yac transgenic model of alzheimer disease
TRANSCRIPT
In many creatures, as diverse as humansand flies, the male-specific Y chromo-some has accumulated genes that areinvolved in that cardinal male-specificfunction, making sperm. Work in the1970s showed that human males withdeletions of the Y long arm were infer-tile, and since then the bright light ofpositional cloning has been shone intothe murky and repeat-ridden world ofYq. Because of the lack of recombina-tion, genetic mapping is impossible, andthe candidate genes DAZ and RBMYhave been isolated by mapping de novo
deletions associated with infertility:they do not tell a simple story. Both aremulticopy, and no cases of causativepoint mutations in infertile males havebeen found. Genotype–phenotype cor-relations are baffling: for DAZ, deletionof the entire gene cluster gives a pheno-typic range from a complete lack ofgerm cells to one case of fatherhood.Heretics might even suggest that DAZ isa non-functional relic.
Slee et al.1 have turned to transgenicsto confirm a spermatogenic role forDAZ. In humans, the gene has an auto-
somal homologue: the mouse possessesonly this autosomal locus, and the infer-tile phenotype of its knockout, withgerm-cell depletion and meiotic failure,shows this gene to be essential in sper-matogenesis. When a single copy ofhuman DAZ was introduced as a trans-gene, although these mice remainedinfertile, germ cells increased in numberand survived into meiosis. Why onlyman and other higher primates requirethe Y-linked gene, and in many copies,remains to be seen.
Sperm tales
1 Slee, R. et al. (1999) A human DAZ transgeneconfers partial rescue of the mouse Dazl nullphenotype. Proc. Natl. Acad. Sci. U. S. A. 96,8040–8045
Mark A. Jobling
Chromosome 21 dosage imbalances inDown syndrome and mutations in theamyloid precursor protein (APP) andpresenilin 1 (PS 1) genes in early-onsetfamilial Alzheimer disease (FAD) resultin the increased production and depos-ition of amyloid β (Aβ) peptides, particu-larly of the 42 amino acid form. Manytransgenic mice have been producedusing cDNA-based expression of APPor PS-1 gene fragments. However, yeastartificial chromosome (YAC) trans-genesis allows for the incorporation ofgenomic sequences that contain all the transcriptional regulatory elementsnecessary for proper spatial and tempo-ral expression, with the appropriatesplice-donor and acceptor sites required
to generate the entire spectrum of alter-natively spliced transcripts and proteinisoforms of APP and PS-1. Thus, YACtransgenic approaches might provideunique insights into mechanism(s) andprogression of AD in humans.
Lamb et al.1 examined the effects ofFAD mutations in vivo by transferring1 000 kb YACs, containing the entiregenomic copy of human APP and/or PS-1 genes harboring FAD mutations,into mouse embryonic stem cells to gen-erate YAC transgenic mice. They reportthat mutant APP YAC transgenic micedevelop Aβ deposits in the frontal cor-tex and the hippocampus, which is the same location observed in FADpatients. This deposition is accelerated
when the animals are mated to homozy-gosity and/or to mutant PS-1 YACtransgenic mice. Interestingly, no mentionhas been made by the authors about thephenotype of these mice. The relation-ship of Aβ deposits in transgenic micewith respect to dementia, neurofibril-lary tangles, neuron loss and other phenotypes associated with AD still,therefore, remains unclear.
YAC transgenic model of Alzheimer disease
1 Lamb, B.T. et al. (1999) Amyloid productionand deposition in mutant amyloid precursorprotein and presenilin-1 yeast artificialchromosome mice. Nat. Neurosci. 2, 695–697
Sharon Nicholson
A double-strand break (DSB) in DNA isa catastrophic event that, if unrepaired,might lead to genome instability and disease, such as cancer. There-fore, eukaryotic cells have developed complex methods to repair DSBs,whereby DNA-damage checkpointsensure cell-cycle arrest until the damageis repaired.
The papers by Mills et al.1 andMartin et al.2 offer a fascinating insightinto the mechanism of DSB repair bythe non-homologous end-joining (NHEJ)pathway in yeast. Notably, when
DSBs are induced, protein complexes,including Ku, Rap1p and Sir proteinstranslocate from their normal locationat telomeres to the sites of DNA dam-age. Ku is a heterodimer that binds dou-ble-strand DNA ends and might targetDNA-repair machinery to DSBs.Previous work suggested a functionalconnection between yKu and the Sirproteins, which mediate transcriptionalsilencing at telomeres. Sir4p physicallyinteracts with yKu70. Also, Ku mutantsexhibit dramatic derepression ofsilenced telomeric genes. Martin et al.2
also show that Ku, tethered to a DNA-binding domain, can silence non-telom-eric genes in the absence of DNA-silencer elements, but in a Sir-dependentmanner.
Ku or SIR mutant yeast strains arehighly sensitive to agents that causeDSBs. In normal cells, yKu80, Sir3p and Sir4p are seen to co-localize attelomere ends and subtelomeric chro-matin by chromatin immunoprecipi-tation (CHIP) and immunofluorescencetechniques. However, when DSBs areintroduced, immunofluorescence
Telomere proteins take a break
Simon Dowell
Outlook JOURNAL CLUB
TIG October 1999, volume 15, No. 10394