H221Y was first reported related to NRTI treatment in 2003 [9
], but in recent study, H221Y was concerned associated with exposed to NNRTIs [6
]. Ceccherini-Silberstein et al. found the prevalence rate of H221Y in isolates from patients failing NVP treatment was 10.3% and the mutation was included in the top 10 and 15 determinants for NVP and EFV resistance, respectively, ranking even above some classical NNRTI resistance mutations, such as K101E, V108I, and G190E [6
]. The group found H221Y was strongly associated with the use of NVP and showed positive interactions with Y181C and was also negatively associated with the use of ZDV and with TAMs (particularly TAMs-2, such as D67N, K70R, K219Q/E, and T215F) and was then associated with an increased susceptibility to ZDV. When H221Y was copresent with TAMs-2, the ZDV susceptibility was even greater than that observed when TAMs-2 were copresent with Y181C. The presence of H221Y along with Y181C was associated with a 12.4-fold increase in NVP resistance. Jiang et al. found that the occurrence of the H221Y was 19.8% in 91 patients receiving nevirapine and lamivudine plus stavudine (57.1%) or zidovudine (42.9%) [10
]. Reuman et al. analyzed viruses from 13039 individuals with sequences containing at least one of 52 published NNRTI-selected mutations; the frequency of H221Y among 1510 viruses from individuals who received nevirapine but no other NNRTI was 12%. H221Y occurred along with Y181C in our study (data not shown) and the virus with mutation pattern K103N/Y181C/H221Y can replicate stably in vitro without drug pressure [8
]. Our study showed that as for the mutation profiles K101Q/Y181C, K101Q, V179D/Y181C, V179D, K103N/Y181C, and K103N, the presence of H221Y, respectively lead to 2.2 ± 0.9, 3.2 ± 0.3, 3.6 ± 0.5, 3.0 ± 0.4, 2.1 ± 0.5, and 2.2 ± 0.1-fold increase in NVP resistance.
Y181C is a very important NNRTIs-related drug resistant mutation, and the mutation can cause high-level resistance to NVP and DLV and low-level resistance to EFV; otherwise, this mutation can increase susceptibility to AZT and TDF. Sungkanuparph group reported the prevalence rate of Y181C was 59.5% among the 158 NNRTI failure patients (for a median NNRTI treatment period of 88 weeks) [11
]. Taiwo et al. found the frequency of Y181C was 42.9% in the patients receiving NNRTI treatment for a median period of 53 weeks [12
]. Y181C presents in 40% of patients failing NVP and has the third-greatest weight in the SVR (support vector regression) model for NVP resistance [6
]. In Reuman group's study [13
], Y181C is the most common resistant mutation among the sequences from patients receiving NVP (48%), and nearly 17% (n
= 2233) sequences contained three or more NNRTIs-resistant mutations and these NNRTIs-resistant profiles often carried Y181C and occurred with one or more thymidine analogue mutations, which suggested these mutation pattern might significantly associated with the function of HIV-1 RT. Many studies have reported the phenotype resistance of Y181C, Bacheler et al. [14
] found the virus with K103N/Y181C and other mutations conferring above 1600-fold resistance to NVP, and Qari et al. [15
] covered the virus containing K101E/Y181C/G190A and other mutations could increase 893-fold resistance to NVP. As we have known [16
], Y181C copresent with TAMs2 could increase the susceptibility to ZDV. In our study, mutation patterns K101Q/H221Y, K101Q, V179D/H221Y, V179D, K103N/H221Y, and K103N, Y181C could, respectively, improve 759.1 ± 41.9, 1297.0 ± 289.1, 390.0 ± 101.6, 312.5 ± 45.3, 41.9 ± 8.4, and 47.9 ± 4.2-fold to NVP resistance.
Although H221Y mutation alone just increases 2.1 ~ 3.6-fold resistance to NVP, the mutation could improve 100.6 ~ 3444.6-fold resistance to NVP when it copresent with Y181C. To be specific, to K101Q, V179D, and K103N, Y181C/H221Y could confer 3444.6 ± 834.5, 1132.6 ± 180.4, and 100.6 ± 32.5-fold resistance to NVP.
Ceccherini-Silberstein et al. [6
] described the resistant characters of 9 novel NNRTI-related mutations including K101Q, I135M/T, H221Y, K223E/Q, L228H/R, and V179I. In these novel mutations, K101Q and I135T copresented with K103N which related to the increase resistance of EFV and NVP. Residue 101 may establish hydrogen bonds with EFV and may directly interact with correlated residue 102 by van der Waals interaction. In previous study, the appearance of K101Q might be based on the occurrence of K103N [17
]. In other study [18
], residue 101 showed negative association with HLA-A2 genotype which suggested, in the presence of HLA-A2 restricted immune response, position 101 may be under negative selective pressure that favors the preservation of the wild-type amino acid. As for residue 179, only V179F mutation did not decrease the susceptibility to etravirine; however, when the mutation combined with Y181C, the viral etravirine susceptibility could be reduced more than 100-fold [19
]. Thus receiving NNRTIs treatment on long term might result in the accumulation of accessory mutations and lead to higher level of drug resistance.
With the upcoming approval of the new NNRTIs, understanding the efficacy of these NNRTIs mutation profiles shows more and more importance. Further in vitro and clinical studies are therefore necessary to confirm the efficacy of these new mutation patterns. These studies will provide information for deeper understanding of the mechanisms of drug resistance.