Balkan Journal of Medical Genetics

MOSAIC INTRACHROMOSOMAL TRIPLICATION OF (12)(p11.2p13) IN A PATIENT WITH PALLISTER-KILLIAN SYNDROME
Yakut S, Mıhcı E, Altiok Clark O, Cetin Z, Keser I, Berker S, Luleci G
*Corresponding Author: Professor Dr. Guven Luleci, Department of Medical Biology and Genetics, School of Medicine, Akdeniz University, Arapsuyu, Antalya, Turkey; Tel.: +90-242-249-69-70; Fax: + 90- 242-227-44-95; E-mail: luleci@akdeniz.edu.tr
page: 61

DISCUSSION

Intrachromosomal triplications are rare events and the middle segment usually being inverted in orientation, as in our case [8]. In the majority of cases, triplications were interstitial, whereas there is only one published report of triplication of a whole chromosome arm [9]. In most cases, triplications had originated from maternal chromosomes [13]. Our case is the second patient with intrachromosomal triplication of a whole chromosome arm. To the best of our knowledge, only two patients with intrachromosomal triplication of the short arm of chromosome 12 have been presented in the literature. In the article by Eckel et al. [8], the 12p11.22- p12.3 region was triplicated and this region does not cover the critical PKS region, which defi ned the chromosomal region 12pter-p12.3. Therefore, the patient’s clinical phenotype was similar to trisomy 12p syndrome rather than the PKS. Unexpectedly, the intrachromosomal triplication was found in all peripheral blood lymphocytes. Also, this intrachromosomal triplication was found in 12% of the mother’s peripheral blood lymphocytes. The clinical fi ndings were compatible with the PKS that were reported by Powis et al. [9]. During conventional cytogenetic analyses, triplicated chromosome 12 was observed in 30% of the cultured fi broblasts but not in peripheral blood lymphocytes. However, array-based comparative genomic hybridization analysis showed a triplication of the 12p in the peripheral blood lymphocytes in a low level mosaicism. A parental transmission of the triplication could not be excluded since parental blood samples were not available [9]. However, this has been excluded in our patient because her parents’ karyotypes were normal. Mechanisms for the formation of the intrachromosomal triplication include; i) fusion of the inverted duplicated supernumerary marker chromosome with the normal homologue; ii) unequal crossover or interhomologue translocation followed by the inverted insertion at the former breakpoint junction; iii) two U-type exchanges among three chromatids [14-16]. Powis et al. [9] speculated that triplication of chromosome 12p could have arisen from telomere to telomere fusion of supernumerary analphoid isochromosome 12p with a normal chromosome 12. Indeed, there are three reports about PKS patients with analphoid inverted duplicated supernumerary marker chromosomes consisting of chromosome 12p [4,17-18]. The clinical fi ndings of the patient reported by Powis et al. [9] included hypotonia, brachycephaly with upslanting palpebral fi ssures, thin upper lip, low nasal bridge, high arched palate, a single transverse palmar crease on each hand and abnormal hair pattern. The clinical differences between this case and our case could result from differences in the degree of mosaicism and in the distribution of abnormal cells in different tissues. As a result, we report here the third patient with intrachromosomal triplication of the short arm of chromosome 12. We conclude that intrachromosomal triplication might be a new mechanism of formation for PKS.

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