Genetic studies of hypospadias

Abstract

Hypospadias is defined as an abnormal opening of the urethra on the underside of the penis. It is a frequently found malformation with an incidence of 3 per 1000 males. The aim of this thesis was to identify genetic and environmental factors in the pathogenesis of hypospadias. For this purpose, a variety of genetic methods were used in a nation-wide material corresponding to half of all registered cases of hypospadias in Sweden.

We identified 18 monozygotic twins discordant for hypospadias. In 16 of these, the twin with lowest birth weight was affected with hypospadias. This shows that low birth weight is important for hypospadias, regardless of the genetic constitution (paper 1).

We investigated the familial rate and analyzed the association with low birth weight in 2138 families with at least one boy with hypospadias. There was a familial rate of 7% and a significant lower birth weight in cases with hypospadias compared with their respective brothers, used as controls (p=5x10-13). An increased frequency of dizygotic as well as monozygotic male-male twins was found, with a skewed distribution towards monozygotic twins. This paper (II) also includes a description of the ethnic background in the material and the distribution of phenotypes.

A complex segregation analysis was performed to define the mode of inheritance in a material consisting of 2005 pedigrees. We found a best fit for the multifactorial model and a heritability of 0.99. This is interpreted as monogenic effects acting in some of the families but a multifactorial cause in the majority (paper III).

A genome-wide linkage analysis based on a non-parametric affected relative pair method was used in 69 families. All available family members were genotyped with 360 polymorphic PCR based microsatellite markers with a mean interval of 9.5 cM. Five chromosomal regions reaching the level of suggestive significance were identified (paper IV). These need to be investigated further to identify hypospadias susceptibility genes.

Linkage analysis and subsequent mutation analysis in a family with autosomal dominant inheritance of hypospadias revealed a novel mutation in the HOXA13 gene (paper V). This suggests the diagnosis of hand-foot-genital syndrome although the phenotype in this family is atypical compared with previously reported families.

In this thesis, several lines of evidence suggesting genetic factors in the pathogenesis of hypospadias are presented, including the identification of five chromosomal regions in which genes for hypospadias are likely to be located and a novel mutation in the HOXA13 gene. It is also shown that low birth weight is an important risk factor for hypospadias.