+ 5 or trisomy 5

In childhood ALL, an extra chromosome 5 is commonly encountered in cases with hyperdiploidy >50 chromosomes. The presence of trisomy 5 in high hyperdiploid childhood ALL is associated with a less favourable clinical outcome. Trisomy 5 as a sole abnormality in ALL is exceedingly rare and described in only 3 cases, including 2 adult ALL and 1 paediatrics case occurring in a 12-year old girl. Trisomy 5 has been described in 19 cases of AML. Gain of chromosome 5 usually occurs in association with other cytogenetics aberrations, although very rarely it may exist as the sole abnormality. A case of AML-M2 with normal karyotype at diagnosis showed trisomy 5 as the sole abnormality at relapse, and a case of AML-M5 showed trisomy 5 as the only chromosome aberration in 3% of 59 metaphases at presentation. A further case of AML-M4 showed a clone with trisomy 5 as the sole abnormality together with a second clone with trisomy 5 and evolutionary change. In the other 16 cases, trisomy 5 was found in association with numerical (n=4), structural changes (n=4), or both numerical and structural changes (n=8). A number of interesting observations with respect to AML and trisomy 5 should be noted. First, an association between trisomy 5 and t(8;21) (n=3) and trisomy 8 (n=6) is observed. Second, five out of 6 cases with concurrent trisomies 5 and 8 show monocytic differentiation and are diagnosed as either AML-M4 or M5. Finally, trisomy 5 has been described in all FAB subtypes of AML except acute promyelocytic leukaemia. Given the rarity of trisomy 5 in AML, it is possible that the associated cytogenetic aberrations such as t(8;21) or trisomy 8 and not trisomy 5 per se that predicts for the myeloid phenotype.