X-linked quantitative trait locus for susceptibility to renal injury

In this issue of the journal, Herrera et al. [1] present an interesting article concerning the X-linked mechanism of susceptibility to renal injury in Dahl rats. They report two major findings. First, the F1 male progeny of R × S Dahl rats had a greater susceptibility to renal injury than the F1 male progeny of S × R Dahl rats. That is, the genetic background of XDR/YDS more likely exhibits a greater degree of renal injury that of XDS/YDR. Second, the authors determined a quantitative trait locus (QTL) for the susceptibility to renal injury in Dahl rats. Using the F2 male cohort, a QTL for the susceptibility was mapped to DXrat98 with a peak LRS of 9.4. Sex-linked hypertension and its organ damage have been investigated over several decades. However, with the recent introduction of QTL analysis, gene research for the role of sex chromosomes in hypertension and organ damage has advanced. The article by Herrera et al. [1] is in the current trend of gene exploration to explain the mechanism of sex-associated renal injury in Dahl rats. It is well acknowledged that, throughout most of their life, males are likely to have higher blood pressure than females. This sexual dichotomy is probably the result of genetic factors including the sex chromosomes and genes encoding sexual hormones. To test this hypothesis, elaborate efforts have been made to explore the genes responsible for rising blood pressure in genetic rat strains. Sexual dichotomy is a well-known factor contributing to hypertension in Dahl salt-sensitive (Dahl S) rats. Rising blood pressure in response to a high-salt diet is blunted in female Dahl S rats compared to male Dahl S rats. Several investigators prefer the X chromosome-mediated mechanism, while others have emphasized the role of Y chromosome. The transfer of the Y chromosome from Wistar–Kyoto rats to spontaneously hypertensive rats of the stroke-prone strain (SHRsp) decreases blood pressure and, in a reciprocal consomic strain, blood pressure increases [2]. Such an increase in blood pressure mediated by the Y chromosome is probably the result of androgen and sympathetic nerve activity. Intriguingly, the Y chromosome-mediated elevation of blood pressure is not associated with progression of renal injury in SHRsp. By contrast, an increasing number of studies are of interest because they have successfully disclosed the role of the X chromosome in hypertension and cardiovascular injury. In addition, this is apparently independent of blood pressure values. With an application of QTL analysis, the mapping data are increasing for hypertension-related genes anchoring the X chromosome in genetic models of hypertension. In 1991, Hilbert et al. [3] mapped two hypertensive genes in SHRsp, BP/SP-1 and BP/SP-2 to rat chromosomes 10 and X, respectively. Similarly, three salt-induced hypertension gene loci are found at DXMgh9-DXMit4, DXMgh12-DXRat8 and DXRat2-DXRat4 in female F2 progeny of the Sabra rat model [4]. More intriguingly, Koike et al. [5] reported that the angiotensin II subtype 2-receptor gene (AII-2R) is mapped to rat and human X chromosome, 18.1 cM distal to the androgen receptor [5]. This locus is hypertension-inducible because the AII-2R knockout increases blood pressure in mice, particularly when angiotensin II is infused or dietary sodium is restricted [6]. Regarding the genes related to organ damage, the +1675 G/A-polymorphism of the AT2-R gene on the X chromosome enhances the susceptibility to cardiac hypertrophy in hypertension through a blood-pressure independent mechanism [7]. Moreover, it is also reported that the ACE2 gene maps to a position between DXMgh12 and DXRat8 in Sabra salt-sensitive rats, and to the BP3 QTL in SHRsp and the BB.Xs QTL in SHR [8]. Surprisingly, ACE2 expression is downregulated in hypertensive rats. Against this background, it is quite interesting that, for the first time, Herrera et al. [1] mapped a QTL for the susceptibility of renal injury in Dahl rats to DXRat98. Comparing the data from the previous studies on QTLs in hypertensive models, this locus is apparently outside of the intervals reported as QTLs for high blood pressure. In fact, the QTL demonstrated by Herrera et al. [1] was associated with a susceptibility to hypertensive renal injury, but not with the blood pressure values per se. QTLs for high blood pressure have been the target of gene exploration for the past few years. In this sense, this QTL for renal injury susceptibility is quite unique and provides a new insight into the genetic mechanism for hypertension-related organ damage. However, it should be noted that the authors detected one such unique QTL anchoring the X chromosome, although this does not necessarily guarantee what the candidate gene is. We will have to wait for the gene(s) to be determined, as was the case for the candidate genes of the previous QTLs. Even so, it surely remains of great concern. By contrast, Dahl S rats are prone to renal injury compared to SHR having similar blood pressure levels. This susceptibility is due not only to haemodynamic stress, but also to hypertension-independent mechanisms. Much evidence strongly suggests that the renin–angiotensin system and/or oxygen radicals specifically contribute to the renal injury in Dahl S rats [9–11]. In view of the proposed map of ATII-2R and ACE2, both ATII-2R and ACE2 genes anchoring the X chromosome are excluded as candidate genes for the QTL demonstrated by Herrera et al. [1]. Personally, I presume that the candidate genes are related to oxygen radicals or bradykinin/nitric oxide mediated by an ATII-2R mechanism. To address these questions directly, the authors should extend their study to determine the candidate gene(s) for the QTL. In their study, the authors used the grade IV Raij score as an indicator of susceptibility to renal injury in Dahl rats [12]. Glomeruli that are determined as having a grade IV Raij score are almost obsolescent in their structure. In renal injury in Dahl S rats, glomeruli with various degrees of sclerotic lesions intermingle, and the number of glomeruli with a grade IV Raij score is much smaller than the number of glomeruli with moderate to mild injury [9,12,13]. Hence, it is unacceptable for the authors to use only the grade IV Raij score as an indicator of renal injury over the whole kidney. The summation of the sclerotic area in the glomeruli examined would be more appropriate. If such a standard scoring was utilized for the QTL analysis, would the authors draw the same conclusions for the QTL? It is also surprising that, in the F2 progeny, renal injury was not correlated with the blood pressure value. The data for blood pressure in the F2 progeny were not made available by the authors. Theoretically, however, it must be widely distributed from high blood pressure to near normal values. It is well known that high blood pressure in Dahl S rats is the major determinant of renal injury [9,14]. Moreover, the authors insisted that the contribution of the QTL to renal injury was only 8% of variance, while the grade IV Raij score in the F2 progeny ranged widely from 0–40% of the glomeruli. If blood pressure was not the determinant of grade VI Raij score, what are the major factors determining a large part of the renal injury? Taking this into consideration, I am skeptical of the sensitivity of the grade IV Raij score alone as an indicator of renal injury in Dahl rats. More sensitive and specific markers reflecting overall renal injury would be useful to clarify these points. The F1 male progeny of R × S Dahl rats had a greater susceptibility to renal injury than the F1 male progeny of S × R Dahl rats. This means that the rats loading sex chromosomes of XDR/YDS more likely exhibit a greater degree of renal injury than those having XDS/YDR. This is in agreement with previous studies indicating that a part of the normotensive gene comprises loci responsible for organ injury in genetic rat strains [15,16], as the authors mention in the text. Indeed, normotensive rats may possess masked genes for susceptibility to renal injury in hypertension; however, how well could the authors neglect the roles of the hypertensive Y and sex-related hormone genes on the X chromosome, the crosstalk of which has been reported in the previous studies? [2,17]. By contrast, Dahl salt-resistant rats are completely free from renal injury when they are fed a high-salt diet. Hence, how was the QTL for the susceptibility unmasked in the F1 progeny? In addition, it should be noted that the degree of injury and its variance were much lower in the F1 than in the F2 progeny, and this also raises the question of whether there are any reasons for the mechanism of susceptibility in the F1 progeny being the same as that in the F2 progeny. The data from the F2 progeny only indicate that the grade IV Raij score in the F2 progeny statistically shows a linkage with DXRat98, and contributes 8% of the variance of the susceptibility seen in the F2 progeny. Considering these points, I would suggest that to further clarify the role of the QTL for susceptibility to renal injury anchoring the X chromosome of normotensive Dahl rats, the transfer of the X chromosome from the Dahl R to the Dahl S background or the reciprocal consomic strain needs to be studied. Nevertheless, because the study by Crackower et al. [8] has demonstrated that the ACE2 expression is rather downregulated in hypertension [8], the possibility that some factors are likely to be expressed in the genetic background of normotension cannot be neglected. If this is true, it means more complicated, still unknown mechanisms must underlie the X chromosome-mediated renal injury in Dahl rats [17]. Finally, following the discovery in 1994 of QTL for the blood pressure in chromosome 2 in Dahl S rats by Deng et al. [18], a series of studies using congenic Dahl rats have consistently found QTLs for blood pressure in chromosomes 1, 2, 7, 10, 12, 13 and 15 [19]. Some QTLs are related to their candidate genes including renin, ATII-1R, Na,K-ATPase, NO synthase, endothelin B receptor, Gsα and adrenergic α1B receptor. By contrast, and as pointed out in this commentary, the contribution of the X chromosome to hypertension and its renal injury is more complicated than expected and, probably, multiple and discrete mechanisms are integrated into part of the regulatory system of hypertension and cardiovascular injury. In this context, the X chromosome-mediated mechanism for the progression/regression of kidney injury in Dahl rats is interesting. The progression of renal injury by the transfer of the normotensive X chromosome, if it is experimentally verified, would certainly lead to an acknowledgement of new mechanisms for the susceptibility to kidney injury in salt-dependent hypertension. More accurately, consomic rats would provide clear evidence for the hypothesis proposed by Herrera et al. [1] and, hopefully, the authors will extend this study to disclose the candidate gene(s) for the QTL near DXRat98.