Letter to the Editor

Reply to the Letter to the Editor by Rivera—“Heteroallelic Twins and Twin Publications”

misanthrope adj. et n. (gr. misein, haır, et anthropos,homme). Qui hait le genre humain. // Par ext. Carac-tere bourru, chagrin.bourru adj. et n. D’une humeur brusque et chagrine. //Vin bourru, vin blanc nouveau quı n’a point fermente.

—Petit Larousse.Paris: Librairie Larousse, 1967.pp. 135, 664.

To the Editor:

Neither Figure 1 nor some data and the discussion inour Letter to the Editor [Cantu et al., 1998] were pub-lished before. The publication [Dıaz-Gallardo et al.,1995; referred to in our letter!], surmised as “twin” inthe letter which is the object of this reply, appeared ina supplement of Archives of Medical Research withsome errors (all the authors’ responsibility, since thepaper was not evaluated by referees), which were latercorrected [Arch Med Res 1996:27:227]; the letter ofRivera [1996], appeared in the next issue repeating theerrata and suggesting unethical procedures; a replywas then considered unnecessary.

The other flaws that Rivera suppose in our letter,deserve the following responses.

1. The reference of Zneimer et al. [1993] was initiallyquoted to support the differences within monozy-gotic (MZ) twins, but our equivocal sentence compo-sition made it misplaced.

2. The fact that there is no evidence to support a prezy-gotic origin to explain discordance in MZ twins doesnot discard the hypothesis. To find a set of tripletswith only one carrying a mutation could support thehypothesis of a prezygotic origin.

3. There is no doubt that MZ twinning can be eitherhomoallelic or heteroallelic. Both forms could eitherhave a prezygotic or postzygotic origin. A prezygoticorigin in homoallelic MZ twins can escape compre-hension if discordance with a parental genotype isnot considered. For instance, prezygotic origin canbe suspected in homoallelic MZ male twins with He-mophilia A, if the mother is homozygous for the wildallele.

4. Our suggestion that there are no genomically iden-tical twins seems more likely. Our letter, although

received and accepted in 1996, was published inMay 1998. In 1997, Broude et al. reported that ge-nome subsets containing specific interspersed tri-nucleotide repeats showed differences within MZtwin pair genomes. If there is heteroallelism be-tween individuals derived from the same zygote,heteroallelic cell lines in a single individual, i.e., mo-saicism, would not be uncommon. There are reportsdemonstrating mosaicism in fragile X affectedmales [Nolin et al., 1994], and in Friedreich’s ataxiapatients [Montermini et al., 1997], indicating exten-sive mitotic instability. On this basis, we wonder if“Dolly” [Wilmut, 1998], has genomic differencesfrom the nuclear mother, if there are, it would berather difficult to obtain exact cloning from adultcells.

To conclude, it is difficult to find better examples ofself-reference than “jargonaut,” and of whistleblowingunder false premises. In 35 years of working in humangenetics I have trusted and been trusted by many col-leagues, students, and technicians. There have beensome disappointing experiences, both ways for sure,which I have to accept as part of the process of learn-ing. The present instance reaffirms my belief in confi-dence as an indispensible ingredient in group work,vital for the future of science.

REFERENCES

Cantu JM, Dıaz-Gallardo MY, Barros-Nunez P, Figuera LE. 1998. Hetero-allelic monozygotic twins and triplets. Am J Med Genet 77:166–167.

Dıaz-Gallardo MY, Barros-Nunez P, Dıaz CA, Hernandez A, Gomez-Espinel I, Leal CA, Fragoso R, Figuera L, Garcıa-Cruz D, Ramırez-Duenas ML, Cantu JM. 1995. Molecular characterization of the Frag-ile-X syndrome in the Mexican population. Arch Med Res 26(sup-pl):S77–S83.

Montermini L, Kish SJ, Jiralerspong S, Lamarche JB, Pandolfo M. 1997.Somatic mosaicism for Friedreich’s ataxia GAA triplet repeat expan-sions in the central nervous system. Neurology 49:606–610.

Nolin SL, Glicksman A, Houck GE Jr, Brown WT, Dobkin CS. 1994. Mo-saicism in fragile X affected males. Am J Med Genet 51:509–512.

Rivera H. 1996. Fragile X and authorship. Arch Med Res 27:587–588.

Wilmut I. 1998. Cloning for medicine. Sci Am 279:58–63.

Zneimer S, Schneider N, Richards S. 1993. In situ hybridization showsdirect evidence of skewed X inactivation in one of monozygotic twinfemales with Duchenne muscular dystrophy. Am J Med Genet 46:601–605.

Jose Maria Cantu*Division de GeneticaCentro de Investigacion Biomedica de OccidenteInstituto Mexicano de Seguro SocialGuadalajara, Jalisco, Mexico

*Correspondence to: Jose Maria Cantu, Division de Genetica,Centro de Investigacion Biomedica de Occidente, Instituto Mexi-cano del Seguro Social, A.P. I-3838, Guadalajara, Jal. 44310,Mexico. E-mail: imcantu@koch.mb.udg.mx

Received 10 March 1999; Accepted 10 March 1999

American Journal of Medical Genetics 86:89 (1999)

© 1999 Wiley-Liss, Inc.