Download as PowerPoint Slide Figure 1. Cellular mosaicism in a glomerulus B from a female, heterozygous for X-linked Alport syndrome, and A normal control. Adapted from reference 19 , with permission. It is likely that a woman's kidney is composed of two populations of cells differing in the X-linked alleles that are expressed. The two kinds of cells comingle at least to some extent and cooperate metabolically, or in other ways.
Such intermingling of cells will determine the ultimate phenotype of her mosaic renal cells. But some women may have very skewed patterns of X inactivation because of the small numbers of cells present when X inactivation occurs, or mutations affecting XIST, or a growth advantage of one of the cell populations. In the case of carriers of mutations affecting the kidney, the nature of the cellular mosaicism will determine whether or not she has significant renal disease. In any case, the X chromosome mosaicism, even if it were not responsible for sex differences in kidney development, is clearly responsible for sex differences in renal diseases.
Previous Section Next Section Sex Differences in Renal Disease Table 1 shows the sex differences in expression of some diseases that affect the kidney; most affect predominantly males and have variable penetrance in carriers. The list is not exhaustive, and it consists mainly of syndromes associated with severe deficiencies of the gene products.
There are more OMIM numbers than the number of mutant genes because the syndromes were described before the genes were known. The identification of mutations has led to a broader appreciation of the spectrum of disorders attributable to mutations in a single gene.
Some of the phenotypic variations are the result of different effects of multiple mutations within the same gene. Generally, the sex differences in the manifestations of X-linked diseases fall into two categories: Either males have a disease, and females have no manifestations or attenuated manifestations, or females uniquely manifest the disease because the mutation is lethal in males.
Carriers with the same mutation survive because enough of their cells express the normal allele to carry out the function of the gene, or the gene product can be transferred from normal to deficient cells. Sharing of gene products takes place in the usual ways that cells communicate with each other, namely, metabolic cooperation through gap junctions, endocytosis, and other intercellular channels. If gene products cannot be shared, then normal cells may outgrow the mutant ones and eventually eliminate them.
Such strategies are carried out in a tissue-specific fashion; those tissues unable to metabolically cooperate may undergo cell selection instead.
These interactions, occurring uniquely in females, ameliorate their phenotype so that they may have no clinical manifestations at all. Cell-to-cell transfer of gene products masks the genotype, as the mutant cells are no longer deficient. Good examples are mutations affecting the metabolism of large intracellular proteins. The enzymes needed to digest such molecules can be transferred from one cell to another by mannose-6 phosphate-mediated endocytosis.
Without sufficient enzyme, the undigested proteins can induce kidney disease, which is the case in Fabry disease. Previous Section Next Section Fabry Disease The lysosomal enzyme alpha galactosidase A GLA is ubiquitously expressed, as it is needed to break down glycosphingolipids present in most cell membranes.
In the absence of sufficient enzyme, the glycosphingolipids accumulate and plug the blood vessels in all tissues, producing severe episodic pain and premature death. The severe kind of renal disease occurs most often in males, who usually require enzyme replacement therapy with human recombinant GLA and kidney transplants for end-stage renal disease ESRD.
In the kidney, the glycosphingolipids colocalize with the lysosomes and are deposited in the glomeruli, renal tubules, and blood vessels.
Fabry nephropathy consists of glomerular sclerosis, tubular fibrosis, and hyalinization of the blood vessels. Clinically, the renal disease manifests as hypertension, moderate proteinuria, lipiduria, and microscopic hematuria. Many heterozygous females manifest some signs of the disease, but in an attenuated form. These affected females usually have a later onset of symptoms, live significantly longer, and their symptoms can be more easily relieved by enzyme replacement than males.
In any case, the symptoms and circulating levels of GLA are more variable in females, and the disease has a slower rate of progression. Clearly, the milder renal disease is the result of the effect of having cells that produce normal amounts of GLA.
Also, the normal cells can export the enzyme by endocytosis to the mutant cells, thereby ameliorating their deficiency. Enzyme transfer is enough to preclude elimination of mutant cells but inadequate to correct the defect, which explains why heterozygotes may be symptomatic. The GLA enzyme is taken up poorly compared with other lysosomal enzymes, perhaps explaining why almost all carriers have the lens opacities characteristic of Fabry disease.
In this case, the skewing is not the result of a growth advantage of the mutant cells but reflects instead stochastic events, or skewing for reasons unrelated to the mutant gene, such as chromosome abnormalities and twinning. The influence of X inactivation is striking in a pair of female MZ twins, who are uniquely prone to skewing. Only one twin had the classic form of Fabry disease, and her cells predominantly express the mutant allele. These boys have an almost complete deficiency of the enzyme hypoxanthine phosphoribosyl transferase HPRT , which is needed, especially in the brain, for the reutilization of breakdown products of DNA, specifically the purine bases hypoxanthine and guanine.
In addition to mental retardation and spastic cerebral palsy, severe HPRT mutations result in uric acid deposits in joints and kidney. However, in some cases, they also result in ESRD, often unrecognized because of the lack of other symptoms. These boys and others who are treated with allopurinal for a long time may also develop xanthinuria and xanthine urolithiasis with staghorn calculi.
In many of their tissues, inosinic acid, the product of the HPRT metabolic reaction, is transferred from the normal to the mutant cells by means of gap junctions. Connecting the cytoplasms of neighboring cells, these channels mediate the transfer of small molecules, such as inosinic acid, across the lipid bilayer of the cell membranes. However, in blood cells both erythroid and leukocytic lineages , which lack gap junctions and hence the ability to transfer inosinic acid , the mutant cells have a growth disadvantage and are completely eliminated after the first decade of life.
As happens in such cases, the translocation chromosome is always the active X, so the mutation is expressed in all her cells. Another affected female is an identical twin with mostly mutant skin cells; her normal co-twin had no skewing. The discordant phenotype observed in monozygotic twins suggests that twinning can trigger skewed X inactivation. Previous Section Next Section Alport Syndrome Alport syndrome is a group of hereditary diseases affecting the kidney that may also cause hearing loss and ocular lesions.
In the kidney, there is hematuria and proteinuria, often leading to ESRD. The molecular defects involve the basement membranes of both tubules and glomeruli, most often caused by deficiency of one of their type IV collagen components. Males with a COL4A5 deficiency almost always end up with ESRD, whereas females have a wide range of phenotypes ranging from asymptomatic to disease as severe as that of males.
Attempts to correlate the phenotype of carriers with their X inactivation patterns in blood cells have been generally unsuccessful. This is expected, as the blood cells are not affected by these mutations. Kashtan and colleagues studied the glomeruli of human females with COL4A5 mutations and female mice carrying a human Alport mutation. Striking is the mosaic pattern in the kidney, with a block of labeled cells, most likely normal clone s , and blocks that are not labeled, most likely mutant clone s.
In mice, the pattern varies, with some glomeruli more labeled than others, 20 suggesting that each glomerulus is composed of progeny of several progenitor cells, and are often mosaic with respect to basement membrane function. In the case of COL4A5 mutations, the deficiency is cell autonomous.
However, one could imagine that other gene products might be passed between glomerular cells. The availability of female mice with the Alport mutation provides the means to determine how much of a glomerulus needs to be functional and how many normal glomeruli are needed for adequate renal function.
The COL4A5 gene is located in the middle of the long arm of the X, not close to the pseudoautosomal region that is homologous on X and Y chromosomes—where the genes are expressed from all sex chromosomes.
Some X-linked genes in other regions of the X are said to escape inactivation as they are expressed from the inactive X to some extent 21 ; their expression is limited because the chromatin of all but the pseudoautosomal region is quite repressed. Severe skewing that by chance favors the mutant allele has been observed in kidney cells from a heterozygote, manifesting severe renal disease.
Females with one copy of the deletion manifest the smooth-muscle tumors as often as males do, but their renal disease tends to be milder with only rare females affected with ESRD. Although not tested, presumably such females are the ones with greater numbers of mutant cells in their kidneys.
As a result of a severe growth disadvantage during embryogenesis, most deleted X chromosomes are inactive in every cell, but these Alport deletions are apparently not large enough to influence the growth of the mutant cell. If they were, then the deleted X would always be inactive, and the mutation would not be expressed at all. Mutations are almost always associated with low-molecular weight proteinuria, aminoaciduria, and progressive renal failure. The disease is the result of inactivating mutations of CLCN5, encoding a member of the CLC family of voltage-gated chloride channels and transporters.
CLCN5 localizes to endosomes of the proximal tubule. In the absence of CLCN5, there are less of the scavenger protein receptors, megalin and cubilin, in the proximal tubules, where they are needed to take up low molecular weight proteins from the glomerular filtrate.
Because random inactivation provides enough normal cells, females are usually asymptomatic, but some may have hypophosphatemia and decreased urine osmolality. Previous Section Next Section Dent 2 Disease Lowe Syndrome Another cause of renal tubular dysfunction is Lowe oculocerebrorenal syndrome, which affects multiple tissues and organs. Lowe syndrome is also called Dent 2 disease because the mechanism of kidney disease is similar to that of Dent 1 disease.
In both cases, the renal tubules lack the receptors that control the exchange of certain small molecules between the urine and the blood. OCRL attaches itself to an adaptor molecule, involved in the sorting and signaling of cell surface receptors in the brain and kidney. In the kidney, that receptor protein is megalin, just as in Dent 1 disease. In any case, lens opacities provide a rather sensitive assay for heterozygotes in both diseases.
A few Lowe heterozygotes are fully affected as males, and in one case this was explained by severe skewing such that only mutant cells were present, with her normal allele mute on her inactive X.
Polycystic kidneys are found in up to half the cases, especially when the OFDI mutation affects a splice site. Deficiency of the protein causes ciliary dysfunction, resulting in early developmental defects in the heart, neural tube, kidney, and affecting laterality because of absent cilia in the embryonic node.
The severity of the polycystic renal disease in heterozygous females varies widely, and undoubtedly their X inactivation status plays a role in determining how severe it will be. It is likely that human heterozygous females are protected to some extent by the small amount of protein produced from the normal allele on their inactive X.
In mice, the gene on the inactive X is completely silent, and all affected females have polycystic kidneys, surviving only a short time after birth. In any case, along with variation in the nature of the mutation, the variability either in expression from the inactive X or in the number of normal cells clearly explains the variability in renal disease among carriers.
Some may have sufficient OFD1 protein for cilium assembly. Because the homologous locus on the Y chromosome is a nonexpressed pseudogene, affected males cannot benefit from the allele on their Y chromosome. It is likely the leaky expression of the normal allele from the inactive X weakens the influence of X inactivation on the severity of the polycystic disease. Diabetes insipidus is a disease of water homeostasis resulting from failure to concentrate urine. Most of the AVPR2 mutations result in V2 receptors that cannot reach the plasma membrane because they are trapped within the renal cell.
Affected males have excessive thirst and produce large quantities of dilute urine, even when treated with exogenous vasopressin. Because most patients are males, the few affected females are usually considered to have a mutation in the autosomal gene.
However, van Lieburg et al. They suggested that the manifesting females had too many mutant cells because of skewed X inactivation. This hypothesis was tested by Nomura et al. Both manifesting females had skewed X inactivation in blood cells; the most severe had the greatest amount of skewing.