A polymorphic gene nested within an intron of the tau gene: implications for Alzheimer's disease

Abstract

A previously undescribed gene, Saitohin (STH), has been discovered in the intron between exons 9 and 10 of the human tau gene. STH is an intronless gene that encodes a 128-aa protein with no clear homologs. The tissue expression of STH is similar to tau, a gene that is implicated in many neurodegenerative disorders. In humans, a single nucleotide polymorphism that results in an amino acid change (Q7R) has been identified in STH and was used in a case control study. The Q7R polymorphism appears to be over-represented in the homozygous state in late onset Alzheimer's disease subjects.

Figure 1

Tau locus, Saitohin location. The vertical red bar shows the physical location of the Saitohin gene in the intron downstream of exon 9 of the tau gene.

Figure 2

The ORF for Saitohin protein. The predicted amino acids (three letter abbreviation) for the nucleotide sequences are shown. The red-boxed glutamine (gln) codon [CAA] at amino acid 7 is where the nucleotide polymorphism (A → G) changes the codon [CGA] to an arginine in Saitohin.

Figure 3

Human expression of Saitohin and Tau in multiple tissues and the CNS. Reverse transcription (RT)-PCR was performed on the Human Tissue Rapid-Scan panel (A and B) and the Human Brain Rapid-Scan panel (C and D), Saitohin expression is shown in the panels as a single band (A and C). (B and D) The expression of the tau isoforms as two bands, the Upper band consists of isoforms with exon 10, and the Lower band contain the isoforms without exon 10. The lanes of multiple tissue panels in A and B are: 1, brain; 2, heart; 3, kidney; 4, spleen; 5, liver; 6, colon; 7, lung; 8, small intestine; 9, muscle; 10, stomach; 11, testis; 12, placenta; 13, salivary; 14, thyroid; 15, adrenal; 16, pancreas; 17, ovary; 18, uterus; 19, prostate; 20, skin; 21, pbl; 22, bone marrow; 23, fetal brain; 24, fetal liver. The lanes for the Human Brain panel are: 1, frontal lobe; 2, temporal lobe; 3, cerebellum; 4, hippocampus; 5, substantia nigra; 6, caudate nucleus; 7, amygdala; 8, thalamus; 9, hypothalamus; 10, pons; 11, medulla; 12, spinal cord.

Figure 4

A representative alleleotyping gel of HinFI-digested PCR products of the genotypes: QQ, QR, and RR. The polymorphism creates a HinF1restriction enzyme site. HinF1-digested PCR product yields two bands (171 bp and 55 bp) in subjects with a Q allele, and three bands (55, 74, and 97 bp) in individuals with an R allele. Two QQ homozygotes, two QR heterozygotes, and two RR homozygotes are shown.

Figure 5

Western blot analysis of normal (NC) and AD subjects with the QQ, QR, and RR genotypes. (A–C) Immunoblots of whole lysates of IPTG-induced bacteria of expressing recombinant 6XHis tagged saitohin (6H-SA) or 6XHis tagged tau or glutatione S-transferase (GST) or GST-saitohin fusion protein (GST-SA) with antibodies to GST (DT12), 6XHIS, and Saitohin (TS6). (D) Immunoblots of partial purified brain homogenates from QQ, QR, or RR genotypes of AD (QQ-AD, AD-QR, and AD-RR) and normal (QQ-NC, QR-NC) subjects with Saitohin monoclonal antibodies 11F11, TS6, and 10B3 (Left, Center, and Right, respectively).

Figure 6

Immunohistochemistry of STH. Immunolabelling of the cytoplasm of neurons and some astrocytes in human brain sections with TS6 and 10B3.