Unique organization of the human BCR gene promoter

MXD1 regulates the imatinib resistance of chronic myeloid leukemia cells by repressing BCR-ABL1 expression

Tyrosine kinase inhibitors have achieved unprecedented efficacy in the treatment of chronic myeloid leukemia (CML); however, imatinib resistance has emerged as a major problem in the clinic. Because the overexpression of BCR-ABL1 critically contributes to CML pathogenesis and drug resistance, targeting the regulation of BCR-ABL1 gene expression may be an alternative therapeutic strategy. In this study, we found that the transcriptional repressor MXD1 showed low expression in CML patients and was negatively correlated with BCR-ABL1. Overexpression of MXD1 markedly inhibited the proliferation of K562 cells and sensitized the imatinib-resistant K562/G01 cell line to imatinib, with decreased BCR-ABL1 mRNA and protein expression. Further investigation using reporter gene analysis showed that MXD1 significantly inhibited the transcriptional activity of the BCR-ABL1 gene promoter. Taken together, these data show that MXD1 functions as a negative regulator of BCR-ABL1 expression and subsequently inhibits proliferation and sensitizes CML cells to imatinib treatment.

BCR/ABL1 and BCR are under the transcriptional control of the MYC oncogene

Background

Chronic Myeloid Leukaemia (CML) is caused by the BCR/ABL1 fusion gene. Both the presence and the levels of BCR/ABL1 expression seem to be critical for CML progression from chronic phase (CP) to blast crisis (BC). After the oncogenic translocation, the BCR/ABL1 gene is under the transcriptional control of BCR promoter but the molecular mechanisms involved in the regulation of oncogene expression are mostly unknown.

Methods

A region of 1443bp of the functional BCR promoter was studied for transcription factor binding sites through in-silico analysis and Chromatin Immunoprecipitation experiments. BCR and BCR/ABL1 expression levels were analysed in CML cell lines after over-expression or silencing of MYC transcription factor. A luciferase reporter assay was used to confirm its activity on BCR promoter.

Results

In the present study we demonstrate that MYC and its partner MAX bind to the BCR promoter, leading to up-regulation of BCR and BCR/ABL1 at both transcriptional and protein levels. Accordingly, silencing of MYC expression in various BCR/ABL1 positive cell lines causes significant downregulation of BCR and BCR/ABL1, which consequently leads to decreased proliferation and induction of cell death.

Conclusions

Here we describe a regulatory pathway modulating BCR and BCR/ABL1 expression, showing that the BCR promoter is under the transcriptional control of the MYC/MAX heterodimer. Since MYC is frequently over-expressed in BC, this phenomenon could play a critical role in BCR/ABL1 up-regulation and blast aggressiveness acquired during CML evolution.

CGGBP1--an indispensable protein with ubiquitous cytoprotective functions

The human genome contains multiple stretches of CGG trinucleotide repeats, which act as transcription- and translation-regulatory elements but at the same time form secondary structures that impede replication and give rise to sites of chromosome fragility. Proteins binding to such DNA elements may be involved in divergent cellular processes such as transcription, DNA damage, and epigenetic state of the chromatin. We review here the work done on CGG repeats and associated proteins with special focus on a factor called CGGBP1. CGGBP1 presents with an interesting example of factors that do not have any single dedicated function, but participate indispensably in multiple processes. Both experimental results and data from cancer genome sequencing have revealed that any alteration in CGGBP1 that compromises its function is not tolerated by normal or cancer cells alike. Based upon a large amount of published data, information from databases, and unpublished results, we decipher in this review how CGGBP1 is a classic example of the 'one factor, divergent functions' paradigm of cytoprotection. By taking cues from the studies on CGGBP1, more such factors can be discovered for a better understanding of the evolution of mechanisms of cellular survival.

BCR and BCR-ABL regulation during myeloid differentiation in healthy donors and in chronic phase/blast crisis CML patients

Chronic myeloid leukemia (CML) is caused by the BCR-ABL hybrid gene. The molecular mechanisms leading from chronic phase (CP) to blast crisis (BC) are not understood. However, both the presence and the levels of BCR-ABL seem to be important for CML progression. BCR-ABL is under the transcriptional control of BCR promoter. Here we focused on the gene expression control of BCR and BCR-ABL upon myeloid differentiation in healthy donors (HDs), CP and BC patients. As previously reported, BCR-ABL is downregulated during myeloid maturation in CP patients. A similar pattern was detected for BCR (but not for ABL) in CP-CML and in HD, thus suggesting that the two genes may be under a similar transcriptional control. In BC this mechanism is similarly impaired for both BCR-ABL and BCR. These data indicate the presence of an 'in trans' deregulated transcription of both BCR and BCR-ABL promoters, associated with CML progression.

TATA box polymorphisms in human gene promoters and associated hereditary pathologies

TATA-binding protein (TBP) is the first basal factor that recognizes and binds a TATA box on TATA-containing gene promoters transcribed by RNA polymerase II. Data available in the literature are indicative of admissible variability of the TATA box. The TATA box flanking sequences can influence TBP affinity as well as the level of basal and activated transcription. The possibility of mediated involvement in in vivo gene expression regulation of the TBP interactions with variant TATA boxes is supported by data on TATA box polymorphisms and associated human hereditary pathologies. A table containing data on TATA element polymorphisms in human gene promoters (about 40 mutations have been described), associated with particular pathologies, their short functional characteristics, and manifestation mechanisms of TATA-box SNPs is presented. Four classes of polymorphisms are considered: TATA box polymorphisms that weaken and enhance promoter, polymorphisms causing TATA box emergence and disappearance, and human virus TATA box polymorphisms. The described examples are indicative of the polymorphism-associated severe pathologies like thalassemia, the increased risk of hepatocellular carcinoma, sensitivity to H. pylori infection, oral cavity and lung cancers, arterial hypertension, etc.

P210 Bcr-abl tyrosine kinase interaction with histone deacetylase 1 modifies histone H4 acetylation and chromatin structure of chronic myeloid leukaemia haematopoietic progenitors

The BCR-ABL fusion gene, originating from the balanced (9;22) translocation, is the molecular hallmark and the causative event of chronic myeloid leukaemia (CML). The interactions of its p210 protein constitutively activated and improperly confined to the cytoplasm with multiple regulatory signals of cell cycle progression, apoptosis and self-renewal, induce the illegitimate enlargement of clonal haematopoiesis and genetic instability that drives its progression towards the fully transformed phenotype of blast crisis. However, its effects on the basic transcription machinery and chromatin remodelling are unknown. Our study underscored histone H4 hyperacetylation associated with p210 tyrosine kinase in vitro and in vivo and its role in BCR-ABL transcription. Histone H4 hyperacetylation proceeds, at least partly, from the 'loss of function' of histone deacetylase 1 protein, a critical component of Rb-mediated transcriptional repression, in consequence of its cytoplasmatic compartmentalisation.

Targeting neighbouring poly(purine.pyrimidine) sequences located in the human bcr promoter by triplex-forming oligonucleotides

Most poly(purine.pyrimidine) [poly(R.Y)] sequences in eukaryotic genomes are interrupted by one or more base pair inversions. When the inversions are centrally located, the poly(R.Y) sequences can be regarded as the sum of two abutting sites, each potentially capable of forming a triple helix. Employing band-shift, footprinting and modeling methods we examined the formation of triple helices at a critical 27 bp poly(R.Y) sequence interrupted by two adjacent CG inversions, and located in the promoter of the human bcr gene at transcription initiation. We designed several 13-mer and 14-mer triplex-forming oligonucleotides (TFOs) capable of binding the bcr abutting sites, thereby generating different base juxtapositions at the triple helical junction, to examine whether triplex formation occurs in a cooperative manner. It is found that in 50 mM Tris/HCl, pH 7.4, 10 mM MgCl2, 2 mM spermine, 37 degrees C, the 13-and the 14-mer TFOs bind to one half of the bcr site with Delta G between -30 and -35 kJ x mol-1. However, when different 13-mer/14-mer combinations of TFOs were directed against the abutting poly(R x Y) sites, triplex formation has been found to be enhanced only for the triple helical junction formed by the 5'-A-T-3' base juxtaposition, in keeping with a partial stacking suggested from modeling analysis. On the other hand, a longer 24-mer TFO, binding noncooperatively to the same abutting sites, forms a much more stable triplex (Delta G = -51 kJ x mol-1), notwithstanding the two T x CG triads in the middle. Modeling investigations reveal that there is no continuity or propagation of base stacking involving adjacent bases of the third strand at the site of base inversion as well as on the 5' side. The data indicate that the entropy penalty of forming a triplex with two oligonucleotides is much higher than the energy gained from base stacking interactions at the triplex junction formed between the two TFOs.

Selection and characterization of BCR-ABL positive cell lines with differential sensitivity to the tyrosine kinase inhibitor STI571: diverse mechanisms of resistance

Targeting the tyrosine kinase activity of Bcr-Abl with STI571 is an attractive therapeutic strategy in chronic myelogenous leukemia (CML). A few CML cell lines and primary progenitors are, however, resistant to this compound. We investigated the mechanism of this resistance in clones of the murine BaF/3 cells transfected with BCR-ABL and in 4 human cell lines from which sensitive (s) and resistant (r) clones were generated by various methods. Although the resistant cells were able to survive in the presence of STI571, their proliferation was approximately 30% lower than that of their sensitive counterparts in the absence of the compound. The concentration of STI571 needed for a 50% reduction in viable cells after a 3-day exposure was on average 10 times higher in the resistant (2-3 μmol/L) than in the sensitive (0.2-0.25 μmol/L) clones. The mechanism of resistance to STI571 varied among the cell lines. Thus, in Baf/BCR-ABL-r, LAMA84-r, and AR230-r, there was up-regulation of the Bcr-Abl protein associated with amplification of the BCR-ABL gene. In K562-r, there was no Bcr-Abl overexpression, but the IC50 for the inhibition of Bcr-Abl autophosphorylation was increased in the resistant clones. Sequencing of the Abl kinase domain revealed no mutations. The multidrug resistance P-glycoprotein (Pgp) was overexpressed in LAMA84-r, indicating that at least 2 mechanisms of resistance operate in this cell line. KCL22-r showed neither Bcr-Abl up-regulation nor a higher threshold for tyrosine kinase inhibition by STI571. We conclude that BCR-ABL–positive cells can evade the inhibitory effect of STI571 by different mechanisms, such as Bcr-Abl overexpression, reduced intake mediated by Pgp, and, possibly, acquisition of compensatory mutations in genes other than BCR-ABL.

Selection and characterization of BCR-ABL positive cell lines with differential sensitivity to the tyrosine kinase inhibitor STI571: diverse mechanisms of resistance

Targeting the tyrosine kinase activity of Bcr-Abl with STI571 is an attractive therapeutic strategy in chronic myelogenous leukemia (CML). A few CML cell lines and primary progenitors are, however, resistant to this compound. We investigated the mechanism of this resistance in clones of the murine BaF/3 cells transfected with BCR-ABL and in 4 human cell lines from which sensitive (s) and resistant (r) clones were generated by various methods. Although the resistant cells were able to survive in the presence of STI571, their proliferation was approximately 30% lower than that of their sensitive counterparts in the absence of the compound. The concentration of STI571 needed for a 50% reduction in viable cells after a 3-day exposure was on average 10 times higher in the resistant (2-3 μmol/L) than in the sensitive (0.2-0.25 μmol/L) clones. The mechanism of resistance to STI571 varied among the cell lines. Thus, in Baf/BCR-ABL-r, LAMA84-r, and AR230-r, there was up-regulation of the Bcr-Abl protein associated with amplification of the BCR-ABL gene. In K562-r, there was no Bcr-Abl overexpression, but the IC50 for the inhibition of Bcr-Abl autophosphorylation was increased in the resistant clones. Sequencing of the Abl kinase domain revealed no mutations. The multidrug resistance P-glycoprotein (Pgp) was overexpressed in LAMA84-r, indicating that at least 2 mechanisms of resistance operate in this cell line. KCL22-r showed neither Bcr-Abl up-regulation nor a higher threshold for tyrosine kinase inhibition by STI571. We conclude that BCR-ABL–positive cells can evade the inhibitory effect of STI571 by different mechanisms, such as Bcr-Abl overexpression, reduced intake mediated by Pgp, and, possibly, acquisition of compensatory mutations in genes other than BCR-ABL.

Exon-skipping in BCR/ABL is induced by ABL exon 2

The BCR/ABL fusion gene is pathognomonic for chronic myelogenous leukaemia (CML). We have previously reported alternative splicing of BCR/ABL, as indicated by the detection of both p190- and p210-encoding transcripts, in about 60% of CML patient samples. These exon-skipping events involved the joining of ABL exon 2 to variable upstream BCR exons. Similarly, ABL exon 2 is alternatively spliced to either of two upstream ABL exons (1a or 1b) in c-ABL. We have constructed BCR and BCR/ABL minigenes to study this phenomenon in more detail. These constructs were transfected into various cell types and splicing was assessed by reverse transcriptase PCR. Whereas the basic BCR minigene expressed exon-inclusive transcripts only, insertion of genomic DNA spanning ABL exon 2 induced exon-skipping but only when expressed in the CML cell lines K562 and EM3. In this study we localized the required sequence element to ABL exon 2 itself. These results mimic the splicing phenotype displayed by most CML patients. We propose a model where a trans-factor present in some CML cells interacts with ABL exon 2 pre-mRNA to promote skipping of upstream BCR exons.

Formation of stable DNA triple helices within the human bcr promoter at a critical oligopurine target interrupted in the middle by two adjacent pyrimidines

Antigene strategies based on the use of triplex-forming oligonucleotides (TFO) as artificial repressors are constrained by the need for genomic targets with a polypurine-polypyrimidine [poly (R.Y)] DNA motif. In this study, we demonstrate that both A/G and G/T motif oligonucleotides recognize and bind strongly to a critical polypurine sequence interrupted in the middle by two adjacent cytosines and located in the promoter of the human bcr gene at the transcription initiation. The interaction between the designed TFO and this irregular poly (R.Y) target has been studied using a number of techniques, including electrophoretic mobility shift assay (EMSA), circular dichroism (CD), DNase I, and dimethyl sulfate (DMS) footprinting. Although CD shows that the 24-mer TFO self-aggregate in solution, they bind to the bcr target at 37 degrees C, forming stable triplexes that do not dissociate during electrophoretic runs performed up to 50 degrees C in 50 mM Tris-acetate, pH 7.4, 10 mM MgCl2, 50 mM NaCl (buffer A). We used EMSA to determine the equilibrium dissociation constants (Kd) for the reaction T <==> D + TFO at 37 degrees C, either in buffer A or in 50 mM Tris-acetate, pH 7.4, 10 mM MgCl2, 5 mM NaCl (buffer B). The triplexes were found to be more stable in buffer B, a behavior that can be rationalized in terms of monovalent and divalent cation competition for binding to DNA. Footprinting experiments showed that the TFO interact with the irregular poly (R.Y) target in a highly sequence-specific way and that the A/G motif oligonucleotide, juxtaposing T to the double CG inversions of the target, formed the most stable triplex (e.g., 1 microM TFO promoted strong footprints at 37 degrees C). These triplexes, except the one containing two A.C.G mismatched triads, are not destabilized under near physiologic conditions, that is, in 50 mM Tris-acetate, pH 7.4, 80 mM KCl, 20 mM NaCl, 2 mM spermidine. Moreover, we found that guanine N7 in T.C.G and guanine N7 in A.C.G are both accessible to DMS and that the first is less reactive than the second. In conclusion, the results of this study indicate that a critical sequence in the human ber promoter may be used as a potential binding site for TFO designed to repress artificially the transcription of the fused bcr/abl gene expressed in leukemia cells.

A composite DNA element in the promoter of the polymeric immunoglobulin receptor regulates its constitutive expression

The polymeric immunoglobulin receptor (pIgR), which is constitutively expressed on the basolateral surface of secretory epithelial cells, mediates external translocation of polymeric IgA and pentameric IgM (collectively called pIg) to exocrine secretions. A high level of synthesis must be maintained because the receptor is continuously cleaved to release bound secretory component (SC) in secretory IgA and secretory IgM, as well as free SC from unoccupied receptor. We have isolated the promoter of the pIgR gene and identified a short activating region that is required for the expression of pIgR promoter-driven reporter genes. This region contained an E-box and an inverted repeat sequence (IRS). Gel electrophoresis mobility shift assays with nuclear extracts from different pIgR-expressing epithelial cell lines demonstrated proteins that bind independently to both the E-box and the IRS sequence of the pIgR promoter. In addition, a DNA probe that contained both the E-box and the IRS gave rise to a larger complex that could not be competed by either element on its own. Binding was confirmed by DNase I footprinting of the E-box and IRS sequences with nuclear extracts, and by dimethyl sulfide footprinting in living HT-29 epithelial cells. Finally, a mutation in the pIgR promoter that inhibited protein binding to the E-box and the formation of the larger complex, abolished activated transcription from the reporter gene.

The chimeric BCR-ABL gene

The 1982 discovery that in chronic myeloid leukaemia (CML) the ABL proto-oncogene is translocated to the BCR gene located on chromosome 22 initiated many studies on the structural organization and function of these genes. The nucleotide sequence of the entire BCR and major parts of the ABL gene has now been determined. However, the actual cause of the fusion of BCR with ABL remains essentially unknown. Mouse models have been helpful to unravel the normal cellular function of BCR and ABL, as well the activity of BCR-ABL, although a single mechanism explaining the transforming activity of the latter has not been discovered. The cause of progression of the disease remains unknown, and no single genetic abnormality has been linked to the blast phase of CML. Much has been learned concerning the molecular biology of CML, but answers to the fundamental questions above may be expected in the coming years in parallel to increasing knowledge of genome structure, signal transduction and cell cycle control.

CAC--the neglected repeat

It is becoming increasingly clear that repetitive DNA is of biological significance as well as experimental importance. Here we review the information available about one type of repetitive DNA, the trinucleotide repeat (CAC)n, and briefly compare it with other trinucleotide repeats. Although much work has been done in analysing DNA fingerprinting patterns produced using the synthetic oligonucleotide (CAC)5 as a probe, there is relatively little information about individual (CAC)n-containing sequences and their abundance, organisation and distribution in mammalian DNA. From the data that is available, it is clear that there are at least two areas that should repay further study: (1) the organisation and generation of long sequences that contain (CAC)n motifs as part of a larger repeating unit (minisatellites) and (2) the distribution of small (CAC)n sequences (microsatellites), in particular their relationship to genes.

Transcriptional regulation of the neuronal L-type calcium channel alpha 1D subunit gene

1. The transcriptional regulation of the rat brain L-type calcium channel alpha 1D subunit (RB alpha 1D) gene was investigated using NG108-15 neuroblastoma-glioma cells. 2. Differentiation of NG108-15 cells in the presence of prostaglandin E1 or retinoic acid resulted in the appearance of mRNA encoding the RB alpha 1D subunit detected using Northern blot analysis. 3. A rat genomic DNA library was screened, and a 15.2-kb clone was isolated and partially sequenced which included part of the 5' upstream sequence through the initial part of intron 2 of the RB alpha 1D gene. 4. Deletion analysis, using a CAT reporter gene and transfected NG108-15 cells, revealed that the 1.2-kb 5'-upstream sequence from the RB alpha 1D gene contains cis-acting positive and negative regulatory elements. A deletion of the 3' end of exon 1 also suggested the presence of regulatory elements in the first exon. 5. DNase footprinting of exon 1 of the RB alpha 1D gene revealed two regions protected from digestion by specific protein binding, and the second region included an (ATG)7 trinucleotide repeat sequence. Electrophoretic mobility shift assays confirmed nuclear protein(s) binding to the (ATG)7 sequence. 6. The (ATG)7 sequence functions as a enhancer when linked to a thymidine kinase promoter and a CAT reporter gene. 7. These results provide the initial description of the transcriptional regulation of the RB alpha 1D gene and identify a novel enhancer that consists of an (ATG)7 trinucleotide repeat sequence.

Differential effects of simple repeating DNA sequences on gene expression from the SV40 early promoter

The influence of simple repeat sequences, cloned into different positions relative to the SV40 early promoter/enhancer, on the transient expression of the chloramphenicol acetyltransferase (CAT) gene was investigated. Insertion of (G)29.(C)29 in either orientation into the 5'-untranslated region of the CAT gene reduced expression in CV-1 cells 50-100 fold when compared with controls with random sequence inserts. Analysis of CAT-specific mRNA levels demonstrated that the effect was due to a reduction of CAT mRNA production rather than to posttranscriptional events. In contrast, insertion of the same insert in either orientation upstream of the promoter-enhancer or downstream of the gene stimulated gene expression 2-3-fold. These effects could be reversed by cotransfection of a competitor plasmid carrying (G)25.(C)25 sequences. The results suggest that a G.C-binding transcription factor modulates gene expression in this system and that promoter strength can be regulated by providing protein-binding sites in trans. Although constructs containing longer tracts of alternating (C-G), (T-G), or (A-T) sequences inhibited CAT expression when inserted in the 5'-untranslated region of the CAT gene, the amount of CAT mRNA was unaffected. Hence, these inhibitions must be due to posttranscriptional events, presumably at the level of translation. These effects of microsatellite sequences on gene expression are discussed with respect to recent data on related simple repeat sequences which cause several human genetic diseases.

Regional localization and developmental expression of the BCR gene in rodent brain

The BCR gene is implicated in the development of Ph-positive leukemia through its fusion with the nonreceptor tyrosine kinase gene ABL. The normal 160 kDa Bcr protein has several functional domains, and recently one specific role for Bcr was established in the regulation of respiratory burst activity in white blood cells. Bcr expression levels are relatively constant throughout mouse development until adulthood in brain and in hematopoietic tissues, a pattern that is distinctly different from that of the functionally related n-chimerin gene. In the present study, RNA in situ hybridization was used to explore the normal cellular function of Bcr in rodent brain and hematopoietic organs. The data pinpoint the high bcr expression in the brain to the hippocampal pyramidal cell layer and the dentate gyrus, and to the piriform cortex and the olfactory nuclei, reflecting a potentially interesting function for Bcr in these highly specialized brain regions.

Identification of a 59 bp enhancer located at the first exon/intron boundary of the human O6-methylguanine DNA methyltransferase gene

The DNA repair enzyme, O6-methylguanine DNA methyltransferase (MGMT) is responsible for repair of damage induced by alkylating agents that produce adducts at O6-guanine in DNA. Although the MGMT gene promoter has housekeeping gene promoter characteristics, unlike these genes which are expressed at a constant level, MGMT transcriptional activity varies between cell types. During an attempt to identify regions of the MGMT regulatory sequence sensitive to variations in transcription factors between cell types, we have identified a 59 bp enhancer which is required for efficient MGMT promoter function. This fragment produced increased transcriptional activity in reporter gene constructs containing either the MGMT or UMP-synthase promoter when transfected into either of two cell lines; it seems therefore that this enhancer may interact with relatively common trans-acting factors. Functional activity is only detected when the enhancer is in 'cis' with respect to the promoter, suggesting that complexes are formed between proteins bound to the enhancer and promoter sequences. We propose that the enhancer-binding protein may be a novel transcription factor since there are no obvious consensus sequences within the 59 bp sequence for known DNA-binding proteins.

Ph-positive leukemia: a transgenic mouse model

The presence of the BCR/ABL chimeric gene is the hallmark of defined types of human leukemia. To increase our knowledge of the oncogenic processes and to develop a model for this type of leukemia we generated a BCR/ABL (P190) transgenic mouse line. Over 95% of mice of this line die of leukemia or leukemia/lymphoma within 35-200 days of age. Karyotypically visible genetic alterations were absent from the early stages of BCR/ABL generated leukemia. A high frequency of aneuploidy was found in advanced leukemia indicating a primary and pivotal role for BCR/ABL in leukemogenesis. Moreover, the data suggest that BCR/ABL has a destabilizing effect on the regulation of the cell cycle. BCR/ABL expression was also found in tissues other than hematopoietic cells. However, this did not result in the development of solid tumors, strongly suggesting that the oncogenicity of BCR/ABL is limited to the hematopoietic lineage.

Human genes containing polymorphic trinucleotide repeats

Expansions of trinucleotide repeats within gene transcripts are responsible for fragile X syndrome, myotonic dystrophy and spinal and bulbar muscular atrophy. To identify other human genes with similar features as candidates for triplet repeat expansion mutations, we screened human cDNA libraries with repeat probes and searched databases for transcribed genes with repeats. From both strategies, 40 genes were identified and 14 characterized. Five were found to contain repeats which are highly polymorphic including the N-cadherin, BCR, glutathione-S-transferase and Na+/K+ ATPase (beta-subunit) genes. These data demonstrate the occurrence of other human loci which may undergo this novel mechanism of mutagenesis giving rise to genetic disease.

Fragile-X syndrome: unique genetics of the heritable unstable element

The fragile site at Xq27.3 is an unstable microsatellite repeat, p(CCG)n. In fragile-X syndrome pedigrees, this sequence exhibits variable amplification, the length of which correlates with fragile-site expression. There is a direct relationship between increased p(CCG)n copy number and propensity for instability: individuals having large amplifications exhibit somatic variation due to increased instability. The instability of the p(CCG)n repeat, when transmitted through affected pedigrees, explains the unusual segregation patterns of fragile-X phenotype, referred to as the Sherman paradox. All individuals of fragile-X genotype were found (where testing was possible) to have a parent with amplified p(CCG)n repeat, indicating that few, if any, cases of fragile-X syndrome are not familial.