Immunodeficiency in Bloom's Syndrome

Abstract

Bloom's syndrome (BS) is an autosomal recessive disease, caused by mutations in the BLM gene. This gene codes for BLM protein, which is a helicase involved in DNA repair. DNA repair is especially important for the development and maturation of the T and B cells. Since BLM is involved in DNA repair, we aimed to study if BLM deficiency affects T and B cell development and especially somatic hypermutation (SHM) and class switch recombination (CSR) processes. Clinical data of six BS patients was collected, and immunoglobulin serum levels were measured at different time points. In addition, we performed immune phenotyping of the B and T cells and analyzed the SHM and CSR in detail by analyzing IGHA and IGHG transcripts using next-generation sequencing. The serum immunoglobulin levels were relatively low, and patients had an increased number of infections. The absolute number of T, B, and NK cells were low but still in the normal range. Remarkably, all BS patients studied had a high percentage (20-80%) of CD4+ and CD8+ effector memory T cells. The process of SHM seems normal; however, the Ig subclass distribution was not normal, since the BS patients had more IGHG1 and IGHG3 transcripts. In conclusion, BS patients have low number of lymphocytes, but the immunodeficiency seems relatively mild since they have no severe or opportunistic infections. Most changes in the B cell development were seen in the CSR process; however, further studies are necessary to elucidate the exact role of BLM in CSR.

Keywords: DNA repair; Immunodeficiency; bloom’s syndrome; class switch recombination; lymphocyte; somatic hypermutations.

Fig. 1

T cell subsets in BS patients. The absolute numbers of CD19+ B and CD16+CD56+ NK cells (A), CD3+ T cells, CD4+ T (CD3+ CD8-) cells, and CD8+ T cells (CD3+ CD8+) (B) are low in the BS patients compared to the healthy controls (HC). [37] The absolute numbers of CD4+ and CD8+ naïve (CD45RO⁻CCR7⁺CD27⁺CD28⁺), central memory (Tcm) (CD45RO⁺CCR7⁺CD27⁺CD28⁺), and effector memory (Tem) (CCR7⁻) are reduced in the BS patients, but the relative distribution is normal in most BS patients (C). Significant values were calculated using the two-tailed Mann-Whitney test and are indicated: *P ≤ 0.05; **P ≤ 0.01

Fig. 2

B cells and serum immunoglobulin levels. The absolute numbers of transitional (IgD⁺CD27⁻CD24⁺⁺CD38⁺⁺) and naïve mature B cells (IgD⁺CD27⁻CD24⁺CD38⁺) are normal or increased in the BS patients compared to the age-matched healthy controls (HC), while the natural effector (IgD⁺CD27⁺IgM⁺) and memory B cells (IgD⁻CD27⁺) are decreased (a). The serum IgM, IgG, and IgA immunoglobulin levels are persistently low in most of the BS patients. The gray area indicates the range of the reference values. Significant values were calculated using the two-tailed Mann-Whitney test and are indicated: *P ≤ 0.05

Fig. 3

SHM frequency and patterns. Frequency of SHM in IGHG and IGHA transcripts are low but in the normal range in the BS patients (a). The mean percentage of mutations located in RGYW/WRCY motives and TW/WA motives (b), and the mean percentage of mutations at A/T or G/C (divided in transitions and transversions) locations (c) is normal in BS patients. The error bars indicate the standard deviation

Fig. 4

Subclass distribution of the IGH transcripts. BS patients have relatively more IGHG1 and IGHG3 transcripts compared to age-matched controls (a). The distribution of the IGHA1 and IGHA2 transcripts is normal (b). The bar graphs of the controls indicate the mean and standard deviation

Fig. 5

Selection characteristics of the IGH transcripts. The median CDR3 length of IGHG and IGHA transcripts in BS patients is similar to IGHG and IGHA transcripts from healthy controls and significantly shorter than the CDR3 length of naïve B cells (a). In the BS patients, the mean percentage of rearrangements using VH4-34 is lower compared to naïve B cells from HC, similarly to switched B cells from the HC (b). The replacement/silent (R/S) ratio in the CDR3 regions of the VH genes is lower in the BS patients (c). The percentage of antigen-selected sequences as determined using IgAT is lower in the BS patients (d). All bar graphs represent the mean and standard deviation. The line in the dotplots represents the median. All columns were compared to the control naïve column, and significant values were calculated using the two-tailed Mann-Whitney test and are indicated: *P ≤ 0.05

Fig. 6

Principal component analysis on the B cell repertoire characteristics. Principal component analysis (PCA) on the data obtained from the IGHG and IGHA transcripts including information on the SHM, subclass distribution, and antigen selection (a). Graphs showing how much each variable contributed to PCA1 and PCA2