HIV integrase variability and genetic barrier in antiretroviral naïve and experienced patients

  • Antonio Piralla1,

    Affiliated with

    • Stefania Paolucci1,

      Affiliated with

      • Roberto Gulminetti2,

        Affiliated with

        • Giuditta Comolli1, 3 and

          Affiliated with

          • Fausto Baldanti1Email author

            Affiliated with

            Virology Journal20118:149

            DOI: 10.1186/1743-422X-8-149

            Received: 19 January 2011

            Accepted: 31 March 2011

            Published: 31 March 2011

            Abstract

            Background

            HIV-1 integrase (IN) variability in treatment naïve patients with different HIV-1 subtypes is a major issue. In fact, the effect of previous exposure to antiretrovirals other than IN inhibitors (INI) on IN variability has not been satisfactorily defined. In addition, the genetic barrier for specific INI resistance mutations remains to be calculated.

            Methods

            IN variability was analyzed and compared with reverse transcriptase (RT) and protease (PR) variability in 41 treatment naïve and 54 RT inhibitor (RTI) and protease inhibitor (PRI) experienced patients from subjects infected with subtype B and non-B strains. In addition, four HIV-2 strains were analyzed in parallel. Frequency and distribution of IN mutations were compared between HAART-naïve and RTI/PI-experienced patients; the genetic barrier for 27 amino acid positions related to INI susceptibility was calculated as well.

            Results

            Primary mutations associated with resistance to INI were not detected in patients not previously treated with this class of drug. However, some secondary mutations which have been shown to contribute to INI resistance were found. Only limited differences in codon usage distribution between patient groups were found. HIV-2 strains from INI naïve patients showed the presence of both primary and secondary resistance mutations.

            Conclusion

            Exposure to antivirals other than INI does not seem to significantly influence the emergence of mutations implicated in INI resistance. HIV-2 strain might have reduced susceptibility to INI.

            Background

            Raltegravir (MK-0518; Isentress, Merck) was the first integrase (IN) inhibitor approved for treatment of HIV infection [1], while other compounds such as GS-9137 [2], S-1360 [3], and L-870,810 [4] are at different stages of development. Raltegravir (RAL) has shown potent and durable antiretroviral activity in both treatment naïve [5, 6] and highly experienced HIV-1-infected individuals. Due to its novel mechanism of action, RAL was shown to be effective also against HIV-1 strains resistant to reverse transcriptase (RT), protease (PR) and entry inhibitors, both in vitro [7] and in vivo [8, 9]. However, it has been observed that failure of highly active antiretroviral therapy (HAART) including RAL might be related to the emergence of drug-resistant virus variants [1015], and amino acid changes associated with resistance to integrase inhibitors (INI) have been reported [1618]. In particular, Y143R/C, N155H and Q148K/R/H have been identified as primary RAL resistance mutations, usually associated with secondary mutations often already present at baseline [10, 12, and 13]. However, the entire panel of mutations associated with RAL resistance has not been fully ascertained. Nor do we fully understand the potential impact of naturally occurring ancillary mutations with respect to: i) promotion of RAL resistance associated mutations, ii) improvement of the activity of mutated IN and iii) HIV-1 replicative capacity. In addition, it is unclear whether drug pressure on the Pol gene by RT and PR inhibitors might influence the emergence of primary or ancillary RAL mutations. Finally, it has been observed that about 20% of new HIV infections are now sustained in Italy by a wide variety of subtype non-B HIV-1 strains and a few HIV-2 strains [19, 20]. Thus, it is important to define the variability of the IN gene in treatment-naïve and HAART-experienced patients in different HIV-1 subtypes.

            The aims of the study were: i) to evaluate IN variability and polymorphism distribution among patients naïve for RAL treatment; ii) to better understand whether previous HAART treatment not including RAL might be associated with the emergence of mutations conferring resistance to INI; iii) to calculate the genetic barrier of primary and secondary mutations associated with INI resistance in different HIV-1 subtypes.

            Materials and methods

            Patients

            IN variability was analyzed using stored plasma samples from 95 consecutive patients infected with HIV-1, as well as four HIV-2 positive patients referred to our Institution in the period December 2008 - December 2009. Patients with no available plasma samples or viral load < 1,000 HIV RNA copies/ml plasma were excluded from the analysis. Eligible patients were stratified on the basis of treatment history as follows: i) HAART-naïve patients, ii) RT and PR inhibitor-experienced but RAL-naïve (RTI/PI-experienced).

            Real-time RT-PCR, RT-PCR and sequencing

            HIV-1 plasma RNA levels were determined using the Versant HIV-1 RNA 3.0 Assay (Bayer, NY, USA), while HIV-2 plasma RNA levels were determined according to an in house developed real-time RT PCR [21]. For IN gene sequencing (codons 1-277), a region of the HIV-1 Pol gene was amplified in a nested-RT-PCR using primers Int1F, 5'- CAT GGG TAC CAG CAC ACA CAA AGG-3' and Int1R, 5'-CCA TGT TCT AAT CCT CAT CCT GTC -3' for the first PCR round, while primers Int2F 5'-GGA ATT GGA GGA AAT GAA CAA GTA GAT -3' and Int2R 5'-GCC ACA CAA TCA TCA CCT GCC ATC-3' were used in the second PCR round [12]. The first nested-RT-PCR reaction was performed in 50 μl using the SuperScript™ III Platinum® One-Step qRT-PCR System (Invitrogen, Carlsbad, CA, USA) with the following thermal profile: 30 min at 50°C and 10 min at 95°C for 1 cycle, 1 min at 95°C, 1 min at 52°C and 1 min and 10 sec at 72°C for 50 cycles followed by 10 min at 72°C. The nested PCR reaction was performed in 100 μl using TaqGold and the relevant buffer (Applied Biosystem, Foster City, CA, USA) with the following thermal profile: 10 min at 95°C for 1 cycle, 1 min at 95°C, 1 min at 50°C, and 1 min and 10 sec at 72°C for 30 cycles, followed by 10 min at 72°C [12]. RT and PR genes were sequenced in parallel [22]. Sequencing of amplicons was performed using an ABI PRISM 3100 Genetic Analyzer® (Applied Biosystem, Foster City, CA, USA) with the ABI PRISM™ Big Dye Terminator Cycle Sequencing Reaction kit.

            Sequence analysis

            IN, RT, and PR sequences were analyzed with the MEGA 4.0 version software [23]. Sequence distances were calculated using the Simmonic sequence editor (version 1.6) program [24], with the Kimura 2-Parameter as a distance estimated method. Divergence was defined as the mean proportion of nucleotide or amino acid differences between all sequence pairs. In each patient, only the predominant virus strain was taken into account to calculate variability. Integrase variability was also calculated in three functional domains: N-terminal domain (NTD), catalytic core domain (CCD), and C-terminal domain (CTD).

            Genotypic resistance and genetic barrier

            Resistance to antiretrovirals was estimated on the basis of the Stanford HIV drug resistance database report (http://​hivdb.​stanford.​edu) and the geno2pheno® report (http://​integrase.​bioinf.​mpi-inf.​mpg.​de/​index.​php). Twenty-seven IN positions related to 28 mutations in INI resistant HIV-1 strains were categorized as follows: i) primary mutations (E92Q, F121Y, E138A, G140A/S, Y143R/C/H, S147G, Q148H/R/K, S153Y, N155H, R263K) ii) ancillary mutations (H51Y, T66I, L74A/M/I, Q95K, T97A, E138K, Q146P V151I, E157Q,G163R, I203M, S230R) and iii) mutations with an uncertain role (V72I, T125K, A128Y, K160D, V165I, V201I; http://​hivdb.​stanford.​edu).

            The genetic barrier to INI-resistance was calculated in 27 IN amino acid positions with a method previously published by van de Vijver et al. [25]. The smallest number of transitions (scored as 1) or transversions (scored as 2.5) were used to calculate the genetic barrier. The genetic barrier was calculated with the sum of scores obtained for each amino acid position.

            Statistical analysis

            Statistical analyses were performed using GraphPad Prism (version 4.0) software (San Diego, CA, USA). To compare the nucleotide and amino acid divergence between groups of patients the Mann Whitney U-test was utilized, while the chi-square test was used for comparing the calculated genetic barrier for major and minor substitutions between groups of patients.

            Results

            Study population

            Nucleotide (nt) and amino acid (aa) integrase variability was analyzed in 41 HAART-naïve patients, and 54 RTI/PI-experienced patients. Four HIV-2 strains were also analyzed. Among HIV-1 strains from HAART-naïve patients, 16 were subtype B and 25 were non-B strains (A, no.3; C, no.5; G, no.3; F, no.6; CRF02AG, no.6; CRF01AE, no.1; CRF12BF, no.1;). Among HIV-1 strains from RTI/PI-experienced patients, 19 were subtype B and 35 were subtype non-B strains (A, no.1; A/K no. 2; C, no.2 D, no.3; G, no.3; D/F, no.1; F, no.11; CRF02AG, no.9; CRF01AE, no.2; CRF 19CPX, no.1). For comparison, RT and PR gene sequence variability was analyzed in HAART-naïve and RTI/PI-experienced patients (Table 1). HIV-2 strains were from RTI/PI-experienced patients.
            Table 1

            Conserved and variable amino acid distribution in Integrase sequences and amino acid divergence among RT, PR and IN sequences in HIV-1

            Category (strain no.)

            Mean nucleotide divergence ± SD

            Mean amino acid divergence ± SD

            IN conserved amino acid (%)

            IN variable amino acid (%)

            IN conserved amino acid in three functional domains (%)

             

            RT gene

            PR gene

            IN gene

            RT gene

            PR gene

            IN gene

             

            Singleton a

            Parsimony b

            NTD (1-50)

            CCD (51-212)

            CTD (213-277)

            HAART-naïve (41)

            9.2 ± 2.3

            10.5 ± 2.9

            8.8 ± 2.5

            6.3 ± 1.8

            11.3 ± 3.5

            7.0 ± 1.8

            180 (65.9)

            36 (13.2)

            57 (20.9)

            26 (52.0)

            114 (70.4)

            40 (62.5)

            RT/PI-experienced (54)

            9.8 ± 2.5

            12.2 ± 3.6

            8.3 ± 2.5

            9.8 ± 2.6

            18.8 ± 7.8

            6.5 ± 1.9

            170 (62.3)

            39 (14.3)

            64 (23.4)

            26 (52.0)

            103 (63.6)

            41 (64.1)

            HAART-naïve vs RT/PI-experienced

            p < 0.001

            p < 0.001

            p < 0.001

            p < 0.001

            p < 0.001

            p < 0.001

            ns

            ns

            ns

            ns

            ns

            ns

            HIV-2 (4)

            ND

            ND

            13.0 ± 1.1

            ND

            ND

            7.9 ± 1.4

            241(88.3)

            9(3.3)

            23 (8.4)

            39 (78.0)

            149 (91.9)

            53 (82.8)

            HIV-2 vs HIV-1

              

            p < 0.001

              

            p < 0.001

            p < 0.001

            p < 0.001

            p < 0.001

            p < 0.001

            p < 0.001

            p < 0.001

            SD: standard deviation; ND: not done; ns: not significant; NTD: N-terminal domain; CCD: catalytic core domain; CTD: C-terminal domain; RT: reverse trancriptase; PR: protease; IN: integrase.

            a A singleton site contains at least two types of amino acid with, at most, one occurring multiple times.

            b Parsimony-informative if it contains at least two types of amino acid, and at least two of these occur with a minimum frequency of two.

            c Significant p-values are reported.

            HIV IN, RT, and PR variability

            Both IN nucleotide and amino acid variability was higher in HAART-naïve patients with respect to RTI/PI-experienced patients (p < 0.001; Table 1). Conversely, RT and PR nucleotide variability was higher in RTI/PI-experienced patients with respect to HAART-naïve patients (p < 0.001).

            Gene variability in the HIV-1 B and non-B subtypes was also analyzed. In subtype B strains, IN amino acid variability was statistically higher in HAART-naïve patients with respect to RTI/PI-experienced patients (6.1 ± 1.5 vs 4.1 ± 1.3; p < 0.001). However, both RT (8.3 ± 2.3 vs 4.5 ± 1.2; p < 0.001) and PR (20.3 ± 7.5 vs 8.9 ± 2.8; p < 0.001) amino acid variability was higher in strains from RTI/PI-experienced patients with respect to HAART-naïve patients. In subtype non-B strains, IN amino acid variability was not statistically different in naïve patients with respect to RTI/PRI experienced patients (6.9 ± 1.9 vs 7.1 ± 1.8; p > 0.05), whereas RT (8.0 ± 2.2 vs 7.0 ± 1.9; p < 0.001) and PR (13.3 ± 6.2 vs 10.8 ± 4.1; p < 0.001) amino acid variability was higher in RTI/PI-experienced patients with respect to HAART-naïve patients.

            In Table 1, the number of conserved and variable amino acid residues of the IN gene in each group of virus strains is shown. The number of conserved residues in sequences from HAART-naïve patients and RTI/PI-experienced patients was comparable (Table 1). No differences between singleton sites and parsimony informative sites were observed among the variable residues in all patient groups (Table 1).

            When the three functional domains of the IN gene were individually analyzed, the amino acid sequences from HAART-naïve patients were slightly more conserved than sequences from RTI/PI-experienced patients. In the analysis of amino acid variability in the three structural domains for all sequence categories, a higher variability in the NTD with respect to the CCD and the CTD was observed (p < 0.001; data not shown).

            Frequency, distribution and genetic barrier of IN resistance mutations

            In Table 2, mutations in positions associated with INI susceptibility are shown. The eight primary mutations associated with RAL or elvitegravir (EGV) resistance were not present in any of the strains from INI-naïve patients.
            Table 2

            HIV-1 and HIV-2 amino acid polymorphisms at positions associated with INI (RAL and EGV) resistance

            Mutation categories

            Known amino acid substitution

            Rate of INI resistance mutations in INI-naïve patients (%)

            HIV-1 amino acid substitution

            HIV-2 clade A ROD subtype consensus

            HIV-2 substitution (4 strains)

               

            HAART-naïve (n = 41)

            RTI/PI-experienced (n = 54)

              

            Primary

            E92Q

               

            E

             

            mutations

            F121Y

               

            F

             
             

            E138A

               

            S

            T3

             

            G140A/S

               

            G

             
             

            Y143R7C/H

               

            Y

             
             

            S147G

               

            S

             
             

            Q148H/R/K

               

            Q

             
             

            S153Y

             

            A 1

             

            A

             
             

            N155H

               

            N

             
             

            R263K

               

            R

             

            Secondary mutations

            H51Y

               

            H

             
             

            T66I

               

            T

             
             

            L74A/M/I

            7 (7.4)

            I2/M1

            I4

            I

             
             

            Q95K

              

            R 1

            R

             
             

            T97A

            2 (2.1)

            S 1

            A2

            T

             
             

            E138K

              

            D 1

            S

            T3

             

            Q146P

               

            N

             
             

            V151I

               

            V

             
             

            E157Q

            3 (3.2)

            Q2

            Q1

            H

             
             

            G163R

              

            E 2 /N 1 /A 1

            S

            G2

             

            I203M

            3 (3.2)

            M2

            M1

            M

             
             

            S230R/M

             

            N 4

            N 2

            G

             

            Polymorphic and non-polymorphic mutations

            V72I

            50 (52.6)

            I22

            I28/D 1

            I

            V2

             

            T125K

             

            A 19 /M 1 /V 1

            A 24 /V 4 /P 1

            E

            D2

             

            A128T

               

            M

             
             

            K160D

             

            Q 1 /R 1

            Q 1

            N

             
             

            V165I

            18 (18.9)

            I8

            I10

            I

             
             

            V201I

            68 (71.6)

            I30

            I38

            I

             

            RTI: reverse trancriptase inhibitor; PI: protease inhibitor; INI: integrase inhibitor.

            New polymorphisms are reported in boldface.

            In the 41 HAART-naïve patients, 6 secondary mutations were found: V72I (22/41, 53.7%), L74A/M/I (3/41, 7.3%), E157Q (2/41, 4.9%), V165I (8/41, 19.5%), V201I (30/41, 73.2%) I203M (2/41. 4.9%) (Table 2). Additionally, five substitutions of unknown significance (T97S, T125A/M/V, S153A, K160Q/R, S230N) were found.

            In the 54 RTI/PI-experienced patients, 7 secondary mutations were found: V72I (28/54, 51.9%), L74A/M/I (4/54, 7.4%), T97A (2/54, 3.7%), E157Q (1/54, 1.9%), V165I (10/54, 18.5%), V201I (38/54, 70.4%) and I203M (1/54, 1.9%; Table 2). Besides, seven substitutions of unknown significance (V72D, Q95R, T125A/V/P, E138D, K160Q, G163E/N/A and S230N; Table 2) were found. No statistical difference in the prevalence of secondary mutations between sequences from HAART-naïve and RTI/PI-experienced patients was observed (p < 0.05)

            Analysis of codon usage distribution between sequences from HAART-naïve and RTI/PI-experienced patients showed that there was no significant difference in the INI resistance mutations with the exception of position 148 (Table 3). At this position, a significant difference in predominant codon usage (CAA vs CAG) between sequences from HAART-naïve and RTI/PI-experienced patients was shown (p = 0.01).
            Table 3

            Codon distribution and calculated genetic barrier at 27 integrase inhibitor susceptible positions in HIV-1 INI-naïve patients

            IN codon position

            Substitution

            Wild type codon

            Codon % distribution

            Mutational resistance codon

            Lower score

            IN codon position

            Substitution

            Wild type codon

            Codon % distribution

            Mutational resistance codon

            Lower scores

               

            HAART-naïve (n = 41)

            RTI/PI-experienced (n = 54)

                 

            HAART-naïve (n = 41)

            RTI/PI-experienced (n = 54)

              

            51

            H51Y

            CAT

            98

            100

            TAT

            1

            143

            Y143C

            TAC

            90

            90

            TGC

            1

              

            CAC

            2

            0

            TAC

            1

              

            TAT

            10

            10

            TGT

            1

            66

            T66I

            ACA

            98

            94

            ATA

            1

             

            Y143R

            TAC

            90

            90

            CGC

            2

              

            ACC

            0

            6

            ATC

            1

              

            TAT

            10

            10

            CGT

            2

              

            ACG

            2

            0

            ATA

            2

            146

            Q146P

            CAA

            98

            98

            CCG

            2.5

            72

            V72I

            GTT

            32

            31

            ATT

            1

              

            CAG

            2

            2

            CCA

            2.5

              

            GTC

            15

            6

            ATC

            1

             

            Q146K

            CAA

            98

            98

            AAA

            2.5

              

            GTA

            0

            6

            ATA

            1

              

            CAG

            2

            2

            AAG

            2.5

              

            GTG

            0

            4

            ATA

            2

            147

            S147G

            AGT

            76

            70

            GGT

            1

             

            I72

            ATT

            41

            45

             

            0

              

            AGC

            24

            30

            GGC

            2

              

            ATC

            12

            2

             

            0

            148

            Q148H

            CAA

            80

            96

            CAT/C

            2.5

              

            ATA

            0

            4

             

            0

              

            CAG

            20

            4

            CAT/C

            2.5

             

            D72

            GAT

            0

            2

            ATT

            3.5

             

            Q148K

            CAA

            80

            96

            AAA

            2.5

            74

            L74I

            CTG

            72

            78

            ATA

            3.5

              

            CAG

            20

            4

            AAG

            2.5

              

            CTA

            10

            15

            ATA

            2.5

             

            Q148R

            CAA

            80

            96

            CGA

            1

              

            TTA

            10

            2

            ATA

            2.5

              

            CAG

            20

            4

            CGG

            1

              

            TTG

            2

            0

            ATA

            3.5

            151

            V151I

            GTA

            71

            72

            ATA

            1

             

            I74

            ATT

            0

            0

             

            0

              

            GTG

            29

            28

            ATA

            2

              

            ATA

            2

            7

             

            0

            153

            S153Y

            TCT

            76

            87

            TAT

            2.5

             

            M74

            ATG

            2

            0

            ATA

            1

              

            TCC

            15

            11

            TAC

            2.5

            92

            E92Q

            GAA

            63

            61

            CAA

            2.5

              

            TCA

            7

            2

            TAT/C

            5

              

            GAG

            37

            39

            CAG

            2.5

             

            A153

            GCC

            2

            0

            TAC

            5

            95

            Q95K

            CAA

            32

            28

            AAA

            2.5

            155

            N155H

            AAT

            90

            93

            CAT

            2.5

              

            CAG

            68

            70

            AAG

            2.5

              

            AAC

            10

            7

            CAC

            2.5

             

            R95

            CGG

            0

            2

            AAG

            3.5

             

            N155S

            AAT

            90

            93

            AGT

            1

            97

            T97A

            ACA

            98

            96

            GCA

            1

              

            AAC

            10

            7

            AGC

            1

             

            A97

            GCA

            0

            4

             

            0

            157

            E157Q

            GAA

            88

            91

            CAA

            2.5

             

            S97

            TCA

            2

            0

            GCA

            2.5

              

            GAG

            7

            7

            CAG

            2.5

            121

            F121Y

            TTC

            90

            98

            TAC

            2.5

             

            Q157

            CAA

            5

            2

             

            0

              

            TTT

            10

            2

            TAT

            2.5

            160

            K160D

            AAA

            85

            93

            GAT/C

            3.5

            125

            T125K

            ACA

            27

            18

            AAG

            2.5

              

            AAG

            11

            5

            GAT/C

            3.5

              

            ACT

            10

            4

            AAA/G

            5

             

            R160

            AGA

            2

            0

            GAT

            4.5

              

            ACG

            12

            24

            AAG

            2.5

             

            Q160

            CAA

            2

            2

            GAT/C

            5

             

            A125

            GCA

            44

            33

            AAA

            3.5

            163

            G163R

            GGA

            63

            57

            AGA

            1

              

            GCG

            2

            6

            AAG

            3.5

              

            GGG

            27

            39

            AGG

            1

              

            GCT

            0

            6

            AAA/G

            6

              

            GGT

            0

            2

            CGC,AGA/G

            3.5

             

            V125

            GTG

            0

            7

            AAG

            3.5

             

            E163

            GAA

            0

            4

            AGA

            2

              

            GTA

            2

            0

            AAA

            3.5

             

            N163

            AAC

            0

            2

            CGC,AGA/G

            3.5

             

            M125

            ATG

            2

            0

            AAG

            2.5

             

            A163

            GCG

            0

            2

            CTG,AGG

            3.5

             

            P125

            CCG

            0

            2

            AAG

            5

            165

            V165I

            GTA

            56

            52

            ATA

            1

            128

            A128T

            GCA

            54

            50

            ACA

            1

              

            GTC

            22

            18

            ATC

            1

              

            GCC

            37

            46

            ACC

            1

              

            GTT

            2

            10

            ATT

            1

              

            GCT

            9

            4

            ACT

            1

              

            GTG

            0

            2

            ATA

            2

            138

            E138K

            GAA

            100

            98

            AAA

            1

             

            I165

            ATA

            20

            18

             

            0

             

            D138

            GAC

            0

            2

            AAA/G

            3.5

            201

            V201I

            GTA

            24

            28

            ATA

            1

            140

            G140S

            GGA

            46

            37

            AGT/C

            3.5

              

            GTG

            2

            2

            ATA

            2

              

            GGC

            44

            46

            AGC

            1

             

            I201

            ATA

            74

            70

             

            0

              

            GGT

            5

            6

            AGT

            1

            203

            I203M

            ATA

            95

            98

            ATG

            1

              

            GGG

            5

            9

            AGT/C

            3.5

             

            M203

            ATG

            5

            2

             

            0

             

            G140A

            GGA

            46

            37

            GCA

            2.5

            230

            S230R

            AGC

            90

            96

            AAC

            1

              

            GGC

            44

            46

            GCC

            2.5

             

            N230

            AAC

            10

            4

             

            0

              

            GGT

            5

            6

            GCT

            2.5

            263

            R263K

            AGA

            85

            96

            AAA

            1

              

            GGG

            5

            6

            GCT/G

            2.5

              

            AGG

            15

            4

            AAG

            1

            The amino acids differing from wild-type or expected mutant are in boldface.

            HIV-2 variability and mutation distribution

            HIV-2 strains showed higher conservation with respect to HIV-1 subtype B and non-B strains (p < 0.001). In detail, in HIV-2 strains, 241/273 (88.3%) conserved sites and 32/273 (11.7%) variable sites were observed (Table 1). The analysis of four HIV-2 integrase genes showed that the mean nucleotide and amino acid variability was 13.0% ± 1.1 and 7.9% ± 1.4, respectively. Three out of four HIV-2 strains analyzed (all from RTI/PI-experienced patients but INI naïve) showed the presence of an E to T polymorphism, in a position where a primary mutation (E138A normally associated with RAL-resistant HIV-1 strains) was detected. Three secondary mutations I72V, E125D and S163D (Table 2) were also detected.

            Discussion

            The introduction of the new INI antiviral-drug class [6, 26] is an important step forward in the treatment of HIV-1 infection [8]. Despite the success of HAART in managing HIV-1 infection, the development and worldwide spread of HIV-1 drug resistant strains remain serious issues. In this study, we analyzed the IN gene variability in parallel with RT and PR variability with the aim of evaluating whether HIV genetic background influences the appearance of IN mutations. The distribution of specific amino acids implicated in INI susceptibility was also compared in different HIV-1 subtypes and in patients naïve for treatment or exposed to RTI and PI. The simultaneous evaluation of RT, PR and IN identity showed that the IN gene had lower amino acid variability. This would confirm the high level of integrase sequence conservation reported by Rhee et al. [27]. Moreover, the analysis of three functional integrase domains showed only a small difference in the catalytic core domain.

            The evaluation of IN variability in different patient categories showed no differences in subtype non-B and a higher divergence in subtype B strains from HAART-naïve compared with RTI/PI-experienced patients. These results are in contrast with the previously reported greater amino acid IN divergence in RT/PI-experienced patients [28]. On the other hand, we observed a greater RT and PR amino acid divergence in both B and non-B strains from RTI/PI-experienced patients with respect to HAART-naïve patients. These findings are consistent with the hypothesis that multiple rounds of positive selection by subsequent HAART regimens including different RTI and PI, may lead to the emergence of a wider number of RT and PR variants in contrast with little or no change in the IN gene.

            In keeping with previous studies [13, 2932] no primary IN mutations associated with INI susceptibility were present in strains from INI-naïve patients. However, in sequences from INI-naïve patients some secondary mutations, which have been shown to contribute to RAL resistance [2831], were found. The low prevalence and the equal distribution of these polymorphisms among the different groups of patients are in contrast with the reported appearance of secondary mutations in association with prior antiretroviral exposure [28, 31]. Thus, our results are in accordance with findings by Garrido et al., [32] and suggest that the emergence of RAL resistance mutations is weakly influenced by prior exposure to antiretrovirals other than INI.

            Finally, new polymorphisms (Q95R, and T97S) in positions related to INI resistance were found. Whether and how these mutations might influence viral fitness or replication remains to be clarified.

            The genetic barrier was calculated for 27 amino acid positions related to INI susceptibility. The majority of these positions were highly conserved. Our analysis extends the results reported by Maïga et al [33] (including only B and CRFD02_AG subtypes) to a wider collection of HIV-1 subtypes which reflects the evolving epidemiology of this infection in our region [19]. Analysis of codon usage distribution between sequences from HAART-naïve and RTI/PI-experienced patients revealed a single position (148), with a predominant difference in codon usage. These findings suggest a marginal yet valid influence of prior antiretroviral exposure on the genetic barrier in our study population [33]. A larger dataset would allow better definition of the role played by previous treatment with RTI and PI on INI susceptibility. On the other hand, the great majority of patients on HAART show complete suppression of peripheral viral load, and the enrollment of 95 viremic patients required 1 year to be completed.

            Due to the small number of HIV-2 sequences, our analysis did not allow us to draw conclusions on HIV-2 variability. However, of particular interest was the detection of mutations in IN positions associated with RAL resistance. This finding confirms and extends a previous observation by Xu et al. [34]. The identification of INI-resistance mutations in INI-naive patients infected with HIV-2 highlights the urgent need for future studies on HIV-2 and may necessitate avoidance of INI in the treatment of these patients.

            In conclusion, primary INI resistance-associated mutations were not present in this population of INI naïve HIV-1 infected individuals. Exposure to antivirals other than INI does not seem to significantly influence the emergence of mutations implicated in INI resistance.

            Declarations

            Acknowledgements

            We thank all the technical staff of the Virology Unit, Laurene Kelly for revision of the English, and Daniela Sartori for editing. This work was partially supported by the Ministero della Salute, Ricerca Corrente 80207 and Programma Nazionale AIDS (convenzione 40H55).

            Authors’ Affiliations

            (1)
            Molecular Virology Unit, Virology and Microbiology Department, Fondazione IRCCS Policlinico San Matteo
            (2)
            Institute of Infectious Diseases, Fondazione IRCCS Policlinico San Matteo
            (3)
            Experimental Research Laboratories, Biotechnology Area, Fondazione IRCCS Policlinico San Matteo

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            © Piralla et al; licensee BioMed Central Ltd. 2011

            This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://​creativecommons.​org/​licenses/​by/​2.​0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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