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Polymorphic human specific Alu insertions as markers for human identification.

Alu sequences represent the largest family of short interspersed repetitive elements (SINEs) in humans with 500 000 copies per genome. Recently, one Alu subfamily was found to be human specific (HS). We originally described the use of polymorphis HS Alu insertions as a tool in population studies and recently as tools in DNA fingerprinting and forensic analysis. In this report, we will use this simple polymerase chain reaction (PCR) base technique for the detection of HS Alu insertion polymorphisms. We will test the resolving power of this DNA profiling approach in both population genetics and paternity assessment. At the population level, we will describe the genotypic distribution of five polymorphic Alu insertions among 3 populations from the American continent, one of African origin, the other two Amerindians. Insight into their relationships will be provided. At the family level, we will examine one European American family of seven individuals and the same pedigree will also be characterized by way of the two systems currently and widely used to ascertain paternity: PCR-sequence specific oligonucleotide probe hybridization (PCR-SSO) and PCR-restriction fragment length polymorphism (PCR-RFLP) of human leucocyte antigen (HLA) class II molecules, and a standard RFLP protocol used in forensic casework and paternity studies. The importance and strengths of the methods as well as its perspectives for future use in filiation studies will be evaluated.

Forensic Medicine↗

Flanking sequences of an Alu source stimulate transcription in vitro by interacting with sequence-specific transcription factors.

An Alu source gene, called the EPL Alu, was previously isolated by a phylogenetic strategy. Sequences flanking the EPL Alu family member stimulate its RNA polymerase III (Pol III) template activity in vitro. One cis-acting element maps within a 40-nucleotide region immediately upstream to the EPL Alu. This same region contains an Ap1 site which, when mutated, abolishes the transcriptional stimulation provided by this region. The flanking sequence, as assayed by gel mobility shift, forms sequence-specific complexes with several nuclear factors including Ap1. These results demonstrate that an an ancestral Alu source sequence fortuitously acquired positive transcriptional control elements by insertion into the EPL locus, thereby providing biochemical evidence for a model which explains the selective amplification of Alu subfamilies.

Base Sequence↗

RNA polymerase III transcripts and the PTB protein are essential for the integrity of the perinucleolar compartment.

The perinucleolar compartment (PNC) is a nuclear substructure present in transformed cells. The PNC is defined by high concentrations of certain RNA binding proteins and a subset of small RNAs transcribed by RNA polymerase III (pol III), including the signal recognition particle RNA and an Alu RNA as reported here. To determine if the PNC is dependent on pol III transcription, HeLa cells were microinjected with the selective pol III inhibitor, Tagetin. This resulted in disassembly of the PNC, whereas inhibition of pol I by cycloheximide or pol II by alpha-amanitin did not significantly affect the PNC. However, overexpression of one of the PNC-associated RNAs from a pol II promoter followed by injection of Tagetin blocked the Tagetin-induced PNC disassembly, demonstrating that it is the RNA rather than pol III activity that is important for the PNC integrity. To elucidate the role of the PNC-associated protein PTB, its synthesis was inhibited by siRNA. This resulted in a reduction of the number of PNC-containing cells and the PNC size. Together, these findings suggest, as a working model, that PNCs may be involved in the metabolism of specific pol III transcripts in the transformed state and that PTB is one of the key elements mediating this process.

Alu Elements↗

Involvement of Alu sequences in the cell-specific regulation of transcription of the gamma chain of Fc and T cell receptors.

The Fc epsilon RI-gamma chains are expressed in a variety of hematopoietic cells where they play a critical role in signal transduction. They are part of the high affinity IgE receptor in mast cells, basophils, Langerhans cells, and possibly other cells; a component of the low affinity receptor for IgG (Fc gamma RIIIA or CD16) in natural killer cells and macrophages; and part of the T cell antigen receptor in subsets of T cells. Here we have investigated the transcriptional regulation of the gamma chain gene by analyzing the 2.5-kilobase sequence upstream of the transcription start site. This sequence contains a promoter specific to cells of hematopoietic lineage. However, the tissue specificity of this promoter is only partial because it is active in all of the hematopoietic cells tested here, regardless of whether they constitutively express Fc epsilon RI- gamma chain transcripts. We have identified two adjacent cis-acting regulatory elements, both of which are part of an Alu repeat. The first (-445/-366) is a positive element active in both basophils and T cells. The second (-365/-264) binds to nuclear factors, which appear to be different in basophils and T cells, and acts as a negative element in basophils and as a positive one in T cells. Thus, this Alu repeat (90% identical to Alu consensus sequences) has evolved to become both a positive and negative regulator.

Amino Acid Sequence↗

DNA sequences complementary to human 7 SK RNA show structural similarities to the short mobile elements of the mammalian genome.

A complementary DNA clone of 7 SK RNA from HeLa cells was used to study the genomic organization of 7 SK sequences in the human genome. Genomic hybridizations and genomic clones show that 7 SK is homologous to a family of disperse repeated sequences most of which lack the 3' end of the 7 SK RNA sequence. Only few of the genomic K sequences are homologous to both 3' and 5' 7 SK probes and presumably include the gene(s) for 7 SK RNA. The sequence of four genomic 7 SK clones confirms that they are in most cases pseudogenes. Although Alu sequences are frequently found near the 3' and 5' end of K DNA, the sequences immediately flanking the pseudogenes are different in all clones studied. However, direct repeats were found flanking directly the K DNA or the K-Alu unit, suggesting that the K sequences alone or in conjunction with Alu DNA might constitute a mobile element.

Base Sequence↗

Ataxia-telangiectasia locus: sequence analysis of 184 kb of human genomic DNA containing the entire ATM gene.

Ataxia-telangiectasia (A-T) is an autosomal recessive disorder involving cerebellar degeneration, immunodeficiency, chromosomal instability, radiosensitivity, and cancer predisposition. The genomic organization of the A-T gene, designated ATM, was established recently. To date, more than 100 A-T-associated mutations have been reported in the ATM gene that do not support the existence of one or several mutational hotspots. To allow genotype/phenotype correlations it will be important to find additional ATM mutations. The nature and location of the mutations will also provide insights into the molecular processes that underly the disease. To facilitate the search for ATM mutations and to establish the basis for the identification of transcriptional regulatory elements, we have sequenced and report here 184,490 bp of genomic sequence from the human 11q22-23 chromosomal region containing the entire ATM gene, spanning 146 kb, and 10 kb of the 5'-region of an adjacent gene named E14/NPAT. The latter shares a bidirectional promoter with ATM and is transcribed in the opposite direction. The entire region is transcribed to approximately 85% and translated to 5%. Genome-wide repeats were found to constitute 37.2%, with LINE (17.1%) and Alu (14.6%) being the main repetitive elements. The high representation of LINE repeats is attributable to the presence of three full-length LINE-1s, inserted in the same orientation in introns 18 and 63 as well as downstream of the ATM gene. Homology searches suggest that ATM exon 2 could have derived from a mammalian interspersed repeat (MIR). Promoter recognition algorithms identified divergent promoter elements within the CpG island, which lies between the ATM and E14/NPAT genes, and provide evidence for a putative second ATM promoter located within intron 3, immediately upstream of the first coding exon. The low G+C level (38.1%) of the ATM locus is reflected in a strongly biased codon and amino acid usage of the gene.

Ataxia Telangiectasia↗

African and Levantine origins of Pakistani YAP+ Y chromosomes.

We surveyed 9 Pakistani subpopulations for variation on the nonrecombining portion of the Y chromosome. The polymorphic systems examined were the Y-chromosome Alu insertion polymorphism (YAP) at DYS287, 5 single nucleotide polymorphisms, and the tetranucleotide microsatellite DYS19. Y chromosomes carrying the YAP element (YAP+) were found in populations from southwestern Pakistan at frequencies ranging from 2% to 8%, whereas northeastern populations appeared to lack YAP+ chromosomes. In contrast to other South Asian populations, several Pakistani subpopulations had a high frequency of the DYS19*B allele, the most frequent allele in West Asian, North African, and European populations. The combination of alleles at all polymorphic sites gave rise to 9 YAP-DYS19 combination haplotypes in Pakistani populations, including YAP+ haplotypes 4-A, 4-B, 5-C, and 5-E. We hypothesize that the geographic distributions of YAP+ haplotypes 4 and 5 trace separate migratory routes to Pakistan: YAP+ haplotype 5 may have entered Pakistan from the Arabian Peninsula by means of migrations across the Gulf of Oman, whereas males possessing YAP+ haplotype 4 may have traveled over land from the Middle East. These inferences are consistent with ethnohistorical data suggesting that Pakistan's ethnic groups have been influenced by migrations from both African and Levantine source populations.

Africa↗

Organization and nucleotide sequence of the rat T cell receptor beta-chain complex.

We have characterized four overlapping genomic clones containing the DA rat TCR C beta complex, which span a total of 23 kb and bear two closely related complexes of gene segments. The D beta 1-J beta 1-C beta 1 and the D beta 2-J beta 2-C beta 2 complexes each contain a single diversity segment, six joining segments and four exons that encode the C region. All gene segments appear to be functional except J beta 2.5, which has a 5-bp frame-shifting deletion. This organizational pattern is identical to that of the mouse, and the homologous rat and mouse coding regions share about 92% nucleotide sequence identity. Our sequence comparisons indicate that a localized gene correction event has homogenized the sequences of the first exons of C beta 1 and C beta 2 in the evolutionary time since rats and mice became separate species. We have identified three repetitive elements, each flanked by short direct repeats, present in the region "brain-specific" identifier (ID) sequences, another is a truncated member of the LINE I class of repetitive elements, and the third is a member of the Alu type 2 family. The insertion of at least two, and probably all, of these elements has occurred since the time of rat/mouse divergence. We have identified a substantial number of "cryptic" rearrangement signals (heptamer/nonamer) in the C beta locus, which match the consensus sequence as well or better than authentic signals, and may represent sites of nonfunctional rearrangements.

Animals↗

Specific hypermethylation of LINE-1 elements during abnormal overgrowth and differentiation of human placenta.

In human post-natal somatic cells, low global levels of DNA methylation have been associated with the hypomethylation of several repetitive elements, a feature that has been proposed to be a surrogate epigenetic marker. These data, mainly derived from the analysis of cancer cells, suggest a potential association between loss of cell-growth control and altered differentiation with hypomethylation of repetitive sequences. Partial hydatidiform moles (PHMs) can be used as an alternative model for investigating this association in a non-tumorigenic context. This gestational disease is characterized by abnormal overgrowth and differentiation of the placenta and spontaneous abortion. Here, we comprehensively analyse the DNA methylation of these trophoblastic tissues in both PHM and normal placenta at global and sequence-specific levels. Analysis of the global 5-methylcytosine content and immunohistochemistry indicate that PHM and normal placenta have identical global levels of DNA methylation. In contrast, bisulfite genomic sequencing shows that, whereas Alu, NBL2 and satellite 2 repetitive elements are equally methylated, LINE-1 sequences are hypermethylated in PHM tissues ( approximately 2-fold relative to normal placenta). Interestingly, altered demethylation is also found in triploid diandric embryos that originate from dispermic fertilization of an oocyte, a common event responsible for most PHMs. In conclusion, alterations of DNA methylation do not seem to be randomly distributed in PHM, as several repeated elements remain unaltered, whereas LINE-1 sequences are hypermethylated. In addition, our findings suggest that the hypomethylation of repetitive elements in cancer is directly linked to the neoplasic process and not a simple consequence of loss of growth control observed in most of the cancer cells.

Cell Differentiation↗

The biased distribution of Alus in human isochores might be driven by recombination.

Alu retrotransposons do not show a homogeneous distribution over the human genome but have a higher density in GC-rich (H) than in AT-rich (L) isochores. However, since they preferentially insert into the L isochores, the question arises: What is the evolutionary mechanism that shifts the Alu density maximum from L to H isochores? To disclose the role played by each of the potential mechanisms involved in such biased distribution, we carried out a genome-wide analysis of the density of the Alus as a function of their evolutionary age, isochore membership, and intron vs. intergene location. Since Alus depend on the retrotransposase encoded by the LINE1 elements, we also studied the distribution of LINE1 to provide a complete evolutionary scenario. We consecutively check, and discard, the contributions of the Alu/LINE1 competition for retrotransposase, compositional matching pressure, and Alu overrepresentation in introns. In analyzing the role played by unequal recombination, we scan the genome for Alu trimers, a direct product of Alu-Alu recombination. Through computer simulations, we show that such trimers are much more frequent than expected, the observed/expected ratio being higher in L than in H isochores. This result, together with the known higher selective disadvantage of recombination products in H isochores, points to Alu-Alu recombination as the main agent provoking the density shift of Alus toward the GC-rich parts of the genome. Two independent pieces of evidence-the lower evolutionary divergence shown by recently inserted Alu subfamilies and the higher frequency of old stand-alone Alus in L isochores-support such a conclusion. Other evolutionary factors, such as population bottlenecks during primate speciation, may have accelerated the fast accumulation of Alus in GC-rich isochores.

Algorithms↗

Inference of human geographic origins using Alu insertion polymorphisms.

The inference of an individual's geographic ancestry or origin can be critical in narrowing the field of potential suspects in a criminal investigation. Most current technologies rely on single nucleotide polymorphism (SNP) genotypes to accomplish this task. However, SNPs can introduce homoplasy into an analysis since they can be identical-by-state. We introduce the use of insertion polymorphisms based on short interspersed elements (SINEs) as a potential alternative to SNPs. SINE polymorphisms are identical-by-descent, essentially homoplasy-free, and inexpensive to genotype using a variety of approaches. Herein, we present results of a blind study using 100 Alu insertion polymorphisms to infer the geographic ancestry of 18 unknown individuals from a variety of geographic locations. Using a Structure analysis of the Alu insertion polymorphism-based genotypes, we were able to correctly infer the geographic affiliation of all 18 unknown human individuals with high levels of confidence. This technique to infer the geographic affiliation of unknown human DNA samples will be a useful tool in forensic genomics.

Alu Elements↗

Short interspersed transposable elements (SINEs) are excluded from imprinted regions in the human genome.

To test whether regions undergoing genomic imprinting have unique genomic characteristics, imprinted and nonimprinted human loci were compared for nucleotide and retroelement composition. Maternally and paternally expressed subgroups of imprinted genes were found to differ in terms of guanine and cytosine, CpG, and retroelement content, indicating a segregation into distinct genomic compartments. Imprinted regions have been normally permissive to L1 long interspersed transposable element retroposition during mammalian evolution but universally and significantly lack short interspersed transposable elements (SINEs). The primate-specific Alu SINEs, as well as the more ancient mammalian-wide interspersed repeat SINEs, are found at significantly low densities in imprinted regions. The latter paleogenomic signature indicates that the sequence characteristics of currently imprinted regions existed before the mammalian radiation. Transitions from imprinted to nonimprinted genomic regions in cis are characterized by a sharp inflection in SINE content, demonstrating that this genomic characteristic can help predict the presence and extent of regions undergoing imprinting. During primate evolution, SINE accumulation in imprinted regions occurred at a decreased rate compared with control loci. The constraint on SINE accumulation in imprinted regions may be mediated by an active selection process. This selection could be because of SINEs attracting and spreading methylation, as has been found at other loci. Methylation-induced silencing could lead to deleterious consequences at imprinted loci, where inactivation of one allele is already established, and expression is often essential for embryonic growth and survival.

CpG Islands↗

Clustering, duplication and chromosomal distribution of mouse SINE retrotransposons.

We analyzed potential mechanisms determining chromosomal distributions of the mouse B1 and B2 non-LTR retrotransposons, also known as SINE elements. We report that young B1 and B2 SINEs are underrepresented on chromosome X relative to autosomes, which is consistent with their integration in male germ lines. As the age of the SINE elements progresses, their densities on chromosome X increase relative to autosomal densities, possibly due to differences in ectopic recombination rates between chromosome X and autosomes. Furthermore, unlike young human Alus that tend to be integrated outside Alu-dense regions, young B1 and B2 elements are found mostly in SINE-rich clusters. The B1- or B2-rich clusters are more likely to contain duplicated elements than B1- or B2-poor chromosomal regions. We also present evidence indicating potential association of B1 and B2 elements with intra-chromosomal segmental duplications. No such association was found with inter-chromosomal duplications. We propose that the accumulation of mouse SINE elements observed in GC-rich regions may be due to the excess of DNA duplications over deletions in gene-rich regions that tend to be GC rich.

Animals↗

Hypermethylation of CpG island loci and hypomethylation of LINE-1 and Alu repeats in prostate adenocarcinoma and their relationship to clinicopathological features.

Promoter CpG island hypermethylation is an important carcinogenic event in prostate adenocarcinoma. Regardless of tissue type, human cancers have in common both focal CpG island hypermethylation and global genomic hypomethylation. The present study evaluated CpG island loci hypermethylation and LINE-1 and Alu repeat hypomethylation in prostate adenocarcinoma, analysed the relationship between them, and correlated these findings with clinicopathological features. We examined 179 cases of prostate adenocarcinoma and 30 cases of benign prostate hypertrophy for the methylation status of 22 CpG island loci and the methylation levels of LINE-1 and Alu repeats using methylation-specific polymerase chain reaction and combined bisulphite restriction analysis, respectively. The following 16 CpG island loci were found to display cancer-related hypermethylation: RASSF1A, GSTP1, RARB, TNFRSF10C, APC, BCL2, MDR1, ASC, TIG1, RBP1, COX2, THBS1, TNFRSF10D, CD44, p16, and RUNX3. Except for the last four CpG island loci, hypermethylation of each of the remaining 12 CpG island loci displayed a close association with one or more of the prognostic parameters (ie preoperative serum prostate specific antigen level, Gleason score sum, and clinical stage). Prostate adenocarcinoma with hypermethylation of each of ASC, COX2, RARB, TNFRSF10C, MDR1, TIG1, RBP1, NEUROG1, RASSF1A, and GSTP1 showed a significantly lower methylation level of Alu or LINE-1 than prostate adenocarcinoma without hypermethylation. In addition, hypomethylation of Alu or LINE-1 was closely associated with one or more of the above prognostic parameters. These data suggest that in tumour progression a close relationship exists between CpG island hypermethylation and the hypomethylation of repetitive elements, and that CpG island hypermethylation and DNA hypomethylation contribute to cancer progression.

Adenocarcinoma↗

Growth hormone locus expands and diverges after the separation of New and Old World Monkeys.

While most mammals including the prosimians have a single copy of the growth hormone (GH) gene, anthropoids possess a cluster of GH-related genes. Throughout the evolution of the main anthropoid groups [New World Monkeys (NWM), Old World Monkeys (OWM), and apes], two features stand out of the GH loci. The first is the appearance of chorionic somatommamotropin hormone (CSH) genes within the OWM lineage and the second is the expansion of the loci intergenic regions in the OWM and apes. In relation with this loci expansion, the NWM possess intergenic regions of homogeneous lengths (3.5 kb). In contrast, heterogeneous lengths (6 and 13 kb) have been reported for species of the OWM. At the present, none of the OWM genomic GH loci organizations have been described. Here, we report the genomic organization of the GH locus in the rhesus monkey, this locus has six GH-related genes separated by five intergenic regions. The 5' end gene (GH-1) encodes for the pituitary GH and is followed by CSH-1, GH-2, CSH-2, CSH-3 and CSH-4 genes. The five intergenic regions have heterogeneous lengths and also present more or less the same Alu distribution as the human GH locus. To analyze the events that contributed to the extension of the intergenic regions of the GH locus and the emergence of the regulatory elements, the five GH locus intergenic regions of the spider monkey (NWM) were sequenced. The results of comparing the loci from both species suggest that the long intergenic regions (13 kb) of the rhesus GH locus share a common ancestor with the 3.5 kb intergenic regions of the spider monkey. However, the observed increased length of the former is due to an insertion (approximately 8.7 kb) at their 3' end. Interestingly in this insert, we discovered a DNA element resembling the enhancer of the CSH genes of the human GH locus. On the other hand, we observed that the short intergenic regions (6 kb) increased by a different recombination event.

Alu Elements↗

Does SINE evolution preclude Alu function?

The evolution, mobility and deleterious genetic effects of human Alus are fairly well understood. The complexity of regulated transcriptional expression of Alus is becoming apparent and insight into the mechanism of retrotransposition is emerging. Unresolved questions concern why mobile, highly repetitive short interspersed elements (SINEs) have been tolerated throughout evolution and why and how families of such sequences are periodically replaced. Either certain SINEs are more successful genomic parasites or positive selection drives their relative success and genomic maintenance. A complete understanding of the evolutionary dynamics and significance of SINEs requires determining whether or not they have a function(s). Recent evidence suggests two possibilities, one concerning DNA and the other RNA. Dispersed Alus exhibit remarkable tissue-specific differences in the level of their 5-methylcytosine content. Differences in Alu methylation in the male and female germlines suggest that Alu DNA may be involved in either the unique chromatin organization of sperm or signaling events in the early embryo. Alu RNA is increased by cellular insults and stimulates protein synthesis by inhibiting PKR, the eIF2 kinase that is regulated by double-stranded RNA. PKR serves other roles potentially linking Alu RNA to a variety of vital cell functions. Since Alus have appeared only recently within the primate lineage, this proposal provokes the challenging question of how Alu RNA could have possibly assumed a significant role in cell physiology.

Alu Elements↗

Superimposing polymorphism: the case of a point mutation within a polymorphic Alu insertion.

The COL3A1 Alu insertion is a member of the AluY subfamily. It has been found to be absent in non-human primates and polymorphic in worldwide human populations. The integration of the element into the human genome seems to have preceded the initial migration(s) of anatomically modern humans out of the African continent. Although the insertion has been detected in populations from all the continents, its highest frequency values are located within sub-Saharan Africa. The sequence alignment of the COL3A1 insertion from several African individuals revealed a bi-allelic single nucleotide polymorphism (SNP) at the downstream terminus of the element's poly-A tract. Once discovered, a selective PCR procedure was designed to determine the frequency of both alleles in 19 worldwide populations. The A-allele in this binary SNP experiences a clinal increase in the eastward direction from Africa to Southeast Asia and Mongolia, reaching fixation in the two latter regions. The T variant, on the other hand, exhibits a westward clinal increase outside of Africa, with its lowest frequency in Asia and achieving fixation in northern Europe. The presence of this internal SNP extends the usefulness provided by the polymorphic Alu insertion (PAI). It is possible that superimposing polymorphisms like this one found in the COL3A1 locus may accentuate signals from genetic drift events allowing for visualization of recent dispersal patterns.

Africa↗

The human Alu SINE sequences--is there a role for selection in their evolution?

The Alu sequence is a SINE (Short INterspersed Element) that is abundant in the human genome. A new analysis (1) reveals an unexpected conservation of some bases in the DNA sequence of the element. The bases involved include those forming an RNA polymerase III promoter. An unresolved question is whether this conservation results from selection for transposability. This, in turn, is related to the larger question of the evolutionary relationship between members of the Alu sequence family.

Consensus Sequence↗