To evaluate if the high Alu density observed in this genomic region had to be considered exceptional in the context of the whole genome, Alu density was calculated in non-overlapping 1 Mb windows on each chromosome. The portion of chromosome 1 including YTHDF2 (chr. 1: 28,000,000-28,999,999) resulted among the only four genomic regions of this length (or longer) with an Alu density above 40% (the others are chr. 7: 73,000,000-74,999,999, chr. 19: 10,000,000-10,999,999, and chr. 19: 17,000,000-17,999,999, on 7q11.23, 19p13.2 and 19p13.11 respectively). These four genomic regions with very high Alu density represent only 0.17% of the genome. The large majority (about two-thirds) of the genome has an Alu content at least four times lower below 12% (Figure 2).

The YTHDF2 gene codes for a protein of 579 amino acids (Swiss-Prot accession no. Q9Y5A9; synonyms: lymphocytic leukemia-associated antigen KW-14). A BLAST analysis conducted comparing the YTHDF2 mRNA reference sequence (GenBank accession no. NM_016258) with the human genome sequence revealed that the YTHDF2 gene spans about 32 kilobases and is composed of five exons.

Extensive analysis conducted on publicly available genomic databases and using sequence analysis tools and in particular a BLAST search (http://www.ncbi.nlm.nih.gov/BLAST) and Clustal W multialignment (http://www.ebi.ac.uk/clustalw) evidenced that YTHDF2 belongs to a gene family that also includes YTHDF1 on chromosome 20 and YTHDF3 on chromosome 8. In fact, the three genes share a structure composed of five exons of similar length and high similarity at cDNA and protein sequence level (data not shown). The search in the Protein Family Database showed the presence of a YTH domain (36) in the protein sequences (in position 457-547 on the YTHDF2 protein). Consequently, given that these three genes were previously lacking an official nomenclature, we proposed to the Human Genome Organization (HUGO) Gene Nomenclature Committee the above-reported gene names (YTHDF1, YTH domain family 1; YTHDF2, YTH domain family 2; and YTHDF3, YTH domain family 3) as official symbols and names, and this proposal was accepted.

A protein-translated sequence tblastn analysis (available at http://www.ncbi.nlm.nih.gov/BLAST) conducted on public databases collecting cDNA anda novel and interesting avenue for future research. The present study provides evidence that the YTHDF2 (TG)12-27 locus may be involved in human longevity, possibly through its role in mRNA processing and regulation. The association of homozygosity at this locus with longevity warrants further investigation, particularly in larger cohort studies and in different populations. The characterization of the molecular mechanisms underlying this association may provide new insights into the biology of aging and longevity.An intriguing hypothesis. In fact, the inverse correlation of longevity and reproductive success is a fundamental assumption of the disposable soma theory (47-49): This prediction has been confirmed in model systems (50), and positive evidence in humans has also been reported (51). The trade-off between fertility and longevity often seems to imply genes involved in carbohydrate homeostasis (52-55). In this regard, it is interesting to note that the YTHDF2 gene is reported to be regulated by high glucose concentration (see GenBank accession no. AF192968). Thus, it could be worthwhile to test the hypothesis of a possible role of YTHDF2 in insulin/longevity-associated variants of this polymorphism. We analyzed the expression of the YTHDF2 gene in cells having different genotypes to investigate its possible association with longevity. In particular, we assessed, by relative quantitative real-time PCR, the YTHDF2 mRNA level in cultured cell lines derived from 10 individuals having the 15-15 genotype and from 41 individuals having different genotypes. Cell lines were Epstein-Barr Virus-immortalized B lymphocytes, which have been proved to be a reliable model in correlating genotype and gene expression (25,40). The results showed that cell lines with the 15-15 genotype have, on average, more than two times higher levels of YTHDF2 mRNA expression in comparison with cell lines with other genotypes. This finding suggests the presence of different functional variants of the YTHDF2 gene, where the 15 allele is probably associated with a variant characterized by a higher expression level, at least in the described experimental conditions. These data are compatible with the hypothesis that the alleles at this locus could be in linkage disequilibrium with still unidentified genetic variants in the promoter or in other regulatory sites, but a direct influence of YTHDF2(TG)12-27 polymorphism itself on gene transcription can not be excluded. Indeed, it is not uncommon to find enhancers and regulatory sites in introns, and polymorphic microsatellites can influence gene expression by modifying the distance between specific regulatory sites (41), or by inducing the DNA to assume left-handed helical structures (Z-DNA) (42,43).

The above-described peculiar expression pattern in normal tissues and of an antigenic activity in cancer cells is typical of a class of genes mainly regulated by epigenetic modifications, the so-called cancer-testis antigens (CT antigens) (56,57). They are a heterogeneous group of proteins, presumably involved in cell-cycle regulation or transcriptional control (58), present as antigens in various cancer cells and predominantly expressed in testis, placenta, and/or ovary. CT genes are often highly expressed in the pancreas (59), which noticeably was the third sample showing a relatively high YTHDF2 expression in our RTPCR assay.