RESUMEN
The 9p trisomy is a relatively frequent disorder, while pure 9p trisomies are less frequent and usually derived from 9;22 translocations, duplications or 9p extra chromosomes. Here we report a patient with pure trisomy 9p derived from a terminal balanced unreciprocal translocation. The patient derived to the genetic service by psychomotor delay, presented at 2 years and 11 months: short stature, open anterior fontanelle, dysplastic ears, facial dysmorphisms, long and broad first toes with hypoplastic nails, central nervous system and skeletal alterations. The patient karyotype was: 46,XY,der(10)t(9;10) (p13.1;qter)mat while the mother karyotype was: 46,XX,t(9;10)(p13.1;qter). The presence of the subtelomeric region of 10q showed by FISH as well as the duplication of 9p subtelomere was further confirmed with multiplex ligation dependent probe amplification (MLPA) for the subtelomeric region of all chromosomes. The mechanism of formation seems to be due to a telomere break in 10q leading to loss of telomeric functions, permitting the 9p fusion; this has been supported with molecular probes showing telomere shortening in interstitial telomeric repeats, which are unable to prevent chromosome fusion. This is one of the few cases reported with terminal translocations (not jumping) preserving the subtelomeric region and highlights the importance of subtelomeric probes in terminal arrangements, and the utility of molecular probes, such as MLPA in defining this kind of abnormalities. In the clinical context, the patient presented a high proportion of 9p trisomy features which is expected considering the large 9p segment involved and the presence of the critical region 9p22.