FAMILIAL ASPECTS OF DIFFUSE RENAL DISEASES

X-Linked Kidney Disorders in Women

A number of genes that cause inherited kidney disorders reside on the X chromosome. Given that males have only a single active X chromosome, these disorders clinically manifest primarily in men and boys. However, phenotypes in female carriers of X-linked kidney conditions are becoming more and more recognized. This article reviews the biology of X inactivation as well as the kidney phenotype in women and girls with a number of X-linked kidney disorders including Alport syndrome, Fabry disease, nephrogenic diabetes insipidus, X-linked hypophosphatemic rickets, Dent disease, and Lowe syndrome.

The variable course of women with X-linked Alport Syndrome

X-linked Alport syndrome (XLAS) arises from mutations in the COL4A5 gene encoding the α5-chain of type IV collagen and is associated with hematuria, ocular abnormalities and high-tone sensorineural hearing loss. Nearly all affected males have decreased kidney function resulting in end-stage renal disease (ESRD) as early as the second decade of life. It was long thought that affected females had a benign outcome; however, in recent decades, it has become quite clear that they too are at risk for developing nephrotic syndrome, decreased kidney function and ESRD. We report two young females presenting with microscopic hematuria and proteinuria diagnosed with XLAS on renal biopsy. Both developed nephrotic-range proteinuria and progressive renal insufficiency. Additionally, both developed extra-renal manifestations of XLAS. The ultrastructural and immunofluorescence features on kidney biopsy were instrumental in making the diagnosis of heterozygous XLAS as neither patient had a family history of AS.

Women and Alport syndrome

X-linked Alport syndrome (XLAS) is caused by mutations in type IV collagen causing sensorineural hearing loss, eye abnormalities, and progressive kidney dysfunction that results in near universal end-stage renal disease (ESRD) and the need for kidney transplantation in affected males. Until recent decades, the disease burden in heterozygous "carrier" females was largely minimized or ignored. Heterozygous females have widely variable disease outcomes, with some affected females exhibiting normal urinalysis and kidney function, while others develop ESRD and deafness. While the determinants of disease severity in females with XLAS are uncertain, skewing of X-chromosome inactivation has recently been found to play a role. This review will explore the natural history of heterozygous XLAS females, the determinants of disease severity, and the utility of using XLAS females as kidney donors.

X-inactivation modifies disease severity in female carriers of murine X-linked Alport syndrome

BACKGROUND

Female carriers of X-linked Alport syndrome (XLAS) demonstrate variability in clinical phenotype that, unlike males, cannot be correlated with genotype. X-inactivation, the method by which females (XX) silence transcription from one X chromosome in order to achieve gene dosage parity with males (XY), likely modifies the carrier phenotype, but this hypothesis has not been tested directly.

METHODS

Using a genetically defined mouse model of XLAS, we generated two groups of Alport female (Col4a5(+/-)) carriers that differed only in the X-controlling element (Xce) allele regulating X-inactivation. We followed the groups as far as 6 months of age comparing survival and surrogate outcome measures of urine protein and plasma urea nitrogen.

RESULTS

Preferential inactivation of the mutant Col4a5 gene improved survival and surrogate outcome measures of urine protein and plasma urea nitrogen. In studies of surviving mice, we found that X-inactivation in kidney, measured by allele-specific mRNA expression assays, correlated with surrogate outcomes.

CONCLUSIONS

Our findings establish X-inactivation as a major modifier of the carrier phenotype in X-linked Alport syndrome. Thus, X-inactivation patterns may offer prognostic information and point to possible treatment strategies for symptomatic carriers.

Fechtner syndrome: clinical and genetic aspects

Fechtner syndrome, a variant of Alport syndrome, was first reported by Peterson et al. [1985]. It is characterized by nephritis, hearing loss, eye abnormalities, macrothrombocytopenia, and leucocyte inclusions, present in varying combinations in several members of the same family. This is the second family reported; 16 relatives are affected. The clinical manifestations of the syndrome are delineated. The pattern of inheritance is autosomal dominant. The hematologic abnormalities are similar to those detected in May Hegglin anomaly. They are present in every affected relative and may be present at birth. The feasibility of prenatal diagnosis is discussed.

Alport's syndrome as a cause of renal failure in Europe

We studied the geographical distribution, male to female ratio, and age at the start of renal replacement therapy (RRT) for end-stage renal failure (ESRF) in 600 patients with hereditary nephritis with nerve deafness (Alport's syndrome) reported to the European Dialysis and Transplant Association Registry since 1975. Annual age- and sex-specific acceptance rates for RRT showed a variable peak incidence according to country, ranging between, 0 and 2.4 patients per million population in males aged 15-24 years, but with only about half this incidence in females. In Scandinavian countries there were very few females who started RRT, and males were older than in the rest of Europe. The overall male to female ratio was 4:1. The median age at the start of RRT was: males (n = 479) 24.3 years (1st quartile 19.5 years; 3rd quartile 31.5 years); females (n = 121) 31.5 years (1st quartile 23.0 years; 3rd quartile 43.2 years). Our study provided confirmation that males reach ESRF earlier than females. In addition, we detected previously unrecognized geographical differences.

Hereditary nephritis (with unusual renal histology): report of a first case from the West Indies

A 21-year-old Grenadian girl undergoing investigation in Trinidad for anaemia was diagnosed as a case of hereditary nephritis. She had the clinical features of a nephropathy, nerve deafness and an ocular defect. Renal histology was exceptional in that in addition to the typical findings of a hereditary nephritis, cystic areas generally associated with medullary cystic disease were noted. Several members of the patient's maternal family were afflicted with either deafness visual distrubances or renal disease.

An inherited kidney disease of mice resembling human nephronophthisis

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Anterior lenticonus and Alport's syndrome

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