Achondroplasia: unexpected familial recurrence

Achondroplasia: a comprehensive clinical review

Achondroplasia is the most common of the skeletal dysplasias that result in marked short stature (dwarfism). Although its clinical and radiologic phenotype has been described for more than 50 years, there is still a great deal to be learned about the medical issues that arise secondary to this diagnosis, the manner in which these are best diagnosed and addressed, and whether preventive strategies can ameliorate the problems that can compromise the health and well being of affected individuals. This review provides both an updated discussion of the care needs of those with achondroplasia and an exploration of the limits of evidence that is available regarding care recommendations, controversies that are currently present, and the many areas of ignorance that remain.

The Clinic Is My Laboratory: Life as a Clinical Geneticist

Clinical genetics is the application of advances in genetics and medicine to real human families. It involves diagnosis, care, and counseling concerning options available to affected individuals and their family members. Advances in medicine and genetics have led to dramatic changes in the scope and responsibilities of clinical genetics. This reflection on the last 50+ years of clinical genetics comes from personal experience, with an emphasis on the important contributions that clinical geneticists have made to the understanding of disease/disorder processes and mechanisms. The genetics clinic is a research laboratory where major advances in knowledge can and have been made.

Gonadal mosaicism of a TAZ (G4.5) mutation in a Japanese family with Barth syndrome and left ventricular noncompaction

TAZ (G4.5) was initially identified as the gene associated with Barth syndrome and left ventricular noncompaction (LVNC). The purpose of this study was to investigate patients with LVNC for disease-causing mutations in TAZ. In 124 Japanese patients, including 50 families, mutation analysis of TAZ was performed using DNA sequencing. A splice donor mutation was identified in two brothers with Barth syndrome and LVNC, and a sister who was asymptomatic. However, the variant was not identified in either parent or the maternal grandparents, all of whom were asymptomatic. Due to the recurrent inheritance of this variant by each of the children we concluded that this was evidence of gonadal mosaicism in the obligate carrier mother, the first reported occurrence of this in Barth syndrome.

Germline mosaicism in achondroplasia detected in sperm DNA of the father of three affected sibs

We describe a sib recurrence for achondroplasia with parents of average stature. The three sibs shared the paternal allele and all carried the same causal mutation in the fibroblast growth factor receptor 3 gene (FGFR3): G > A nt1138 (Gly380Arg). We were able to identify this mutation on sperm DNA confirming paternal germinal mosaicism. Our family shows that a more precise definition of the recurrence risk is feasible using this approach, based on a single DNA test, which could be offered in selected cases.

Mutations in fibroblast growth-factor receptor 3 in sporadic cases of achondroplasia occur exclusively on the paternally derived chromosome

More than 97% of achondroplasia cases are caused by one of two mutations (G1138A and G1138C) in the fibroblast growth factor receptor 3 (FGFR3) gene, which results in a specific amino acid substitution, G380R. Sporadic cases of achondroplasia have been associated with advanced paternal age, suggesting that these mutations occur preferentially during spermatogenesis. We have determined the parental origin of the achondroplasia mutation in 40 sporadic cases. Three distinct 1-bp polymorphisms were identified in the FGFR3 gene, within close proximity to the achondroplasia mutation site. Ninety-nine families, each with a sporadic case of achondroplasia in a child, were analyzed in this study. In this population, the achondroplasia mutation occurred on the paternal chromosome in all 40 cases in which parental origin was unambiguous. This observation is consistent with the clinical observation of advanced paternal age resulting in new cases of achondroplasia and suggests that factors influencing DNA replication or repair during spermatogenesis, but not during oogenesis, may predispose to the occurrence of the G1138 FGFR3 mutations.

Mutation rate: a simple concept has become complex

The factors that cause new mutations or affect the rate at which they occur have important implications for many areas of genetics. But recent work on phenomena such as premeiotic mutations, which yield a cluster of identical new mutants at the some time, led us to realize that researchers are using the term "mutation rate" in different, and sometimes contradictory, ways. One premeiotic genetic change may ultimately yield several new mutant offspring, but should this be considered one new mutation or many? The way the data are handled in analyses can have a significant effect on the results. How, then, does one handle clusters in the estimation of mutation rates? We explore this question and propose that geneticists begin to distinguish clearly between three different phenomena that to this point have been given the same name: the initial prerepair "genetic damage rate," the postrepair "mutational event rate," and the observed "mutation rate" as it is expressed in the proportion of new mutant offspring. We believe that all new mutant offspring should be counted when estimating mutation rate, irrespective of when in the developmental cycle it is believed that the initial mutational event occurred.

Unexpected familial recurrence in Angelman syndrome

We report on two instances of familial recurrence of Angelman syndrome which, from pedigree analysis, appear incompatible with currently known mechanisms of inheritance of this disorder. In these two families, deletion-positive Angelman syndrome has recurred in cousins. Several established mechanisms for deletion-positive familial recurrence have been ruled out. In each family, molecular cytogenetic studies show typical chromosome 15 deletions, and DNA methylation analysis verifies the maternal origin of the deleted chromosomes in all four individuals. Since the mothers of the affected individuals in each family are not known to be related, these recurrences appear to be secondary to coincidental, de novo events. This conclusion is consistent with direct and indirect estimates of the population frequency of Angelman syndrome.

Sibs with cleidocranial dysplasia born to normal parents: germ line mosaicism?

Cleidocranial dysplasia (CCD) is classically an autosomal dominant disorder. However, the possibility of an autosomal recessive form of CCD has been suggested based on a report of 2 consanguineous families, one with a single affected child, the second with affected sibs, born to normal parents. We present a family with sibs with CCD born to normal parents, and suggest germ line mosaicism as the more likely mechanism for this occurrence.

Exclusive paternal origin of new mutations in Apert syndrome

Apert syndrome results from one or other of two specific nucleotide substitutions, both C-->G transversions, in the fibroblast growth factor receptor 2 (FGFR2) gene. The frequency of new mutations, estimated as 1 per 65,000 live births, implies germline transversion rates at these two positions are currently the highest known in the human genome. Using a novel application of the amplification refractory mutation system (ARMS), we have determined the parental origin of the new mutation in 57 Apert families: in every case, the mutation arose from the father. This identifies the biological basis of the paternal age effect for new mutations previously suggested for this disorder.

Two distinct mutations in a single dystrophin gene: identification of an altered splice-site as the primary Becker muscular dystrophy mutation

A single base change in the 5' splice-site of intron 19 has been identified as the cause of the Becker muscular dystrophy in a family which had previously been deduced to carry both a major deletion and another, at that stage unidentified, mutation in the same dystrophin gene [Laing et al., 1992]. RNA from a muscle biopsy of one of the Becker muscular dystrophy patients in the family was analysed using the reverse transcriptase-polymerase chain reaction (RT-PCR) to study the mature gene transcript. Exon 19 was deleted from the dystrophin mRNA but present at the genomic level. The loss of exon 19 in the mature mRNA was found to be associated with an A to C mutation in the 5' splice site of intron 19. Deletion of exon 19 should alter the reading frame of the mRNA and be associated with a severe form of muscular dystrophy; however, low levels of normal-size dystrophin message and dystrophin were present in this patient. The distance between the splice-site mutation and the secondary deletion in the dystrophin gene is such that it would seem unlikely that the initial base change could act as a premutation for the deletion. Specific primers to detect the splice-site mutation have been designed and used to genotype all relatives.

Two distinct mutations in a single dystrophin gene: chance occurrence or premutation?

We report on a kindred segregating 2 distinct mutations of a dystrophin gene. DNA analysis showed that the second mutation, a deletion, arose in the same gene carrying the primary defect which produced a Becker phenotype in the affected males. The DNA data for this family are reported and the alternative explanations of chance occurrence and premutation are discussed to explain these unusual findings.

Theoretical considerations on germline mosaicism in Duchenne muscular dystrophy

A newly formulated mutation selection equilibrium for lethal X linked recessive traits such as Duchenne muscular dystrophy is presented, which allows for both male and female germline mosaicism. Estimates of the additional parameters used are given, thus allowing the incorporation of germline mosaicism into the calculation of genetic risks.

Achondroplasia in sibs of normal parents

A new case of recurrent achondroplasia in sibs of normal parents is reported. Two sisters and a half sister were affected. Various mechanisms can be postulated to account for unexpected recurrence of achondroplasia in the same sibship. Germinal mosaicism and unstable premutation are discussed here.

Genetic counselling in unexpected familial recurrence of achondroplasia

Two additional pedigrees with familial recurrence of achondroplasia are described. Genetic risk for children of sibs of affected individuals or premutation carriers seems to be low, but it is advisable to monitor at risk pregnancies by midtrimester ultrasonography to diagnose fetal achondroplasia.

Gonadal mosaicism in pseudoachondroplasia

We report on a family in which a brother and sister have pseudoachondroplasia and normal parents. The brother married a normal woman, and they have 2 daughters; one of them has typical changes of pseudoachondroplasia, the other is normal. The most likely explanation in this family is gonadal (germinal cell) mosaicism in one of the grandparents. Other reports of possible autosomal recessive pseudoachondroplasia are reviewed. It is likely that gonadal mosaicism is responsible for a small percentage of cases with what appears to be a new mutation for pseudoachondroplasia.

Inflammatory linear verrucous epidermal nevus (ILVEN) in a mother and her daughter

Inflammatory linear verrucous epidermal nevus (ILVEN) is a rare skin disease with characteristic clinical and histological changes. The approximately 100 cases so far reported include only one familial occurrence involving a woman and her nephew. We report the occurrence of typical ILVEN in a 47-year-old mother and her 17-year-old daughter. So far, no clear-cut genetic interpretation of this observation can be given. Apart from coincidence, various genetic explanations are considered, including X-linked inheritance with extreme lyonization.

Speculation on the role of transposable elements in human genetic disease with particular attention to achondroplasia and the fragile X syndrome

We suggest that mutations for fragile X-positive Martin-Bell syndrome, and perhaps also for achondroplasia, may result from the insertion of transposable elements (TE's). Loss of genetic function could result from either the insertion of TE's within or adjacent to a normal chromosomal gene or, in the case of fragile X, from the loss of genes distal to the site of TE insertion following subsequent TE excision without ligation of the resulting discontinuity. The phenotypically and often cytogenetically normal transmitting males in fragile X pedigrees are interpreted not as "nonpenetrant" transmitters of a fully formed fragile X but rather as transmitters of some or all of the factors necessary for TE insertion at Xq27. We consider it likely that such insertion frequently first occurs, both in soma and especially in the germline, in their daughters. Our models predict that father to son transmission of causative factors would be a common occurrence in fragile X pedigrees. The absence of documented father to son transmission either points to a flaw in the models or reflects systematic bias in the collection of pedigree information.

Familial recurrence of achondroplasia

A family is reported among whom 2 second-cousins-once removed (two half or half third cousins) demonstrate clinical and radiological features of classical achondroplasia.