Eleven ancestral haplotypes were found in SCA2 families. The most frequent ones were A-G-C-C-C (46.7 % of families), G-C-C-C-C (24.6 %) and A-C-C-C-C (10.3 %) and their mean (sd) CAGexp were 41.68 (3.55), 40.42 (4.11) and 45.67 (9.70) (p = 0.055), respectively. In contrast, the mean (sd) CAG lengths at normal alleles grouped per haplotypes G-C-G-A-T, A-G-C-C-C and G-C-C-C-C were 22.97 (3.93), 23.85 (3.59), and 30.81 (4.27) (p < 0.001), respectively. The other SCA2 haplotypes were rare: among them, a G-C-G-A-T lineage was found, evidencing a G allele in rs695871.

CONCLUSION

We identified several distinct ancestral haplotypes in SCA2 families, including an unexpected lineage with a G allele at rs695871, a variation never found in hundreds of SCA2 patients studied worldwide. SCA2 has multiple origins in South America, and more studies should be done in other regions of the world.

The sequence of the repetitive motif influences the frequency of multistep mutations in Short Tandem Repeats

Microsatellites, or Short Tandem Repeats (STRs), are subject to frequent length mutations that involve the loss or gain of an integer number of repeats. This work aimed to investigate the correlation between STRs' specific repetitive motif composition and mutational dynamics, specifically the occurrence of single- or multistep mutations. Allelic transmission data, comprising 323,818 allele transfers and 1,297 mutations, were gathered for 35 Y-chromosomal STRs with simple structure. Six structure groups were established: ATT, CTT, TCTA/GATA, GAAA/CTTT, CTTTT, and AGAGAT, according to the repetitive motif present in the DNA leading strand of the markers. Results show that the occurrence of multistep mutations varies significantly among groups of markers defined by the repetitive motif. The group of markers with the highest frequency of multistep mutations was the one with repetitive motif CTTTT (25% of the detected mutations) and the lowest frequency corresponding to the group with repetitive motifs TCTA/GATA (0.93%). Statistically significant differences (α = 0.05) were found between groups with repetitive motifs with different lengths, as is the case of TCTA/GATA and ATT (p = 0.0168), CTT (p < 0.0001) and CTTTT (p < 0.0001), as well as between GAAA/CTTT and CTTTT (p = 0.0102). The same occurred between the two tetrameric groups GAAA/CTTT and TCTA/GATA (p < 0.0001) - the first showing 5.7 times more multistep mutations than the second. When considering the number of repeats of the mutated paternal alleles, statistically significant differences were found for alleles with 10 or 12 repeats, between GATA and ATT structure groups. These results, which demonstrate the heterogeneity of mutational dynamics across repeat motifs, have implications in the fields of population genetics, epidemiology, or phylogeography, and whenever STR mutation models are used in evolutionary studies in general.

FAN1 exo- not endo-nuclease pausing on disease-associated slipped-DNA repeats: A mechanism of repeat instability

Ongoing inchworm-like CAG and CGG repeat expansions in brains, arising by aberrant processing of slipped DNAs, may drive Huntington's disease, fragile X syndrome, and autism. FAN1 nuclease modifies hyper-expansion rates by unknown means. We show that FAN1, through iterative cycles, binds, dimerizes, and cleaves slipped DNAs, yielding striking exo-nuclease pauses along slip-outs: 5'-C↓A↓GC↓A↓G-3' and 5'-C↓T↓G↓C↓T↓G-3'. CAG excision is slower than CTG and requires intra-strand A·A and T·T mismatches. Fully paired hairpins arrested excision, whereas disease-delaying CAA interruptions further slowed excision. Endo-nucleolytic cleavage is insensitive to slip-outs. Rare FAN1 variants are found in individuals with autism with CGG/CCG expansions, and CGG/CCG slip-outs show exo-nuclease pauses. The slip-out-specific ligand, naphthyridine-azaquinolone, which induces contractions of expanded repeats in vivo, requires FAN1 for its effect, and protects slip-outs from FAN1 exo-, but not endo-, nucleolytic digestion. FAN1's inchworm pausing of slip-out excision rates is well suited to modify inchworm expansion rates, which modify disease onset and progression.

Toward allele-specific targeting therapy and pharmacodynamic marker for spinocerebellar ataxia type 3

Spinocerebellar ataxia type 3 (SCA3), caused by a CAG repeat expansion in the ataxin-3 gene (ATXN3), is characterized by neuronal polyglutamine (polyQ) ATXN3 protein aggregates. Although there is no cure for SCA3, gene-silencing approaches to reduce toxic polyQ ATXN3 showed promise in preclinical models. However, a major limitation in translating putative treatments for this rare disease to the clinic is the lack of pharmacodynamic markers for use in clinical trials. Here, we developed an immunoassay that readily detects polyQ ATXN3 proteins in human biological fluids and discriminates patients with SCA3 from healthy controls and individuals with other ataxias. We show that polyQ ATXN3 serves as a marker of target engagement in human fibroblasts, which may bode well for its use in clinical trials. Last, we identified a single-nucleotide polymorphism that strongly associates with the expanded allele, thus providing an exciting drug target to abrogate detrimental events initiated by mutant ATXN3. Gene-silencing strategies for several repeat diseases are well under way, and our results are expected to improve clinical trial preparedness for SCA3 therapies.

Selective forces acting on spinocerebellar ataxia type 3/Machado-Joseph disease recurrency: A systematic review and meta-analysis

Spinocerebellar ataxia type 3/Machado-Joseph disease (SCA3/MJD) is a dominant neurodegenerative disease caused by the expansion of a CAG repeat tract in ATXN3. Anticipation and worsening of clinical picture in subsequent generations were repeatedly reported, but there is no indication that SCA3/MJD frequency is changing. Thus, we performed a systematic review and meta-analysis on phenomena with potential effect on SCA3/MJD recurrency in populations: instability of CAG repeat transmissions, anticipation, fitness, and segregation of alleles. Transmission of the mutant allele was associated with an increase of 1.23 CAG repeats in the next generation, and the average change in age at onset showed an anticipation of 7.75 years per generation; but biased recruitments cannot be ruled out. Affected SCA3/MJD individuals had 45% more children than related controls. Transmissions from SCA3/MJD carriers showed that the expanded allele was segregated in 64% of their children. In contrast, transmissions from normal subjects showed that the minor allele was segregated in 54%. The present meta-analysis concluded that there is a segregation distortion favoring the expanded allele, among children of carriers. Therefore, further studies on transmissions and anticipation phenomena as well as more observations about fertility are required to clarify these selective forces over SCA3/MJD.

Genome-wide association study identifies genetic factors that modify age at onset in Machado-Joseph disease

Machado-Joseph disease (MJD/SCA3) is the most common form of dominantly inherited ataxia worldwide. The disorder is caused by an expanded CAG repeat in the ATXN3 gene. Past studies have revealed that the length of the expansion partly explains the disease age at onset (AO) variability of MJD, which is confirmed in this study (Pearson’s correlation coefficient R² = 0.62). Using a total of 786 MJD patients from five different geographical origins, a genome-wide association study (GWAS) was conducted to identify additional AO modifying factors that could explain some of the residual AO variability. We identified nine suggestively associated loci (P < 1 × 10⁻⁵). These loci were enriched for genes involved in vesicle transport, olfactory signaling, and synaptic pathways. Furthermore, associations between AO and the TRIM29 and RAG genes suggests that DNA repair mechanisms might be implicated in MJD pathogenesis. Our study demonstrates the existence of several additional genetic factors, along with CAG expansion, that may lead to a better understanding of the genotype-phenotype correlation in MJD.

Is the High Frequency of Machado-Joseph Disease in China Due to New Mutational Origins?

Machado-Joseph disease (MJD, also known as spinocerebellar ataxia 3 or SCA3) is the most common dominant ataxia worldwide, with an overall average prevalence of 1–5/100,000. To this date, two major ancestral lineages have been found throughout the world. In China, the relative frequency of MJD among the SCAs reaches as high as 63%, however, little is known about its mutational origin in this country. We analyzed 50 families with MJD patients in two or more generations to study the hypothesis that new mutational events have occurred in this population. Haplotypes based on 20 SNPs have shown new genetic backgrounds segregating with MJD mutations in our cohort from China. We found the “Joseph-derived” lineage (Joseph lineage with a G variant in rs56268847) to be very common among Chinese MJD patients. Moreover, we estimated the time for the origin of this MJD SNP background based on STR diversity flanking the (CAG)_n of ATXN3. It was surprising to find that the Chinese MJD population originated from 8,000 to 17,000 years ago, far earlier than the previous literature reports, which will be an important evidence to explain the origin, spread and founder effects of MJD.

A Pipeline to Assess Disease-Associated Haplotypes in Repeat Expansion Disorders: The Example of MJD/SCA3 Locus

At least 40 human diseases are associated with repeat expansions; yet, the mutational origin and instability mechanisms remain unknown for most of them. Previously, genetic epidemiology and predisposing backgrounds for the instability of some expanding loci have been studied in different populations through the analysis of diversity flanking the respective pathogenic repeats. Here, we aimed at developing a pipeline to assess disease-associated haplotypes at oligonucleotide repeat loci, combining analysis of single nucleotide polymorphisms (SNPs) and short tandem repeats (STRs). Machado-Joseph disease (MJD/SCA3), the most frequent dominant ataxia worldwide, was used as an example of a detailed procedure. Thus, to identify genetic backgrounds that segregate with expanded/mutated alleles in MJD, we selected a set of 26 SNPs and 7 STRs flanking the causative CAG repeat. Key criteria and steps for this selection are described, and included (1) haplotype blocks minimizing the occurrence of recombination (for SNPs); and (2) match scores to increase potential for polymorphic information content of repetitive sequences found in Tandem Repeats Finder (for STRs). To directly assess SNP haplotypes in phase with MJD expansions, we optimized a strategy with preferential amplification of normal over expanded alleles, in addition to SNP allele-specific amplifications; this allowed the identification of disease-associated SNP haplotypes, even when only the proband is available in a given family. To infer STR haplotypes, we optimized a multiplex PCR, including 7 STRs plus the MJD_CAG repeat, followed by analysis of segregation or the use of the PHASE software. This protocol is a ready-to-use tool to assess MJD haplotypes in different populations. The pipeline designed can be used to assess disease-associated haplotypes in other repeat-expansion diseases. This should be of great utility to study (1) genetic epidemiology (population-of-origin, age and spreading routes of mutations) and (2) mechanisms responsible for de novo expansions, in these neurological diseases; (3) to detect predisposing haplotypes and (4) phenotype modifiers; (5) to help solving cases of apparent homoallelism (two same-size normal alleles) in diagnosis; and (6) to identify the best targets for the development of allele-specific therapies in ethnically diverse patient populations.

Physiological and pathophysiological characteristics of ataxin-3 isoforms

Ataxin-3 is a deubiquitinating enzyme and the affected protein in the neurodegenerative disorder Machado-Joseph disease (MJD). The ATXN3 gene is alternatively spliced, resulting in protein isoforms that differ in the number of ubiquitin-interacting motifs. Additionally, nonsynonymous SNPs in ATXN3 cause amino acid changes in ataxin-3, and one of these polymorphisms introduces a premature stop codon in one isoform. Here, we examined the effects of different ataxin-3 isoforms and of the premature stop codon on ataxin-3's physiological function and on main disease mechanisms. At the physiological level, we show that alternative splicing and the premature stop codon alter ataxin-3 stability and that ataxin-3 isoforms differ in their enzymatic deubiquitination activity, subcellular distribution, and interaction with other proteins. At the pathological level, we found that the expansion of the polyglutamine repeat leads to a stabilization of ataxin-3 and that ataxin-3 isoforms differ in their aggregation properties. Interestingly, we observed a functional interaction between normal and polyglutamine-expanded ATXN3 allelic variants. We found that interactions between different ATXN3 allelic variants modify the physiological and pathophysiological properties of ataxin-3. Our findings indicate that alternative splicing and interactions between different ataxin-3 isoforms affect not only major aspects of ataxin-3 function but also MJD pathogenesis. Our results stress the importance of considering isoforms of disease-causing proteins and their interplay with the normal allelic variant as disease modifiers in MJD and autosomal-dominantly inherited diseases in general.

Parametric fMRI of paced motor responses uncovers novel whole-brain imaging biomarkers in spinocerebellar ataxia type 3

Machado-Joseph Disease, inherited type 3 spinocerebellar ataxia (SCA3), is the most common form worldwide. Neuroimaging and neuropathology have consistently demonstrated cerebellar alterations. Here we aimed to discover whole-brain functional biomarkers, based on parametric performance-level-dependent signals. We assessed 13 patients with early SCA3 and 14 healthy participants. We used a combined parametric behavioral/functional neuroimaging design to investigate disease fingerprints, as a function of performance levels, coupled with structural MRI and voxel-based morphometry. Functional magnetic resonance imaging (fMRI) was designed to parametrically analyze behavior and neural responses to audio-paced bilateral thumb movements at temporal frequencies of 1, 3, and 5 Hz. Our performance-level-based design probing neuronal correlates of motor coordination enabled the discovery that neural activation and behavior show critical loss of parametric modulation specifically in SCA3, associated with frequency-dependent cortico/subcortical activation/deactivation patterns. Cerebellar/cortical rate-dependent dissociation patterns could clearly differentiate between groups irrespective of grey matter loss. Our findings suggest functional reorganization of the motor network and indicate a possible role of fMRI as a tool to monitor disease progression in SCA3. Accordingly, fMRI patterns proved to be potential biomarkers in early SCA3, as tested by receiver operating characteristic analysis of both behavior and neural activation at different frequencies. Discrimination analysis based on BOLD signal in response to the applied parametric finger-tapping task significantly often reached >80% sensitivity and specificity in single regions-of-interest.Functional fingerprints based on cerebellar and cortical BOLD performance dependent signal modulation can thus be combined as diagnostic and/or therapeutic targets in hereditary ataxia. Hum Brain Mapp 37:3656-3668, 2016. © 2016 Wiley Periodicals, Inc.

Diversity in the androgen receptor CAG repeat has been shaped by a multistep mutational mechanism

The androgen receptor (AR) gene encodes a type of nuclear receptor that functions as a steroid-hormone activated transcription factor. In its coding region, AR includes a CAG repeat, which has been intensely studied due to the inverse correlation between repeat size and AR transcriptional activity. Several studies have reported different (CAG)n sizes associated with the risk of androgen-linked diseases. We aimed at clarifying the mechanisms on the origin of newly CAG sized alleles through a strategy involving the analysis of the associated haplotype diversity. We genotyped 374 control individuals of European and Asian ancestry, and reconstructed the haplotypes associated with the CAG repeat, defined by 10 SNPs and 6 flanking STRs. The most powerful SNPs to tag AR lineages are rs7061037-rs12012620 and rs34191540-rs6625187-rs2768578 in Europeans and Asians, respectively. In the most frequent AR lineage, (CAG)18 alleles seem to have been generated by a multistep mutation mechanism, most probably from longer alleles. We further noticed that the DXS1194-DXS1111 haplotype, in linkage disequilibrium with AR-(CAG)n expanded alleles responsible for spinal bulbar muscular atrophy (SBMA), is rare among our controls; however, the haplotype strategy here described may be used to clarify the origin of expansions in other populations, as in future association studies.

Toward understanding Machado-Joseph disease

Machado-Joseph disease (MJD), also known as spinocerebellar ataxia type 3 (SCA3), is the most common inherited spinocerebellar ataxia and one of many polyglutamine neurodegenerative diseases. In MJD, a CAG repeat expansion encodes an abnormally long polyglutamine (polyQ) tract in the disease protein, ATXN3. Here we review MJD, focusing primarily on the function and dysfunction of ATXN3 and on advances toward potential therapies. ATXN3 is a deubiquitinating enzyme (DUB) whose highly specialized properties suggest that it participates in ubiquitin-dependent proteostasis. By virtue of its interactions with VCP, various ubiquitin ligases and other ubiquitin-linked proteins, ATXN3 may help regulate the stability or activity of many proteins in diverse cellular pathways implicated in proteotoxic stress response, aging, and cell differentiation. Expansion of the polyQ tract in ATXN3 is thought to promote an altered conformation in the protein, leading to changes in interactions with native partners and to the formation of insoluble aggregates. The development of a wide range of cellular and animal models of MJD has been crucial to the emerging understanding of ATXN3 dysfunction upon polyQ expansion. Despite many advances, however, the principal molecular mechanisms by which mutant ATXN3 elicits neurotoxicity remain elusive. In a chronic degenerative disease like MJD, it is conceivable that mutant ATXN3 triggers multiple, interconnected pathogenic cascades that precipitate cellular dysfunction and eventual cell death. A better understanding of these complex molecular mechanisms will be important as scientists and clinicians begin to focus on developing effective therapies for this incurable, fatal disorder.

Machado-Joseph Disease: from first descriptions to new perspectives

Machado-Joseph Disease (MJD), also known as spinocerebellar ataxia type 3 (SCA3), represents the most common form of SCA worldwide. MJD is an autosomal dominant neurodegenerative disorder of late onset, involving predominantly the cerebellar, pyramidal, extrapyramidal, motor neuron and oculomotor systems; although sharing features with other SCAs, the identification of minor, but more specific signs, facilitates its differential diagnosis. MJD presents strong phenotypic heterogeneity, which has justified the classification of patients into three main clinical types. Main pathological lesions are observed in the spinocerebellar system, as well as in the cerebellar dentate nucleus. MJD's causative mutation consists in an expansion of an unstable CAG tract in exon 10 of the ATXN3 gene, located at 14q32.1. Haplotype-based studies have suggested that two main founder mutations may explain the present global distribution of the disease; the ancestral haplotype is of Asian origin, and has an estimated age of around 5,800 years, while the second mutational event has occurred about 1,400 years ago. The ATXN3 gene encodes for ataxin-3, which is ubiquitously expressed in neuronal and non-neuronal tissues, and, among other functions, is thought to participate in cellular protein quality control pathways. Mutated ATXN3 alleles consensually present about 61 to 87 CAG repeats, resulting in an expanded polyglutamine tract in ataxin-3. This altered protein gains a neurotoxic function, through yet unclear mechanisms. Clinical variability of MJD is only partially explained by the size of the CAG tract, which leaves a residual variance that should be explained by still unknown additional factors. Several genetic tests are available for MJD, and Genetic Counseling Programs have been created to better assist the affected families, namely on what concerns the possibility of pre-symptomatic testing. The main goal of this review was to bring together updated knowledge on MJD, covering several aspects from its initial descriptions and clinical presentation, through the discovery of the causative mutation, its origin and dispersion, as well as molecular genetics aspects considered essential for a better understanding of its neuropathology. Issues related with molecular testing and Genetic Counseling, as well as recent progresses and perspectives on genetic therapy, are also addressed.

Severe α-1 antitrypsin deficiency caused by Q0(Ourém) allele: clinical features, haplotype characterization and history

α-1 Antitrypsin deficiency (AATD) caused by null alleles is associated with the total lack of protein and generally it translates into more severe clinical features of pulmonary disease. This is the case of Q0(Ourém) , a rare variant found in several families of Central Portugal caused by the L353fsX376 mutation. A total of 41 patients carrying at least one copy of Q0(Ourém) were evaluated for SERPINA1 levels, respiratory function values and lung parenchyma status (chest X-ray and computerized tomography scan). Q0(Ourém) haplotype background was characterized using seven microsatellites flanking SERPINA1 and Q0(Ourém) age was estimated by a statistical method relying on the decay of haplotype sharing at linked markers (DHSMAP). Homozygous patients showed a compromised lung function and extensive emphysema. SQ0(Ourém) , although having serum levels below the 11 µM threshold, did not necessarily result in signs of disease. MQ0(Ourém) were found to be a heterogeneous group, mainly composed of normal individuals. Eight Q0(Ourém) haplotypes were identified and the allele was estimated to have arisen 650 years ago. Q0(Ourém) was associated with mild to severe AATD and has a single origin, probably linked to the major Ourém settlements where the occurrence of severe AATD may not be explained by recent consanguinity.

Common origin of pure and interrupted repeat expansions in spinocerebellar ataxia type 2 (SCA2)

The spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant neurodegenerative disease characterized by gait and limb ataxia. This disease is caused by the expansion of a (CAG)(n) located in the ATXN2, that encodes a polyglutamine tract of more than 34 repeats. Lately, alleles with 32-33 CAGs have been associated to late-onset disease cases. Repeat interruptions by CAA triplets are common in normal alleles, while expanded alleles usually contain a pure repeat tract. To investigate the mutational origin and the instability associated to the ATXN2 repeat, we performed an extensive haplotype study and sequencing of the CAG/CAA repeat, in a cohort of families of different geographic origins and phenotypes. Our results showed (1) CAA interruptions also in expanded ATXN2 alleles; (2) that pathological CAA interrupted alleles shared an ancestral haplotype with pure expanded alleles; and (3) higher genetic diversity in European SCA2 families, suggesting an older European ancestry of SCA2. In conclusion, we found instability towards expansion in interrupted ATXN2 alleles and a shared ancestral ATXN2 haplotype for pure and interrupted expanded alleles; this finding has strong implications in mutation diagnosis and counseling. Our results indicate that interrupted alleles, below the pathological threshold, may be a reservoir of mutable alleles, prone to expansion in subsequent generations, leading to full disease mutation.

cDNA cloning and tissue expression analyses of the encoding regions for three novel porcine genes- MJD1, CDC42 and NECD

The cDNAs for Machado-Joseph disease protein 1 homolog (MJD1), cell division control protein 42 homolog precursor(CDC42) and necdin (NECD) genes of pig were amplified using the reverse transcriptase polymerase chain reaction (RT-PCR) based on the conserved coding sequence information of the MJD1, CDC42, and NECD genes from mouse and other mammals and the referenced porcine EST sequence information. Tissue expression analysis showed the swine MJD1, CDC42, and NECD genes were obviously differentially expressed in different tissues including muscle, heart, liver, backfat, kidney, lung, small intestine, and large intestine. Our experiment established the primary foundation for further research on these three swine genes.

Cis-acting factors promoting the CAG intergenerational instability in Machado-Joseph disease

In repeat expansion disorders, the size of pathological alleles is the most relevant factor accounting for the disease severity and age-at-onset, emphasizing the clinical significance of their underlying intergenerational instability. In one of these diseases, Machado-Joseph disease (MJD), the sex of transmitting progenitor and the C(987)GG/G(987)GG polymorphism are the best studied factors acting on intergenerational instability of expanded alleles. Here, we assessed the influence of other cis and inter-allelic acting factors, at the ATXN3 locus, through the analysis of MJD lineages, flanking STR-based haplotypes, the initial repeat size and parental age. A total of 100 transmissions of the expanded MJD allele were analyzed according to the sex of the transmitting parent. We have shown that independent origin mutations (identified by intragenic SNP-based haplotypes) behave differently, as the status of instability (contraction, no change or further expansion) is concerned. Indeed, 72% of expansions were associated to the worldwide spread TTACAC lineage, whereas the GTGGCA displayed 75% of all contractions observed. The analysis of flanking recombinant haplotypes did not suggest any further distant cis elements acting up- or downstream the ATXN3 locus. Considering the increased amplitude of expansions seen in older transmitting fathers, a repair-based mechanism may be suggested for the meiotic instability at this locus; furthermore, the lack of correlation between the initial repeat size and degree of instability did not support a replication-based mechanism. In summary, our findings point to different mechanisms of instability underlying male and female meioses, as well as contraction and expansion processes in MJD.

Segregation distortion of wild-type alleles at the Machado-Joseph disease locus: a study in normal families from the Azores islands (Portugal)

Machado-Joseph disease (MJD) is caused by an expansion of a triplet repeat with a CAG motif at the ATXN3 gene. The putative segregation ratio distortion (SRD) of alleles can play an important role in the non-Mendelian behaviour of triplet repeat loci. To study the stability and infer the segregation patterns of wild-type MJD alleles, the size of the (CAG)(n) tract was analysed in 102 normal sibships, representing 428 meioses. No mutational events were detected during the transmission of alleles. Segregation analysis showed that the smaller alleles were preferentially transmitted (56.9%). Considering maternal meioses alone, such preference was still detected (55.7%) but without statistical significance. A positive correlation was observed for the difference in length between the two alleles constituting the transmitters' genotype (D) and the frequency of transmission of the smaller alleles. The results suggest that small D values are not enough to modify the probability of allele transmission. When transmissions involving genotypes with D <or= 2 were excluded, SRD in favour of the smaller allele became significant for both maternal and paternal transmissions. Therefore, the genotypic composition of the transmitters in a sample to be analysed should influence the ability to detect SRD, acting as a confounding factor.