Uniparental disomy (UPD). Genomic imprinting and a case for new genetics (prenatal and clinical implications: the "Likon" concept).

Uniparental disomy (UPD) is often the result of an aneuploid event masquerading under the features of diploidy. As such, it may never be recognized, being at 2 opposite phenotypic poles, harmless to the bearer, or, if harmful, eventually responsible for uncharacteristic although perhaps serious conditions. UPD can also be associated with problems such as recessiveness or mosaicism. This article considers the chances of unmasking UPD, in the course of CVS or AC prenatal diagnosis, by reviewing the main cytogenetic signals and major familial or personal antecedents raising its suspicion. Once suspected, the lead toward UPD may or may not be followed through appropriate molecular studies. UPD for either maternal or paternal chromosomes 13, 21 and 22 may not have consistent, common deleterious effects, while other identified UPD's are too rare to call. Unconditionally, main, consistent or near consistent damages to the phenotype have been traced to specific chromosome pairs such as 15 mat (Prader-Willi syndrome), 15 pat (Angelman syndrome), 11 pat (Wiedemann-Beck with syndrome), 14 mat and pat (multiple cogenital and developmental anomalies [MCDA]-several rather constant) and 7 mat (Russel-Silver [RS] and Growth-failure [GF]). The above problems all stem from an alteration of the normal, developmentally important genomic imprinting processes and most of them may recognize several etiopathogenic paths, other than UPD, none of which abides by straight Mendelian rules. In this very area, therefore, a new, non-traditional type of inheritance is confronting genetic counselling. In this paper, for want of appropriate semantic language, the neologism "likon" (or "laïkon") is coined to make reference to the hemizygously expressed sequences of the genomic parts imprinted in the somatic tissues. Broadening the definition, the word is then applied to the 4 possible epigenotypes of imprinted domains, which depend on the parental sex-of-origin: germinally "resting" (R), or "acting" (A), to be made somatically silent, that is to say "unexpressed" (U), or transcribed and "expressed" (E), thus abbreviated as EA, ER, UA and UR. Entire pedigrees may then be analyzed accordingly in health and in disease. Examples are presented herewith.