Mosaicism in pseudoachondroplasia

Co-occurrence of neurofibromatosis type 1 and pseudoachondroplasia - a first case report

BACKGROUND

Neurofibromatosis type 1 and pseudoachondroplasia are both rare autosomal dominant disorders, caused by pathogenic mutations in NF1 and COMP genes, respectively. Both neurofibromin 1 and cartilage oligomeric matrix protein (COMP) play a role in the development of the skeleton. Carrying both germline mutations has not been previously reported; however, it can affect the developing phenotype.

CASE PRESENTATION

The index patient, an 8-year-old female presented with several skeletal and dermatologic anomalies resembling the coexistence of multiple syndromes. Her mother had dermatologic symptoms characteristic for neurofibromatosis type 1, and her father presented with distinct skeletal anomalies. NGS-based analysis revealed a heterozygous pathogenic mutation in genes NF1 and COMP in the index patient. A previously unreported heterozygous variant was detected for the NF1 gene. The sequencing of the COMP gene revealed a previously reported, pathogenic heterozygous variant that is responsible for the development of the pseudoachondroplasia phenotype.

CONCLUSIONS

Here, we present the case of a young female carrying pathogenic NF1 and COMP mutations, diagnosed with two distinct heritable disorders, neurofibromatosis type 1 and pseudoachondroplasia. The coincidence of two monogenic autosomal dominant disorders is rare and can pose a differential diagnostic challenge. To the best of our knowledge, this is the first reported co-occurrence of these syndromes.

Genomic mosaicism: A neglected factor that promotes variability in asthma diagnosis

To elucidate the genetic architecture of asthma continues to be a challenge for molecular biologists and medical researchers. However, powerful genomic technologies are at disposal to help decipher complete human genomes; the genetic variability in asthma hinders the discovery of common molecular markers for this disease. In this context, we purpose to explore genomic mosaicism on asthma cells' biology as a strategy to discover key mechanisms, which can complement or re-define asthma diagnosis. Recent evidences showed that genomic mosaicism could be a normal event. In brains, each neuron may harbor hundreds of genetic alterations, which may contribute to neuronal diversity. Thus, can mosaicism be a natural motor of diversity in asthma? Why this genetic event is little described in scientific literature? To discuss these questions, we perform a critical review about the normality of genomic mosaicism; moreover, we examine the difficulty of current experimental approaches to detect different genotypes in cell populations of one individual.

Novel therapeutic interventions for pseudoachondroplasia

Pseudoachondroplasia (PSACH), a severe short-limbed dwarfing condition, is associated with life-long joint pain and early onset osteoarthritis. PSACH is caused by mutations in cartilage oligomeric matrix protein (COMP), a pentameric matricellular protein expressed primarily in cartilage and other musculoskeletal tissues. Mutations in COMP diminish calcium binding and as a result perturb protein folding and export to the extracellular matrix. Mutant COMP is retained in the endoplasmic reticulum (ER) of growth plate chondrocytes resulting in massive intracellular COMP retention. COMP trapped in the ER builds an intracellular matrix network that may prevent the normal cellular clearance mechanisms. We have shown that accumulation of intracellular matrix in mutant-COMP (MT-COMP) mice stimulates intense unrelenting ER stress, inflammation and oxidative stress. This cytotoxic stress triggers premature death of growth plate chondrocytes limiting long-bone growth. Here, we review the mutant COMP pathologic mechanisms and anti-inflammatory/antioxidant therapeutic approaches to reduce ER stress. In MT-COMP mice, aspirin and resveratrol both dampen the mutant COMP chondrocyte phenotype by decreasing intracellular accumulation, chondrocyte death and inflammatory marker expression. This reduction in chondrocyte stress translates into an improvement in long-bone growth in the MT-COMP mice. Our efforts now move to translational studies targeted at reducing the clinical consequences of MT-COMP and painful sequelae associated with PSACH.

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.

Pathologic diagnosis of achondrogenesis type 2 in a fragmented fetus: case report and evidence-based differential diagnostic approach in the early midtrimester

As a group, lethal genetic skeletal disorders (GSDs) usually result in death within the perinatal period. Because lethal GSDs are often ultrasonographically detectible by early midtrimester, dilation and evacuation (D&E) is the method of choice for elective termination of pregnancy in many institutions. However, because the diagnosis of the lethal GSDs relies heavily upon radiologic examination of fetal remains, reaching an accurate diagnosis in this setting can be challenging. We report an autopsy case of a fetus delivered by D&E at 15 4/7 weeks gestation with radiologic, histologic, and genetic findings compatible with achondrogenesis type 2 and discuss an evidence-based differential diagnostic approach to lethal GSDs terminated by early midtrimester D&E.

A novel COMP mutation in a Chinese patient with pseudoachondroplasia

A 2.75-year-old Chinese boy presented with typical clinical features of pseudoachondroplasia, including disproportionate short-limb short stature, brachydactyly, genu varus and waddling gait. Radiologically, tubular bones were short with widened metaphyses, irregular and small epiphyses; anterior tonguing or beaking of vertebral bodies were characteristic. DNA sequencing analysis of the COMP gene revealed a heterozygous mutation (c.1511G>A, p.Cys504Tyr) in the patient but his parents were unaffected without this genetic change. The missense mutation (c.1511G>A) was not found in 100 healthy controls and has not been reported previously. Our findings expand the spectrum of known mutations in COMP leading to pseudoachondroplasia.

Pseudoachondroplasia and multiple epiphyseal dysplasia: a 7-year comprehensive analysis of the known disease genes identify novel and recurrent mutations and provides an accurate assessment of their relative contribution

Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED) are relatively common skeletal dysplasias resulting in short-limbed dwarfism, joint pain, and stiffness. PSACH and the largest proportion of autosomal dominant MED (AD-MED) results from mutations in cartilage oligomeric matrix protein (COMP); however, AD-MED is genetically heterogenous and can also result from mutations in matrilin-3 (MATN3) and type IX collagen (COL9A1, COL9A2, and COL9A3). In contrast, autosomal recessive MED (rMED) appears to result exclusively from mutations in sulphate transporter solute carrier family 26 (SLC26A2). The diagnosis of PSACH and MED can be difficult for the nonexpert due to various complications and similarities with other related diseases and often mutation analysis is requested to either confirm or exclude the diagnosis. Since 2003, the European Skeletal Dysplasia Network (ESDN) has used an on-line review system to efficiently diagnose cases referred to the network prior to mutation analysis. In this study, we present the molecular findings in 130 patients referred to ESDN, which includes the identification of novel and recurrent mutations in over 100 patients. Furthermore, this study provides the first indication of the relative contribution of each gene and confirms that they account for the majority of PSACH and MED.

A familial case of achondrogenesis type II caused by a dominant COL2A1 mutation and "patchy" expression in the mosaic father

Achondrogenesis type II (ACG2) is the most severe disorder that can be produced by dominant mutations in COL2A1. We report on four pregnancies of an apparently healthy, nonconsanguineous young couple. The father had scoliosis as a child, and has slight body disproportion with short trunk. The first child was born at 32 weeks and died neonatally. In the second pregnancy, short limbs and fetal hygroma were noted on ultrasound at 17 weeks' gestation. Similar findings were observed in the third fetus. Clinical, radiological, and histological evaluation of the fetuses after termination of the pregnancies showed findings consistent with ACG2. Molecular analysis of genomic DNA extracted from amniotic cells of the second and third fetuses revealed heterozygosity for a 10370G > T missense mutation (G346V) in the COL2A1 gene. This mutation was also found in the father, as a mosaic. The couple had a fourth pregnancy, and at 11 weeks fetal hydrops with a septated cystic hygroma were obvious. DNA from CVS demonstrated the same COL2A1 mutation.

The presence of germ line mosaicism in cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is typically an autosomal dominant condition. The possibility of alternative causes, such as an autosomal recessive form or germ line mosaicism, have been suggested in some families with CCD, but not proven. We present a family consisting of a mother having three sons affected with CCD. One of the affected boys is a half brother to the other two affected children. The diagnosis of CCD was confirmed by DNA analysis of the RUNX2 gene in all three of the boys in blood; however, initial DNA testing in the mother's blood did not detect the presence of a RUNX2 mutation in the mother. Further testing through heteroduplex analysis applying high-resolution melting analysis followed by subcloning detected low-level mosaicism in DNA isolated from maternal blood and buccal swab, confirming low-level mosaicism in somatic cells. We present the first case of confirmed germ line mosaicism in CCD.

Mosaicism of a thyroid hormone receptor-beta gene mutation in resistance to thyroid hormone

CONTEXT

Heterozygous mutations in thyroid hormone receptor-beta (TRbeta) gene are the cause of resistance to thyroid hormone (RTH) in more than 85% of families having the syndrome. In 23% of the families, TRbeta gene mutations occur de novo. Of the 141 families with RTH investigated by us, 21 (15%) had no TRbeta gene mutations detectable by sequencing from genomic DNA (gDNA) or cDNA (non-TR RTH).

OBJECTIVE

The objective of the study was to investigate the genotype of a family with RTH and correlate it to the phenotype.

DESIGN

The DNA was isolated from different tissues, and the sequence of the TRbeta gene was determined. Clinical studies involved the administration of incremental doses of T(3).

SETTING

The study was conducted at a referral pediatric endocrinology clinic in Turkey and an academic medical center in the United States.

MAIN OUTCOME AND MEASURES

Measurement included markers of thyroid hormone action and sequencing of TRbeta revealing a R338W mutation. Patients and Family: We studied two siblings with short stature, panic disorder, psychosis, and high free iodothyronine concentrations with nonsuppressed TSH and their father with similar thyroid function tests without growth or psychiatric abnormalities.

RESULTS

Direct sequencing of gDNA obtained from the father's leukocytes, buccal mucosa cells, and prostate tissue showed less amplification of the mutant allele (R338W) than the normal allele as confirmed by PCR/restriction fragment length polymorphism analysis. No sequence abnormalities were detected in gDNA from fibroblasts. Similar results were found in mRNA from the leukocytes and fibroblasts. The sensitivity of various tissues to thyroid hormone was not uniform. The progeny had equal amounts of mutant and wild-type gDNA in leukocytes and skin.

CONCLUSIONS

The father has a mosaicism for the R338W mutation as it was present in some cell lineages, including his germline, because it was transferred to his children but not in fibroblasts. This indicates that the mutation occurred de novo in early embryonic life. Here is the first report of mosaicism in RTH. The possibility of mosaicism should be considered in subjects with RTH without apparent mutations in the TRbeta gene.

A disorder resembling pseudoachondroplasia but without COMP mutation

Pseudoachondroplasia (PA) is an autosomal dominant skeletal dysplasia characterized by disproportionate short stature, generalized ligamentous laxity, irregular epi-metaphyseal ossification, and vertebral anomalies that regress with age. It usually manifests in the second year of life or later. The clinically and radiographically variable disorder is caused by mutations in the COMP gene. Parental gonadal mosaicism may lead to recurrence of the disorder in children of unaffected parents. Here, we describe sibs with bone changes similar to those seen in very severe PA born to clinically and radiographically unaffected parents. Sequencing of all 19 exons of the COMP gene failed to disclose a mutation. The sibs appear to be affected by a disorder resembling PA but resulting from a defect of an extracellular matrix protein other than COMP. It may be suspected in patients with unusually severe dwarfism, severe epi-metaphyseal abnormalities, and persistent platyspondyly.

Role of TSP-5/COMP in pseudoachondroplasia

Pseudoachondroplasia (PSACH) is a well-characterized dwarfing condition associated with disproportionate short stature, abnormal joints and osteoarthritis requiring joint replacement. PSACH is caused by mutations in cartilage oligomeric matrix protein (COMP). COMP, the fifth member of the thrombospondin (TSP) gene family, is a pentameric protein found primarily in the extracellular matrix of musculoskeletal tissues. Functional studies have shown that COMP binds types II and IX collagens but the role of COMP in the extracellular matrix remains to be defined. Mutations in COMP interfere with calcium-binding and protein conformation. PSACH growth plate and growth plate chondrocytes studies indicate that COMP mutations have a dominant negative effect with both COMP and type IX collagen being retained in large rER cisternae. This massive retention causes impaired chondrocyte function with little COMP secreted into the matrix and premature loss of chondrocytes. Deficiency of linear growth results from loss of chondrocytes from the growth plate. Secondarily, the matrix contains minimal COMP, which may be normal and/or mutant, and little type IX collagen. This deficiency results in abnormal joints that are easily eroded and cause painful osteoarthritis. Unlike other misfolded proteins that are targeted for degradation, much of the retained COMP escapes degradation, compromises cell function, and causes cell death. Gene therapy will need to target the reduction of COMP in order to restore normal chondrocyte function and longevity.

Mesomelic dwarfism in pseudoachondroplasia

Pseudoachondroplasia (PSACH) is associated with mutations in the cartilage oligomeric matrix protein (COMP) gene and the clinical characteristics include short stature, deformities of the extremities involving the epiphyses and metaphyses, early onset arthritis, and ligament laxity. PSACH has been considered a rhizomelic form of dwarfism. So far no previous report has described mesomelic shortening of the limbs in PSACH. We reviewed nine patients with a diagnosis of PSACH based on clinical and radiographic examination and mutation analysis of the COMP gene. The mean height in the adults was 116 cm. All patients showed mesomelic dwarfism. The average ratios of radial length to humeral length and tibial length to femoral length were 0.62 and 0.63, respectively. The tibia and the radius showed more severe bony deformity than the femur and humerus. The degree of short stature was related to the site of the mutation in the COMP gene, but there was no correlation between bony deformity and height or gene mutation.

Recurrence of achondrogenesis type II within the same family: evidence for germline mosaicism

Achondrogenesis type II is a lethal skeletal dysplasia caused by new dominant mutations within the type II collagen gene (COL2A1). Here we report on two pregnancies of a healthy, nonconsanguineous young couple. In the first pregnancy, severe micromelia and generalized edema were noted on ultrasound at 21 weeks' gestation. Clinical, radiological, and histological evaluation of the fetus after termination of the pregnancy showed typical findings of achondrogenesis type II. In the second pregnancy, fetal hygroma was noted at 11 weeks' gestation. Similar clinical, radiographic, and histologic findings were observed in the second fetus, suggesting the recurrence of achondrogenesis II within this family. Molecular analysis of genomic DNA extracted from amniotic cells of the second fetus revealed heterozygosity for a 1340G > A missense mutation (G316D) in the COL2A1 gene. This mutation was not found in the parents. Although, we could not evaluate the presence of this mutation in the first fetus, we strongly believe that our data are in favor of germline mosaicism as the most likely explanation for the recurrence of type II achondrogenesis in both sibs.

Mechanisms and consequences of somatic mosaicism in humans

Somatic mosaicism -- the presence of genetically distinct populations of somatic cells in a given organism -- is frequently masked, but it can also result in major phenotypic changes and reveal the expression of otherwise lethal genetic mutations. Mosaicism can be caused by DNA mutations, epigenetic alterations of DNA, chromosomal abnormalities and the spontaneous reversion of inherited mutations. In this review, we discuss the human disorders that result from somatic mosaicism, as well as the molecular genetic mechanisms by which they arise. Specifically, we emphasize the role of selection in the phenotypic manifestations of mosaicism.

Pseudoachondroplasia and multiple epiphyseal dysplasia: New etiologic developments

Pseudoachondroplasia (PSACH) (OMIM#177170) and multiple epiphyseal dysplasia (MED) are separate but overlapping osteochondrodysplasias. PSACH is a dominantly inherited disorder characterized by short-limb short stature, loose joints, and early-onset osteoarthropathy. The diagnosis is based on characteristic clinical and radiographic findings. Only mutations in the cartilage oligomeric matrix protein (COMP) gene have been reported in PSACH, and all family studies have been consistent with linkage to the COMP locus on chromosome 19. Multiple epiphyseal dysplasia (MED) is a relatively mild chondrodysplasia but like PSACH, MED causes early-onset joint degeneration, particularly of the large weight-bearing joints. Given the clinical similarity between PSACH and MED, it was not surprising that the first MED locus identified was the COMP gene (EDM1). Mutations causing MED have now been identified in five other genes (COL9A1, COL9A2, COL9A3, DTDST, and MATN3), making MED one of the most genetically heterogeneous disorders. This article reviews the clinical features of PSACH and MED, the known mutations, and the pathogenetic effect of COMP mutations on the cartilage extracellular matrix.

Pseudoachondroplasia and multiple epiphyseal dysplasia: mutation review, molecular interactions, and genotype to phenotype correlations

Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED) constitute a bone dysplasia family, which is both genetically and phenotypically heterogeneous. The disease spectrum ranges from mild MED, which manifests with pain and stiffness in the joints and delayed and irregular ossification of the epiphyses, to the more severe PSACH, which is characterized by marked short stature, deformity of the legs, and ligamentous laxity. PSACH is almost exclusively caused by mutations in cartilage oligomeric matrix protein (COMP) whereas various forms of MED are caused by mutations in the genes encoding COMP, type IX collagen (COL9A1, COL9A2, and COL9A3), matrilin-3 (MATN3), and solute carrier member 26, member 2 gene (SLC26A2). In this review we discuss specific disease-causing mutations and the clustering of these mutations in functionally and structurally important regions of the respective gene products, genotype to phenotype correlations, and the diagnostic relevance of mutation screening in these osteochondrodysplasias.

Occurrence of thanatophoric dysplasia type I (R248C) and hypochondroplasia (N540K) mutations in two patients with achondroplasia phenotype

We report two patients with clinical and radiological findings of achondroplasia, who had the most common FGFR3 mutation occurring in thanatophoric dysplasia type I and hypochondroplasia, respectively. Thanatophoric dysplasia is usually a lethal condition, but the patient carrying this mutation is alive and presents a medical history similar to that of patients with achondroplasia. The events leading to such a discrepancy between genotype and phenotype are unclear. These rare cases may influence an appropriate medical and genetic counseling.

Mutations in cartilage oligomeric matrix protein causing pseudoachondroplasia and multiple epiphyseal dysplasia affect binding of calcium and collagen I, II, and IX

Mutations in type 3 repeats of cartilage oligomeric matrix protein (COMP) cause two skeletal dysplasias, pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED). We expressed recombinant wild-type COMP that showed structural and functional properties identical to COMP isolated from cartilage. A fragment encompassing the eight type 3 repeats binds 14 calcium ions with moderate affinity and high cooperativity and presumably forms one large disulfide-bonded folding unit. A recombinant PSACH mutant COMP in which Asp-469 was deleted (D469 Delta) and a MED mutant COMP in which Asp-361 was substituted by Tyr (D361Y) were both secreted into the cell culture medium of human cells. Circular dichroism spectroscopy revealed only small changes in the secondary structures of D469 Delta and D361Y, demonstrating that the mutations do not dramatically affect the folding and stability of COMP. However, the local conformations of the type 3 repeats were disturbed, and the number of bound calcium ions was reduced to 10 and 8, respectively. In addition to collagen I and II, collagen IX also binds to COMP with high affinity. The PSACH and MED mutations reduce the binding to collagens I, II, and IX and result in an altered zinc dependence. These interactions may contribute to the development of the patient phenotypes and may explain why MED can also be caused by mutations in collagen IX genes.

Somatic mosaicism and variable expressivity

For more than 50 years geneticists have assumed that variations in phenotypic expression are caused by alterations in genotype. Recent evidence shows that 'simple' mendelian disorders or monogenic traits are often far from simple, exhibiting phenotypic variation (variable expressivity) that cannot be explained entirely by a gene or allelic alteration. In certain cases of androgen insensitivity syndrome caused by identical mutations in the androgen receptor gene, phenotypic variability is caused by somatic mosaicism, that is, somatic mutations that occur only in certain androgen-sensitive cells. Recently, more than 30 other genetic conditions that exhibit variable expressivity have been linked to somatic mosaicism. Somatic mutations have also been identified in diseases such as prostate and colorectal cancer. Therefore, the concept of somatic mutations and mosaicism is likely to have far reaching consequences for genetics, in particular in areas such as genetic counseling.

Identification of nine novel mutations in cartilage oligomeric matrix protein in patients with pseudoachondroplasia and multiple epiphyseal dysplasia

Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (EDM1) are allelic disorders caused by mutations in the gene encoding cartilage oligomeric matrix protein (COMP). PSACH is a dominant condition characterized by disproportionate short stature, joint laxity, and early-onset osteoarthritis. EDM1 is a less severe skeletal dysplasia associated with average to mild short stature, joint pain, and early-onset osteoarthritis. COMP is an extracellular matrix protein present in cartilage, ligament, and tendon tissues. Here, we report on nine novel mutations in COMP causing PSACH and EDM1. Four of these mutations are in exons 13C and 14 where no previous mutations had been reported. One of those mutations was identified in two separate EDM1 families. In addition, we have identified the first case of PSACH resulting from an expansion of the five aspartates in exon 17B. We are also reporting a mutation in a third PSACH family with somatic/germline mosaicism. Therefore, this report increases the range of mutations that cause PSACH and EDM1 and provides additional regions to target for mutational analysis.

Identification of twelve mutations in cartilage oligomeric matrix protein (COMP) in patients with pseudoachondroplasia

Pseudoachondroplasia (PSACH) is an autosomal dominant dwarfing condition characterized by disproportionate short stature, joint laxity, and early-onset osteoarthrosis. PSACH is caused by mutations in the gene encoding cartilage oligomeric matrix protein (COMP). We are reporting on mutations in COMP in 12 patients with PSACH, including ten novel mutations. Eleven of the mutations are in exons 17A, 17B, and 18A, which encode the calcium-binding domains, and one mutation is in exon 19, which encodes part of the carboxy-terminal globular domain. Two of the mutations identified are the common delGAC(1430-1444) in exon 17B, which accounts for 36% of identified PSACH mutations. This report increases the range of mutations in COMP that cause PSACH and provides additional evidence for the importance of the calcium-binding domains and the globular domain to the function of COMP.

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.