The Chd subfamily of SNF2 ATPases is characterized by the presence of tandem chromodomains and a SNF2-like helicase [21
]. Members of this family have been implicated in a variety of cellular processes and have also been correlated with human diseases [22
]. CHD3 and CHD4 have been implicated in dermatomyositis, a connective tissue disease involving the inflammation of both the skin and muscular systems [22
]. Mutations of CHD5 have been found in patients with neuroblastoma, a malignant neoplasm of the peripheral nervous system [38
]. Mutations in CHD7 have been observed in patients afflicted with CHARGE syndrome, an acronym for a specific group of coexistent congenital anomalies [40
]. Finally, a patient with a balanced translocation between the CHD2 locus on chromosome 15 and the X chromosome presented with scoliosis and multiple other abnormalities [30
]. Though this patient had no reported kidney abnormalities, the differences between the observed phenotypes and those described for the mutant Chd2 mouse strains could be due to the differences in the alterations in the respective Chd2 loci, as the balanced 15:X translocation in humans is genetically distinct from the hypomorphic allele created by the insertion in the mouse Chd2 locus. Regardless, the data collectively suggest a link between the CHD proteins and normal development.
Our previous characterization of Chd2
mutant mice demonstrated gross alterations and histological abnormalities in the kidneys of heterozygous mice [20
], a finding that was recently independently confirmed [30
]. Here, we further investigated the phenotypes of the kidneys in end-stage adult mutant mice. Histopathological analyses revealed that the morbidity and mortality of Chd2+/mut
mice was associated with significant kidney disease affecting the glomerulus in most animals and the tubules in more than half of the animals examined. In particular, the changes in the glomerulus were characterized by increased thickening of the basement membrane, including an increased deposition of collagen (fig. , ). These changes correlated with the loss of normal kidney function. It remains unclear why a small percentage (~15%) of the heterozygous mutant mice did not present with any obvious kidney phenotypes, though we suspect that variations due to the mixed genetic background of these mice are responsible.
Generally, glomerular diseases interfere with the normal function of the glomerular filtration barrier, thus permitting protein loss. Histological analysis showed prominent protein casts within the Bowman's space and within the dilated tubules (fig. ). As detailed in table , protein was detected in the urine samples of Chd2 heterozygous mice. Additionally, urinary albumin excretion was also observed in Chd2+/mut mice (fig. ). Collectively, these data implicate that Chd2 is necessary to obtain a normal renal phenotype and maintain normal renal excretory activities.
Proteinuria can trigger the progression of CRD. We measured the balance of serum electrolytes, as they are often affected by kidney disease. Normally, the kidneys regulate fluid absorption and excretion and maintain a narrow range of electrolyte fluctuation. The three most common serum electrolytes are bicarbonate, sodium, and, potassium. Bicarbonate is the major buffer in the body that helps to maintain the proper blood pH. Decreased levels of serum bicarbonate, coupled with an acidic pH, implies metabolic acidosis and kidney dysfunction in the analyzed heterozygous Chd2
mice (table ). Measurements of blood gases in Chd2+/mut
mice showed decreased levels of CO2
(table ). Together with acidic pH and decreased levels of bicarbonate, the abnormal breathing patterns initially described in the Chd2+/mut
] was likely due to partial respiratory compensation through hyperventilation.
Imbalances of sodium and potassium are also suggestive of poor kidney function. We did not detect significant differences in the level of sodium; however, we did detect a significant increase in the potassium levels in Chd2+/mut mice in comparison to the wild-type controls (table ). Furthermore, the significant elevation of blood urea nitrogen (BUN) and serum creatinine suggests a decrease in renal function in Chd2+/mut mice (fig. ). Collectively, these data demonstrate that the decrease in renal function is a consequence of the pathologic abnormalities observed in the Chd2+/mut mice.
Kidney diseases are frequently associated with a number of systemic diseases. Diabetic nephropathy is the primary cause of kidney disease [2
]. An elevated level of blood glucose is the central feature of diabetes. We did not observe elevated blood glucose, the central feature of diabetes mellitus, nor did we observe elevated urine glucose levels in Chd2+/mut
mice, suggesting that diabetic nephropathy was not a consequence of the Chd2
mutation (data not shown). Hypertension is also frequently associated with kidney disease [2
]; however, we did not observe any indications of arterial hypertension in the Chd2+/mut
mice (data not shown). Further examination would be needed to completely rule out the association of diabetes and/or hypertension with the kidney defects observed in the Chd2+/mut
Anemia is a common subsequent symptom in patients with CKD [41
]. The decrease in EPO mRNA in the Chd2+/mut
kidney samples, which correlates with the decreased levels of hemoglobin and hematocrit, can explain the anemia seen in the Chd2+/mut
mice (fig. ). Thus, the anemia in Chd2+/mut
mice might be a secondary consequence of renal failure caused by the destruction of the kidneys. Alternatively, the anemia may be due to an intrinsic defect in erythropoiesis or to a combination of kidney and erythropoietic dysfunction. Further studies will be required to understand the precise role of Chd2
We did not directly address the requirement for Chd2
during embryonic and postnatal development, although examination of kidneys from one litter taken at postnatal day 21 indicated that the kidneys from the three heterozygous individuals present were discolored and pale, as observed in the adults [20
]. Microscopic examination indicated a disorganized tissue structure (data not shown). However, the authors of a recently published report describing mice with the same insertional mutation in the Chd2 locus indicated that no kidney abnormalities were observed in the neonates. This difference may be due to differences in genetic background, or may indicate that the first indications of kidney abnormalities occur between birth and day 21. Additional studies will be needed to address this issue.