NATURE MEDICINE • VOLUME 5 • NUMBER 4 • APRIL 1999 383

NEWS & VIEWS

SINCE 1991, TWELVE neurological dis-eases have been shown to be caused

by unstable expansions of trinucleotiderepeats (CAG/CTG, GAA, CGG/CCG),or by a GC-rich dodecamer in the caseof progressive myoclonus epilepsy1.Seven neurodegenerative diseases, in-cluding Huntington disease and fourtypes of dominantly inherited spin-ocerebellar ataxias (SCA1, 2, 3 and 7) arecaused by expansions of a CAG repeatcoding for a polyglutamine in the targetprotein. This confers a gain of toxicproperty to these proteins, correlatedwith their aggregation in ubiquitinatednuclear inclusions2,3.

In these diseases, the CAG repeatshows a tendency to expand, mostly onpaternal transmission, that accounts forvarying degree of clinical anticipation1,4

(earlier onset and increased severity insuccessive generations). Another domi-nant ataxia (SCA6) is causedby moderate and stableCAG/polyglutamine expan-sions in the alpha-1A voltage-dependent calcium channel.This expansion is likely toproduce a non-functionalchannel, as there is some clin-ical overlap with episodicataxia type 2 caused by loss offunction mutations in thesame gene5.

Another CAG/CTG expan-sion for spinocerebellar ataxiais now reported in the Aprilissue of Nature Genetics6. Theexpansion was initially de-tected using an elegantmethod called RED (repeat ex-pansion detection), devised in1993 to screen in genomicDNA for long CAG/CTG re-peats7. This technique hasbeen widely applied to neu-rodegenerative and even psychiatric dis-eases, in which anticipation has beenclaimed. But interpretation of RED re-sults is complicated by the frequent oc-currence of long repeats in the healthypopulation, which can be almost en-tirely accounted for by two recentlycharacterized loci8.

Ranum and colleagues applied RED topatients with familial SCAs lacking

known mutations6. In one family, twopatients showed a RED product of about80 CAGs, which was cloned using an en-richment strategy. This allowed PCRanalysis of the repeat in 214 SCA fami-lies. In one large family, all the affectedmembers carried an allele with morethan 107 CAG/CTGs, and co-segrega-tion with the disease could be demon-strated formally by lod scorecalculation, thus defining a new locus—SCA8. By studying 600 control individu-als, the authors found a very lowfrequency (0.25%) of alleles in the rangeof 75 to 91 CAG/CTGs; all others weresmaller. Koob’s data indicate that theexpanded repeat is indeed pathogenic.Ironically the first family analyzed may

have an unrelated SCA, as its SCA8 alleleis shorter than the pathological thresh-old of 100 CAG/CTGs, as defined by theother families, and overlaps with theend of the distribution in the normalpopulation.

However, SCA8 is not a polygluta-mine disease, and it presents some curi-ous features that set it apart from theother SCAs. The authors clearly estab-

lished that the repeat is not protein-coding and they present evidence thatit is transcribed (as a CUG stretch)within a non-coding RNA. However,caution is advised in interpreting thisinformation, as this transcript was de-tected after numerous cycles of PCR—conditions that may lead to detectionof RNA species of very low abundancelacking biological significance.Moreover, the pattern of repeat insta-bility in SCA8 differs considerably fromthe other SCAs in that it shows mater-nal expansions (with some very largejumps over 200 repeats) and frequentpaternal retractions. The tendency toexpand is variable in other SCAs, but isalways higher on paternal than on ma-ternal transmission (see Fig.).

In fact, the SCA8 pattern is more sim-ilar to that observed for longer alleles inmyotonic dystrophy (DM), a disease

caused by large expansion ofan untranslated CTG repeat.DM also shows a high degreeof anticipation, and the mostsevere congenital forms ofthe disease occur almost ex-clusively through maternaltransmission. Frequent re-traction in male transmis-sions in parental alleles largerthan 600 CTGs are believedto be due to the deleteriouseffect of large expansions onspermatogenesis, and the re-sulting selection of smallerrepeats. In the large SCA8family, all the patients inher-ited the expansion from theirmother. This maternal bias ofpenetrance seems consistentwith retractions through pa-ternal transmission. Yet thisfeature may not be absoluteand needs to be confirmed in

additional families. Indeed, two af-fected individuals in a small pedigreeinherited a paternal expansion (L.Ranum, personal communication).

Unlike other pathogenic CAG/CTGrepeats, the SCA8 expansion does notpresent any clear correlation between itssize and the age at onset of the illness,suggesting an ‘all or none’ effect. Suchan effect is found in Fragile X full muta-

Variation on a trinucleotide themeFour of the spinocerebellar ataxias (SCAs) are due to CAG triplet repeats; now SCA8 is found to be the

exception to the rule.

GAËL YVERT &JEAN-LOUIS MANDEL

+1000

+100

+10

0

–10

–100

–1000DM<600 DM>600 SCA1 SCA2 SCA3 SCA7 SCA8

Maternal transmissionsPaternal transmissions

Cha

nge

in C

AG

/CTG

rep

eat

leng

th

Fig. Parental bias on instability in various CAG/CTG expansiondiseases. Average size changes are indicated. For myotonic dystrophy,this is shown for parental alleles in the range of 50–600 CTGs, and forthose larger than 600 repeats. (For DM alleles in the 50–200 range, thesize increase is greater on paternal transmission.)

© 1999 Nature America Inc. • http://medicine.nature.com©

199

9 N

atu

re A

mer

ica

Inc.

• h

ttp

://m

edic

ine.

nat

ure

.co

m

384 NATURE MEDICINE • VOLUME 5 • NUMBER 4 • APRIL 1999

NEWS

ONE OF THE LONG-STANDING, controver-sial issues in virology is the nature

and origin of the virus that caused theso-called ‘Spanish’ flu pandemic of1918, an outbreak that claimed the livesof as many as 1 in 100 of the world’spopulation of the time. With thethought still fresh in mind that if it werenot for the slaughtering of 1.5 millionchickens and other poultry in the 1997Hong Kong H5N1 ‘bird flu’ incident, wemight have have seen another pan-demic, any retrospective insight intoevents surrounding the 1918 outbreakcould prove useful in future preventionand control strategies. Until now,progress towards a better understandingof the 1918 event has been bedevilledby the unavailability of viruses from thepandemic. Now, Reid et al.1 describe for-malin-fixed, paraffin-embeddedarchival lung tissue from two US sol-diers, and formalin-fixed, frozen lungtissue from an Inuit woman buried inpermafrost in Alaska, all of whom suc-cumbed to the 1918 pandemic.

It is known that substitution of the HA

gene of a prevailing human influenza Avirus through reassortment is the basicrequirement for the genesis of a newpandemic virus. Reid and colleagueshave determined the entire HA sequence

of 1701 nucleotides in 22 overlappingfragments of one archival sample andthe HA1 domain of the HA gene of theother two, and found a high degree ofconservation. Phylogenetic analyses ofthe sequence indicate that it is mam-malian and is located at the root ofhuman and swine clades. However, itseems to be more closely related to avianthan any other known mammalian HAsequence, and it may have been intro-duced into humans around 1915.

Assuming that the sequence of the HAgene that Reid and colleagues presentedfaithfully represents the 1918 virus (andit had not been passaged in the labora-tory) and that the gene had been under-going rapid evolution in the way newlyintroduced H2, H3 and H5 HA aviangenes have done2,3, there is in fact everypossibility that the virus was introducedinto the human host well before 1915and had an avian ancestry. This suggeststhat the 1918 virus had been ‘smolder-ing’ in, or adapting to, the human host,and offers some support to the earliersuggestion that the human and swine

The 1918 ‘Spanish’ flu: pearls from swine?An unexpected marriage between modern biological technology and past records suggests that the influenza A

(H1N1) viruses are a long-established family from China, not Spain.

KENNEDY F. SHORTRIDGE

NEWS & VIEWS

tions (defined as more than 200 CGGrepeats in the first exon of the fragile Xmental retardation gene), in which theabnormal methylation associated withlarge expansions shuts off the expres-sion of the target gene.

The next few months should witnessa series of studies that will establish theepidemiological importance of SCA8 asa cause of spinocerebellar ataxia in fa-milial and sporadic cases in differentpopulations. The loci previously charac-terized account for 60 to 80% of SCAfamilies (and for a much smaller propor-tion of sporadic cases), and three addi-tional SCA loci mapped in singlefamilies remain to be cloned (SCA4, 5and 10)(refs. 9,10). Such studies willalso establish on firmer grounds thetransmission properties of SCA8 and therelations between repeat size and clini-cal features, such as penetrance, severityand age at onset. This will be essentialfor applications to diagnosis and ge-netic counseling.

A much more difficult problem willbe determining why the repeat is patho-genic. If, as with fragile X or progressive

myoclonus epilepsy, the SCA8 expan-sion causes a loss of function of a singlegene, one would expect to find conven-tional mutations in that gene causing asimilar clinical phenotype. The possibil-ity of a toxic protein mechanism as inother SCAs is excluded because the re-peat is untranslated. Koob and col-leagues suggest that the SCA8expansion affects a low abundance anti-sense transcript that could in turn regu-late expression of an overlappingprotein coding gene. However, a de-tailed transcript map of the region islacking, and it is possible that anothernearby gene is affected by the mutation.Finally, we must remember the soberingexample presented by DM: 7 years afterthe characterization of the mutation forthis disease, its exact mode of action isstill not fully understood.

1. Genetics Instabilities and Hereditary NeurologicalDiseases. (eds. Wells, R.D. & Warren, S.T.)(Academic, San Diego,1998).

2. Lunkes, A. & Mandel, J-L. Polyglutamines, nuclearinclusions and neurodegeneration. Nature Med.3, 1201–1202 (1997).

3. Kim, T-W. & Tanzi, R.E. Neuronal Intranuclear in-clusions in polyglutamine diseases: Nuclear

Weapons or Nuclear Fallout? Neuron 21, 657–659(1998).

4. David, G. et al. Molecular and clinical correlationsin autosomal dominant cerebellar ataxia withprogressive macular dystrophy (SCA7). Hum. Mol.Genet. 7, 165–170 (1998).

5. Jodice, C. et al. Episodic ataxia type 2 (EA2) andspinocerebellar ataxia type 6 (SCA6) due to CAGrepeat expansion in the CACNA1A gene on chro-mosome 19p. Hum. Mol. Genet. 6, 1973–1978(1997).

6. Koob, M.D. et al. An untranslated CTG expansioncauses a novel form of spinocerebellar ataxia(SCA8). Nature Genet. 21, 379–384 (1999).

7. Schalling, M. et al. Direct detection of novel ex-panded trinucleotide repeats in the humangenome. Nature. Genet. 4, 135–139 (1993).

8. Lindblad, K. et al. Two commonly expandedCAG/CTG repeat loci: involvement in affectivedisorders? Mol. Psychiatry 3, 405–410 (1998).

9. Takano, H. et al. Close Associations betweenprevalences of dominantly inherited spinocere-bellar ataxias with CAG-repeat expansions andfrequencies of large normal CAG alleles inJapanese and Caucasian populations. Am. J. Hum.Genet. 63, 1060–1066 (1998).

10. Moseley, M.L. et al. Incidence of dominant spin-ocerebellar and Friedreich triplet repeats among361 ataxia families. Neurology 51, 1666–1671(1998).

Institut de Genetique et de Biologie Moleculaireet CellulaireCNRS/INSERM/Universite Louis PasteurBP163, 67404 Illkirch Cedex, CU StrasbourgFrance

© 1999 Nature America Inc. • http://medicine.nature.com©

199

9 N

atu

re A

mer

ica

Inc.

• h

ttp

://m

edic

ine.

nat

ure

.co

m