X chromosome imprinting and inactivation in the early mammalian embryo

PERSPECTIVE

X chromosomeimprinting and inactivationin the early mammalianembryo

M a n y organisms, including nematodes, insects and mammals, exhibit dosage compensation to correct for differences in sex chromosome composition between the sexes. Mammalian dosage compensation is achieved by inactivation (XCD and heterochromatization of one X chromosome in females and is affected by genomic imprinting, which renders maternal and pater- KEmt E. IATfiAM nal chromosomes functionally non-equivalent. Paternal X chromosomes (XP) are preferentially inactivated in f 2 ~ a t t v e ~fferen, ees ~ X-Uzf~ g e ~ e x p r e s s ~ marsupials and in certain extraembryonic lineages of eutherians 1-5. The mechanisms leading to XCI and how imprinting affects XCI have not been fully character- e ~ o s proade ct~es i~o tbe raes of g e m ~ i = t ~ t ~ ized. DNA methylation might maintain the inactive state and the X:maosome ratto im~ X cbromosome once it is established~-6, and specific methylation sites fMa~r~ durUg deaCopme~ Thesedata and ma~y (e.g. in the Xist gene) could control the initial inacti- others caa be a c c o m t ~ for by a r~ewmodel of vation process7. The Xist gene is expressed from the x . c ~ h ~ t t v a : ~ OtCt). F.vpresaom of the x ~ inactive X chromosome, maps to the Xic, and is P~'Af ~ m d patemalX c ~ s during devdopm~t believed to be involved in XCI (Ref. 8). Xist RNA is ln~tmptataat~ leads to represstot of ger,es ~ear the expressed exclusively from the XP chromosome, well X . c ~ - i ~ a ~ center (Xic). Otber g e ~ s ate before implantation and XC! (Refs 9-11). X/st-gene repressed as a result of ~ of the ~ but imprinting, which is correlated with methylation at oftly l~ embryos with at ieast two X cbromosome~ XY three specific sites on the maternal allele in oocytes, amirogemmes are om'ydeflcieat in expresskm of gewes eggs and early emf, "os (but not in sperm), could direct near the Xic atul canform blastocysts, whereas X~ selective inactivation of the XP chromosomea.lZ-lL At • amlrogemmes completely imuatvate both X c ~ s implantation, Xist expression and XCI become random and die before the blastocyst stag~ The X:autosome in somatic cells l°. regulates XCI solely by ~ the spread of Little else is known of how imprinting affects XCI, inactivation awayfrom the Xic ol chromosomes that whether imprinting effects are coordinated , dth other express Xtst MetbylatioB of the maternalXist gene is factors, how other genetic elements on the X chromo- retained in extraembryo~ tissue~ so that g j n ~ e m z ~ s some affect imprinting and inactivation, and to what and parfbe~oger, o ~ s cannot express .~st do rot m~dergo extent disruptions in XCI affect tile early embryo. A use- XCi in those tissue$~arid so have exlrewmbr3~tic defects. ful system for addressing these questions is the eady This model should be relet,ata to understaadl~ how (preimplantation and peri-implantation) mouse embryo, aberrant X chromosome regulatio~ might occur and in which X chromosome imprinting and dosage com- how this might contribute to distorffon of the pensation are first manifested, and in which the imprint X.chromosome4rat~missio~ r a t ~ sex ratio distortio~ must be erased in future somatic cells as a prelude to and diseas~ random XCI Dosage compensationand imprinting in the .early

embryo

Male embryos initially develop faster than female embryos 1"~-17. This might be due to overexp~cssion of X-linked genes in females, which would imply that the X chromosome is not dosage compensated initially, or that dosage compensation is it:complete and that a lack of dosage compensation in the early" embryo is detrimental. Consistent with this, embryos with a single maternal X chromosome (XMO) develop faster and are tx)rn larger than XX embryos TM, and gynogenones with one X chromosome (XO) develop better than XX gynogenones and parthenogemmes19.2¢~. Female-embryoderived embryonic stem cells frequently survive only after partial or entire deletion of an X chromosome21. Dosage compensation begins in the preimplantation embryo and affects some genes (e.g. Pgkl) by the 8-cell stage and other genes (e.g. Hprt) slightly later. This difference between genes might relate to proximity to the Xic (Refs 22, 23) (Fig. 1). Paternal alleles are selectively repressed, indicating that imprinting regulates eady dosage compensation. Consistem with this, X.~tOembryos develop more rapidly than normal female embryos, whereas XPO embryos are retarded 18.24.Aneuploid (41, XXY or 41, XXX) embryos with an extra XMchromosome

die, whereas those with an extra XP chromosome can develop to term z~. Thus. Xv and XMchromosomes differ in their expression in the early embryo. Early X chromosome function in androgenones and

gynogenones Our laboratory provided additional evidence from androgenetic and gynogenetic embryos that imprinting affects X chromosome function during the preimplantation periodg. Pgkl expression was severely reduced (more than 85%) in 8-cell androgenones and remained so through the blastocyst stage. By contrast, the expression of other genes (e.g. Hprt, Pdhal and Prpsl) located far away from the Xic was reduced in androgenones by only 40-50% at the 8--cell and momla stages, and was unaffected at the blastocyst stage. The Xist RNA, which is expres~d in all androgenonesz6, was expressed at about twice the normal abundance throughout tile preimplamation period, indicating expression from all XP chromosomes9. Thus, factol~ inherited from the egg or expressed as part of the early developmental program must promote expression of any under-methylated, paternally inherited Xist gent', and this affects expression of the XP genes in a chromosome-position-dependent manner.

"FIG APRIL1996 VOL. 12 NO. 4 C~,pytight ~ 1996 Ei~".'ier Science t;d. All rights re~er.'t~. 01t~8-952~ q6 $15 0q PII: SO168-9"~2~ ~6 H fin I7-2

134

PERSPECTIVE

(a)

Normal

Xu

~

(b)

Normal

xM

~

~

~

Pgkl Eratmm of imprint

[

No erasure of impdnt

Random inact~ttmon

xP°rM

x P°'"

I | ~

Pgkl Non-random klact~aUon

~

Pok~ Somatic tissues (c)

XY Androgenones

Extraembryonic tissues (d)

XX Androgenones

xist XP

xP

~ " /Pgkl

I~

r-"" (

Y

X:A ratio sensing

! /

~

~ Pgkl x~t

ratio sensing

No spreadingof

X:A

inactivation

I OgklBoth XP ~ inactivated

~ x, ~ x,

Viable to blastocyst

Not viable

Fmum L A model to account for X chromosome imprinting effects in androgenones, gynogenones and normal embryos. The upper portion of each box shows tile situation at the 8-cell stage for different types of embryos. The lower portions show the fates of those embq,os 9r their tissues. Differential imprinting of the Xtst gene leads to preimplantation repression of genes near the X-inactivation center (Xic) (e.g. Pgkl) on all XP chromosomes as eady as the 8-cell stage due to a localized effect of Xist RNA transcribed in c/s. In androgenones, repression of genes further away from the Xic requires other factors to be expressed in response to a 1 : 1 ratio of X:autosomes between the 8-cen and blastocyst stages. These factors mediate the spreading of inactivationdistallyonly on chromosomes expressing Xist. Due to the spreading of XCI only in embryos with two x chromosomes, XX androgenones inactivate both X chromosomes and die before the blastocyst stage, leaving only XY androgenones, which express the more distal genes at about the same level as normal embryos. The requirement for spreading of XCI to inactivate other genes is consistent with eadier studies that indicate that paternal Hprt repression occurs after the 8-.cellstage. In gynogenones and pa~henogenones, the selective maintenance of the maternal Xist gene methylation pattern on both of the XMchromosomes in trophectoderm and extraembryonic tissues prevents Xist expression in those cells, blocking dosage compensation, and leading to typical pert-implantation gynogenetic and parthenogenetic defects. the Xist gene at the morula stage as

a result of a requirement for a positive regulator of Xist that was expressed exclusively from the maternal genome. That negative result xM r- ~ r--~ xM ~ ~ r--" could have been a consequence of a technical limitation of the assay xM r- ~ r---~_ xM __t7 -~ ~ used. Given the more recent positive result, there is little reason to question whether Xist expression Erasure of / One XM No erasure No XM impdnt ~ inactivated requires a product encoded by a of imprint l inactivated paternally imprinted gene, We also observed reduced exXM r--)- r--~ r--~ XM r--" ~ pression of the Xist RNA in gynogenones throughout file preimplantxM ~ ~ i- ~ ation period, consistent with earlier studiesg-tz, even at the blastocyst Pg,w stage when Xist is induced as the Extraembryonic gynogenones initiate dosage comtissue d e f e c t s Somatic tissues pensation in some of their cellsz6. In addition, dosage compensation in Our results for X~st RNA expression in androgenones gynogenones v,-~s delayed or incomplete9. Consistent differ from those reported previously26. Based on negawith this, earlier studies reported a smaller than normal tire RT-PCR results obtained with single embryos, the fraction of cells with inactive X chromosomes in parthenoeadier study concluded that androgenones downregulate genetic blastocysts as compared with normal hlastocy.,;ts 27.

(e)

Gynogenones

(0

Gynogenones

[

TIG APRIL 1996 VOL. 12 NO. 4

PERSPECTIVE

mtd~setmze A diploid embryo with two paternal genonms produced by

pronuclearl~nsplamar~on A diploid embryo with two "rotemai getxmles produced by ptonuclear transplantation

B/n.t Germencoding hypoxanthinephosphoribosyltransferase A diplo~ embryo obtained by l ~ n e t i c a l l y unfertilized eggs

activating

~ngkl Gene encoding phosphoglycerateIdr~se type 1

X:ammotmae The =rio of the number of X chromosomesto the number of ~ of autosomes XCI X-chromosomeinactivation The X-chromosome-inactivationcenter XM Maternal X chromosome XMO An embe/o with one maternal X chromosome and no Y chromosome XI. Paternal X chromosome

XI'O An embryo with one paternal X chromosome and no Y chromosotne A role for X chromosome function in androgenetic and gynogenetic defeos Our data, and others").-", indicate that the effects of X chromosome imprinting and dosage compensation are chromosome-position dependent and time dependent. Genes near the Xic are affected earliest and most strongly. More distal genes (relative to the Xic) require a longer period of time to become repressed. Thus, XCI is not an abrupt process that rapidly silences the entire X chromosome, but rather a gradual prcx:ess. Our data, however, cannot be completely accounted for solely on the basis of chromosome position and time. Of special interest is the initial partial (40-50%) repression of the more distal genes in androgenones, followed by recovery of expression to mRNA abundances that are equal to those of normal embryos, by the blastocyst stage. There are three explanations for this result. First, development might I~e retarded in androgenones, so that the mRNAs do not accumulate with the same kinetics as in normal embryos. T~lis explanation might apply to the 8-cell stage, when the Hprl gene is not dosage compensated in normal embryos22. A second explanation is that the spreading ,ff XCI away from the Xic is slower in androgenones than in normal embryos. This is unlikely because androgenetic expression of these genes relative to their expression in normal embryos does not decline with continued

developmem, but instead increases during development to the blastocyst stage. A third explanation becomes apparent when considering the eadier finding that XX androgenones are selectively eliminated before the blastocyst stage26. Specifically, these genes might be repressed between the 8-cell and blastocyst stages, but only in XX androgenones, for which the genes on both X chromosomes would be inactivated because both Xist genes are active tFig. 1). This would cause a 50% reduction in the apparent abundance of these mP~'qAswhen RNA is extracted from a group of androgenones, before the loss of any of the XX androgenones. By the blastocyst stage, when dosage compensation has occurred in normal XX embryos and all of the XX androgenones have been eliminated, expression should be roughly equal between the remaining XY androgenones and normal embryos, as observed. This interpretation is appealing because it accounts for the selective loss of XX androgenones. A critical test of this model, however, will be to perform quantitative analyses of X-linked gene expression in individual androgenones and gynogemmes of known sex chromosome composition. If this interpretation is correct, then X chromosome imprinting and Xist RNA expression in all androgenonesz6 must not be sufficient to silence the entire X chromosome completely. Complete silencing must require other factors to be expressed in response to a 1:1 ratio of X:autosomes. Thus. the Xist RNA might fulfill a very limited role in the initial XCI process, by promoting repression of genes lo~ted near the Xic and signifying which chromosomes to inactivate. Expression of Xist from all Xe chromosomes would, thus. account for the severe repression of Pgkl in androgenones and of the paternal Pgkl allele in normal embryos9.2223. A localized effect of Xist RNA would be consistent with its very low abundance, which is about 1000copies per cell in adult tissues and only about 10copies per cell in the early embryo').'-a. Eady partial XCI might also explain why XPO embryos are viable but developmentally retarded, with a lower than expected birth rate. The viability of many XPO embryos would also indicate that some mechanism must overcome the early repression when a single XP is the only X chromosome present. X chromosome imprinting can also account for delayed XCI and early defects of gynogenones and parthenogenones. These embryos show the most severe defects in the development of extraembryonic membranes29.-~, which, in normal embryos, retain the maternal Xist gene methylation imprint and undergo selective XI' chromosome inactivation. Although earlier reports argued for a delay in XCI followed by an erasure of the XL~t~gene imprint in parthenogenones26.27, the data can be explained equally well by selective retention of the maternal Xist-gene imprint in extraembryonic tissues, so that neither X chromosome undergoes XCI. Emhryos with extra XMchromosomes2'~ or X'x!chromosome disomy51 show lethal defects specifically in extraembryonic tissues. The enhanced development of XO gynogenonestg.'° also supports this idea. Thus, there is ample precedent to propose that XM chromosomes retain their methylation imprints and resist XCI specifically in extraembt'yonic tissues, leading to deleterious overexpression of X-linked genes, and gynogenetic

TIG APRIL1996 VOL. 12 NO. 4 136

PERSPECTIVE

and parthenogenetic defects. This idea was also suggested previously25. The reported XCI in the extraembryonic membranes of some parthen~g~aones 32could represent exceptional cases in which tl~." XM chromosome imprint was fortuitously erased in cells that generated emraembryonic as well as embryonic tissues. Most parthenogenones fail to implant or cease development shortly after implantation33, indicating that extraembryonic cell function is impaired and that allocation, survival, or proliferation of cells in the extraembryonic tissues might be reduced. Viable humans with extra Xi chromosomes might result from mosaic development or exceptional cases in which Xi imprinting is lost. To summarize, all XP chromosomes express the Xist gene by default, thus, repressing genes near the Xic (Fig. 1). Repression of genes located further away from the Xic requires factors that are expressed specifically in embryos with at least two X chromosomes (e.g. in XX androgenones or XX normal embryos). Thus, XX androgenones have both X chromosomes completely inactivated and die around the morula stage. XY androgenones underexpress just the genes near the Xic and can form blastocysts. The X:autosome-mtio-sensing mechanism operates strictly to promote the spread of heterochromatization away from the XCI, and this occurs from any X chromosome expressing Xist. Because of the requirement for Xist expression, and because XM chromosomes retain their maternal Xist methylation patterns specifically in certain extraembryonic tissues, XCI will not occur in those tissues in gynogenones and partbenogenones, leading to deleterious overexpression of X-linked genes and extraembryonic defects. The same will occur in aneuploid embryos with extra Xm chromosomes. The absence in somatic cells of factors that maintain the matemal Xist-gene imprint permits: imprint erasure, random de novo methylation of one Xist gene copy, and random XCI. The role of the X: autosome ratio in controBing X chromosome inactivation If this model is correct, XCl can be viewed as a twophase process. The first phase is mediated by a local effect of the Xtst RNA on genes within the immediate vicinity of the Xtc. The second phase (spreading of XCI) is mediated by other factors that operate either independently of the Xist RNA or in conjunction with it. These factors are expressed only in cells with two X chromosomes. Thus, the X:autosome ratio would primarily affect the second phase (i.e. the spreading of XCI away from the Xic). This is an important point, as it helps to define the role of the X:autosome ratio in the process more precisely. One implication of this model is that, once the X:autosome-ratio-sensing mechanism has initiated the second phase of XCI, the process will inactivate all X chromosomes expressing Xist This is important for considering po~ible mechanisms of selecting which chromosome and how many X chromosomes will be inactivated, decisions that will most likely rest with the methylation status of the Xisl alleles. This view is consistent with the recent results of experiments in which a null mutat on was introduced into the x i s t gene by homologot;s recombination 34. Those experiments indicated that the Xgst gene probably does not participate in either counting the number of

X chromosomes or in selecting which chromosome to inactivate, but is instead a mediator in cis of XCI. Cells that selected the X chromosome with the defective X/st allele for inactivation were inhibited from undergoing XCI. Interestingly, there appeared to be selection against those cells. This is consistent with the idea that a failure to undergo XCI in parthenogenetic cells would be lethal. It will be of interest to dete .m~,ine how the methylation status of the Xist gene promoter is altered in the somatic cells of: (a) gynogenetic and parthenogenetic embryos; (b) embryos that are heterozygous for different alleles of the X-controlling element35; and (c) embryos that have extra X chromosomes.

Other considerations The local effect of Xist expression in cis might affect the function of the X chromosome during spermatogenesis when the Xist gene undergoes demethylation and becomes expressed 12. Genes near the X i c might be selectively inactivated. The resulting loss of function might be compensated by activation of an autosomal gene encoding a functionally similar product, such as the testis-specific Pgk2 gene, which is transcriptionally induced before inactivation of the P g k l gene36. The model is also relevant when considering instances of sex-ratio distortion in mice and in humans. Failure to erase and re-imprint X chromosomes properly during each generation would result in embryos that are functionally androgenetic or gynogenetic for their X chromosomes. The model predicts that this would reduce the viability of embryos that inherit an improperly imprinted X chromosome, particularly female embryos with two maternally imprinted X chromosomes, leading to sex ratio distortion, in the laker case a preponderance of male progeny as observed for patients affected by sporadic bilateral retinoblastoma 37. Further tests of the model and the possible stepwise nature of XCI should improve our understanding of normal and aberrant X chromosome regulation.

Acknowledgemen~ I thank Davor Solter and Sue Varmuza for critical comments on the manuscript.This work was supoorted by grants from the NIH [GM 49489 to K.E.L and an NCI Cancer Center Support Grant (P30 CA 12227) to the Fels Institute] and the Wendy Will Case Cancer Fund. This review is dedicated to the memoryof Prof. Veme M. Chapman. References I Takagi, N. and Sasaki, M. (1975) Nature256, 640-647 2' West,J.D. etal. (1977) Celll2, 873-882 J Harper, M.I. (1982)./'. Embryol. Exp. Morph. 67,127-135 4 Kratzer, P.G. el al. (1983) Cell33, 37-42 5 Lock, L.F. et al. (1987) Cell48, 39-46 6 Park,J-G. and Chapman, V.M.(1994) Mot Cell. Biol. 14, 7975-7983 7 Grant, M. et ai. (1992) Nat. Genet. 2,161-166 8 Brockdorff,N. et al. (1992) Cell71, 527-542 .9 Latham,K.E. and Rambhatla,L. (1995) Dev. Genet. 17, 212-222 10 Kay,G.F. et al. (1993) Cell72,171-182 I I Latham,K.E. et al. (1993) GenesDev.8, 290-299 12 Ariel, M. et al. (1995) Nat. Genet. 9, 312-315 /3 Zuccotti, M. and Monk, M. (1995) Nat. Genet 9, 316-320 14 Nonis, D.P. et al. (1994) Cell77, 41-51 15 Tsunoda, Y. et al. (1985) Gamete Res. 12, 301-304

"FIGAPRIL1996 VOL. 12 NO. 4

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16 Gardner. D.K. and Let_.-sc.H.J. ( 1987)]. Fxp. Zool. 2q2.

Buzin. C.H. et at. t 19941 Devetopment. 120. 3529-2536 29 Barton. 5.C et aL ( 19841 Xature 311, 37-t-376 30 Kaufman, M.H. et at. ( 19,"71 Xature 265, 53-55 31 Takagi. N. and Abe, K. (1990) Det'etopme:tt 109.189-201 32 Rastan. 5. eta/. ( 19801 ,'~kUnre288.172-173 33 Varmuza. S. etaL(19931 IX,t,. Genet. 14. 239-248 34 Penny. G.D. et at. ( 19961Xature379.131-139 35 Courtier, B. el at. (199~) Proc. ,X2:ttAcad. Sci. USA 92. 3531-3555 36 McCarrey.J.R. el at. t 19921 IX,v. BioL 15.t. 160-168 3 7 Naumova. A. and 5apienza. C. ( 19941 Am.J. Hum. GeneL "34.2(~-273 28

103-105 17 Ray, P.F. et at. (1995)]. Reprod. Fert. 104, 165-171 18 ThomhilL A.R. and Burgoyne. P.5. ( 1"993;,Development 118. 171-174 19 Mann.J.R. and Lovell-Badge. R.H. ( 19871 Development 99, 41 l-a,16 20 Mann,J.R. and Lovell-Badge. R.H. ¢19881 IX,velopment 103. 129-136 21 Roberts, m, EJ. el aL ( 19831J. k'mb~.~,L Exp. Morphol. 7-L 297-309 22 5inter-Sam, J. el a]. (1992) Prrm. A2a/A¢'ad. ,$g'i. I'SA 89, 11M69-10473 23 ?,ltx~re,T.F. and Whittingham, I).G. 119921 IX,t'elopment 11"5, 1011-1016 24 Burgoyne. P.S. et al. ( 19831.]. RepnM. bert. 68. 387-393 25 Shao, C. and Takagi. N. ( ITS)0}IX,velopment 110. %9-975 26 Kay, G.F. et al. ( 199.¢~ Cell77. 639-6-5{) 2"7 Tad;. T. and Takagi. N. (1c~12)MoL Reprod. Dev. 31, 20-27

K.E. LATHAM (keith@cleo~fel&temple.edu) Is IN THE FEIS INSTilI,TE FOR Cak~CERPdgSEARCllALVDMOLECULAR BIOLOGY, *LVOTHE IDEPARTML~rOF BIOCIIF~IISTRY,TEMPLE [L~'iVERSiTI"SCHOOL OF MEDICIXE. 3307 NORTH BROAD SIREET,PHll.tDE1.PHI~PA 19140, USA.

REVIEWS

RETmutati0ns in human disease

The RET proto-oncogene enccxles a cell-surtace glycoprotein related to the family of receptor tyrosine kinases (RTK) L" whose ligand is still unknmvn. As its acronym suggests. RkT{ rearranged during tmnsfection) was c'kmed as a chimeric oncogene during a classical BARBARAPASINI,ISABEllACECCHERINIAND GIOVANNIROMEO NIH3T3 transformation assay where the novel tmnsfi~mling gene originated from an anefactual reamtngeThe RETproto-oncogene is at the origin o f one o f the ment that occurred iH l'llrD I. Until a number of linkage studies l~K'alized the k~.'i fi)r most interesting models o f human disease caused by mutations in a receptor tyros,he kitw.se g~te. Somatic MEN type 2 and Ihe autosomal dominant HSCR withi,1 rearrangements o f RKr are iul~olved in the a e t ~ of a a small genomic region surrounding RFT (Refs 3. -it. variable proportion o f poptllary thyroid carcinomas the only known involvement of this novel prom(PTC), the most common type o f thyroid turnout whose oncogene in human disease was its activation through somatic rearrangements in papillaD" thyroid carcinomas prevalence is increasing in areas heartO, exposed to radtoactive fal~ut after the C ~ , ! acctdeM o f 198~ (for definitions see Box 1 ). Three diflL'rent rcammgcd versions of R k T ( n a m e l y RET-tr/'CI. RkT"-IxI'C2 and RF.T- Moreover, germlinc RET mutations are associated with the three variants o f the inherited cancer syndrome knou~t as Irl"C~) (Refs -3-8) have been descril'u.xl in a variable proportion of PTC depending on the different get)- multiple endocrine neoplasia type 2 (MEN2& MEN2B and graphic ;treats investigated (Table 1 ) and on the sensi- FMTC). Finally, RET mutations or heterozygoas deletions of the u,ho~ gene cause the autosomal domiwa~ form of tivitx' of the detection taetht~l used u. In the three types Htrschsprung disease (HSCR), a congenital disorder o f of r~:arrangenmnt desctiDed so far. tile genomic region enctKling tile intracdhdar domain of RET(Fig. lb) is the enteric nervous system (ENS). juxtaposed to different activating genes (t14, Riot and FLEt. respectively1 constitntively expressed in the thyroid. By fusing with the intmcellular domain of RI:7~ human cell lines upon X irradiation II and R/T-/r/E" these three genes contribute a novel N-terminal portion. oncogenes have been identified in more than (0% of which is able to dimerizc in the c.~aoplasm, thus. escapPTC from young patients exposed to radiation fallout in ing the lit;hal-dependent tyrosine kinase activation~'. 13yelomssia following tile 1986 Chemobyl accident". Although several types of tumours have tx*en tested While radiatkm exposure and rearrangements of for RI:T-IrFC activation, this phenomenon seems to be RETare well d(x:umented in the aetiology of a number restricted to the papillary histotype of thyroid cancers 1° of cases of PTC. the issue of inherited predisposition 1o (Box 1 ). This finding could be explained either by a dif- non-medullary thyroid carcinomas (NMTCL and PTC in ferential susceptibility to the rearrangements of RL:T particular, was raised epis~xlically in the past to account among different tissues, or by the ability, of the oncogenic R~r familial clustering but was never addressed systemativersions of R / T t o activate a mitogenic pathway only in cally. In a recent study, the Utah population database specific subtTpes of cells. Interestingly, the RET-PTCI was analysed for familial clu.,tering of 28 distinct cancer activated x'emion of REThas been obtained in vitro in sites U. Although the incidence of NblTC was found to TIG APRIL1996 VOL 12 No. 4 C,,p~,righl• I'~, l!l~'ier~iemx- IJd Allnght~ r~,nL~l i)I(@952-1~, ~l~I~

13