Molecular mechanisms underlying hemophilia A phenotype in seven females

BACKGROUND

Hemophilia A (HA) in females is a rare observation. Here we describe various genetic mechanisms that result in phenotypic expression of HA in seven females.

METHODS

The F8 gene was examined in all patients and relatives by direct sequencing. Multiplex ligation-dependent probe amplification (MLPA) was performed for large deletion screening. X chromosome inactivation was studied by PCR analysis of a polymorphic CAG repeat in the first exon of the human androgen receptor (HUMARA) gene.

RESULTS

In two females sequencing of the F8 gene revealed homozygous missense mutations (Arg593Cys and Tyr1680Phe) as a consequence of consanguineous marriage. The third case was due to compound heterozygosity comprising the missense mutation Leu412Phe inherited from the carrier mother, together with a de novo large deletion spanning exon 9-22, probably originating from the germ cells of the healthy father. Three further cases shared a common mechanism representing heterozygous mutations in the F8 gene (Arg1781His, Arg327His, small deletion in exon 10) combined with non-random inactivation of the X chromosome. The final case describes a coincidental inheritance of HA and Coffin-Lowry syndrome in the same family. The HA phenotype results from a heterozygous small deletion affecting the F8 gene (c.6872 del CT leading to Thr2272fs) and a complete inactivation of the maternal X chromosome, which segregates with Coffin-Lowry syndrome in the two brothers of the proposita.

CONCLUSIONS

In conclusion, molecular genetic analysis represents an essentially valuable tool in elucidating the nature of the molecular mechanisms underlying the HA phenotype in females.

Clinical spectrum of SIX3-associated mutations in holoprosencephaly: correlation between genotype, phenotype and function

BACKGROUND

Holoprosencephaly (HPE) is the most common structural malformation of the human forebrain. There are several important HPE mutational target genes, including the transcription factor SIX3, which encodes an early regulator of Shh, Wnt, Bmp and Nodal signalling expressed in the developing forebrain and eyes of all vertebrates.

OBJECTIVE

To characterise genetic and clinical findings in patients with SIX3 mutations.

METHODS

Patients with HPE and their family members were tested for mutations in HPE-associated genes and the genetic and clinical findings, including those for additional cases found in the literature, were analysed. The results were correlated with a mutation-specific functional assay in zebrafish.

RESULTS

In a cohort of patients (n = 800) with HPE, SIX3 mutations were found in 4.7% of probands and additional cases were found through testing of relatives. In total, 138 cases of HPE were identified, 59 of whom had not previously been clinically presented. Mutations in SIX3 result in more severe HPE than in other cases of non-chromosomal, non-syndromic HPE. An over-representation of severe HPE was found in patients whose mutations confer greater loss of function, as measured by the functional zebrafish assay. The gender ratio in this combined set of patients was 1.5:1 (F:M) and maternal inheritance was almost twice as common as paternal. About 14% of SIX3 mutations in probands occur de novo. There is a wide intrafamilial clinical range of features and classical penetrance is estimated to be at least 62%.

CONCLUSIONS

Our data suggest that SIX3 mutations result in relatively severe HPE and that there is a genotype-phenotype correlation, as shown by functional studies using animal models.

Somatic FGFR3 and PIK3CA mutations are present in familial seborrhoeic keratoses

BACKGROUND

Seborrhoeic keratosis (SK) represents one of the most common benign skin tumours. Familial occurrence of multiple SKs has been reported, but the genetic basis of these SKs has not been investigated so far. We present a German family with at least seven affected members in two generations and occurrence of high numbers of SKs at an unusually young age, suggesting a hereditary background.

OBJECTIVES

Because FGFR3 and PIK3CA mutations have been reported to be involved in the pathogenesis of sporadic SK, we analysed five SKs of an affected family member for hotspot mutations of these genes.

METHODS

A SNaPshot multiplex assay was used for analysis of 11 previously described FGFR3 hotspot mutations. In addition, exon 9 of PIK3CA was directly sequenced and the H1047R hotspot mutation in exon 20 was analysed by a SNaPshot assay.

RESULTS

FGFR3 mutations were present in three of five SKs. One SK with a FGFR3 mutation additionally showed a hotspot PIK3CA mutation. None of these mutations was present in the germline.

CONCLUSIONS

The results show that this case of familial SK reveals the same mutational spectrum as sporadic SK. Because FGFR3 and PIK3CA germline mutations can be excluded as an underlying genetic basis, alternative mechanisms have to contribute to familial SK such as inherited susceptibility factors predisposing to the acquisition of somatic FGFR3 and PIK3CA mutations in skin, or increased exposure of the family members to yet unknown environmental risk factors causing these mutations.

Frequency of CFTR gene mutations in males participating in an ICSI programme

A higher prevalence of cystic fibrosis transmembrane regulator (CFTR) gene mutations has been suggested both in men affected by congenital aplasia of the vas deferens, and in individuals presenting with reduced sperm quality. In this case, an increased risk for offspring being affected by cystic fibrosis (CF) can be expected in couples who are planning to undergo intracytoplasmic sperm injection (ICSI), since most of the male partners suffer from infertility. In order to determine the risk for these couples more precisely, we offered them a test for the most frequent CF mutations prevalent in the German population. The frequency of mutations within the CFTR gene in the female group was in the same range as expected for the general population (six out of 150). In 10 out of 207 males tested, infertility could be explained by exogenous factors not related to CFTR. Among the remaining 197 males with idiopathic infertility, we detected 13 heterozygotes for a mutation within the CFTR gene. This slightly, but significantly (P = 0.014), elevated rate could indicate that infertile males have, compared with the general population, an increased risk of being a carrier of a CFTR gene mutation.