Variable expressivity of the phenotype in two families with brachydactyly type E, craniofacial dysmorphism, short stature and delayed bone age caused by novel heterozygous mutations in the PTHLH gene

A novel heterozygous mutation in PTHLH causing autosomal dominant brachydactyly type E complicated with short stature

BACKGROUND

Brachydactyly type E (BDE) is a general term characterized by variable shortening of metacarpals and metatarsals, with phalanges affected frequently. It can occur as an isolated form or part of syndromes and manifest a high degree of phenotypic variability. In this study, we have identified the clinical characteristics and pathogenic causes of a four-generation pedigree with 10 members affected by BDE and short stature.

METHODS

After the informed consent was signed, clinical data and peripheral blood samples were collected from available family members. Karyotype analysis, array-CGH, next-generation sequencing, and Sanger sequencing were employed to identity the pathogenic candidate gene.

RESULTS

No translocation or microdeletion/duplication was found in karyotype analysis and array-CGH; hence, a novel heterozygous mutation, c.146dupA. p.S50Vfs*22, was detected by next-generation sequencing in PTHLH gene, leading to a premature stop codon. Subsequently, the mutation was confirmed by Sanger sequencing and co-segregation analysis.

CONCLUSION

In this study, we described a novel heterozygous mutation (c.146dupA. p.S50Vfs*22) of gene PTHLH in a Chinese family. The mutation could induce a premature stop codon leading to a truncation of the protein. Our study broadened the mutation spectrum of PTHLH in BDE.

A novel mutation in PTHLH in a family with a variable phenotype with brachydactyly, short stature, oligodontia and developmental delay

Mutations in PTHLH (PTH-like hormone), cause brachydactyly type E (BDE) characterized by shortening of metacarpals, metatarsals and/or phalanges with short stature. In this report we describe three siblings and their mother with a novel heterozygous mutation c.25 T > C, p.Trp9Arg in exon 2 of the PTHLH gene. Beside the known clinical features of PTHLH mutations all had a delay in speech and language development, unknown if this is related to the mutation. Patients with PTHLH mutation may have a variable phenotypic presentation.

TMCO3, a Putative K+ :Proton Antiporter at the Golgi Apparatus, Is Important for Longitudinal Growth in Mice and Humans

Isolated short stature, defined as short stature without any other abnormalities, is a common heterogeneous condition in children. Exome sequencing identified the homozygous nonsense variant c.1832G>A/p.(Trp611*) in TMCO3 in two sisters with isolated short stature. Radiological studies, biochemical measurements, assessment of the skeletal status, and three-dimensional bone microarchitecture revealed no relevant skeletal and bone abnormalities in both sisters. The homozygous TMCO3 variant segregated with short stature in the family. TMCO3 transcript levels were reduced by ~50% in leukocyte-derived RNA of both sisters compared with controls, likely due to nonsense-mediated mRNA decay. In primary urinary cells of heterozygous family members, we detected significantly reduced TMCO3 protein levels. TMCO3 is functionally uncharacterized. We ectopically expressed wild-type TMCO3 in HeLa and ATDC5 chondrogenic cells and detected TMCO3 predominantly at the Golgi apparatus, whereas the TMCO3W611* mutant did not reach the Golgi. Coordinated co-expression of TMCO3W611* -HA and EGFP in HeLa cells confirmed intrinsic instability and/or degradation of the mutant. Tmco3 is expressed in all relevant mouse skeletal cell types. Highest abundance of Tmco3 was found in chondrocytes of the prehypertrophic zone in mouse and minipig growth plates where it co-localizes with a Golgi marker. Knockdown of Tmco3 in differentiated ATDC5 cells caused reduced and increased expression of Pthlh and Ihh, respectively. Measurement of long bones in Tmco3tm1b(KOMP)Wtsi knockout mice revealed significant shortening of forelimbs and hindlimbs. TMCO3 is a potential member of the monovalent cation:proton antiporter 2 (CPA2) family. By in silico tools and homology modeling, TMCO3 is predicted to have an N-terminal secretory signal peptide, forms a dimer localized to the membrane, and is organized in a dimerization and a core domain. The core domain contains the CPA2 motif essential for K+ binding and selectivity. Collectively, our data demonstrate that loss of TMCO3 causes growth defects in both humans and mice. © 2023 American Society for Bone and Mineral Research (ASBMR).

Structural pharmacology of PTH and PTHrP

Parathyroid hormone (PTH) and PTH-related peptide (PTHrP) regulate extracellular phosphate and calcium homeostasis as well as bone remodeling. PTH is a classic endocrine peptide hormone whose synthesis and negative feedback by multiple factors control release from the parathyroid glands. PTHrP is ubiquitously expressed (pre- and postnatally) and acts in an autocrine/paracrine manner. This review considers the structural pharmacology and actions of PTH and PTHrP, biological consequences of inherited mutations, engineered analogs that illuminate similarities and differences in physiologic actions, and targeted therapeutic opportunities.

Novel Pathogenetic Variants in PTHLH and TRPS1 Genes Causing Syndromic Brachydactyly

Skeletal disorders, including both isolated and syndromic brachydactyly type E, derive from genetic defects affecting the fine tuning of the network of pathways involved in skeletogenesis and growth-plate development. Alterations of different genes of this network may result in overlapping phenotypes, as exemplified by disorders due to the impairment of the parathyroid hormone/parathyroid hormone-related protein pathway, and obtaining a correct diagnosis is sometimes challenging without a genetic confirmation. Five patients with Albright's hereditary osteodystrophy (AHO)-like skeletal malformations without a clear clinical diagnosis were analyzed by whole-exome sequencing (WES) and novel potentially pathogenic variants in parathyroid hormone like hormone (PTHLH) (BDE with short stature [BDE2]) and TRPS1 (tricho-rhino-phalangeal syndrome [TRPS]) were discovered. The pathogenic impact of these variants was confirmed by in vitro functional studies. This study expands the spectrum of genetic defects associated with BDE2 and TRPS and demonstrates the pathogenicity of TRPS1 missense variants located outside both the nuclear localization signal and the GATA ((A/T)GATA(A/G)-binding zinc-containing domain) and Ikaros-like binding domains. Unfortunately, we could not find distinctive phenotypic features that might have led to an earlier clinical diagnosis, further highlighting the high degree of overlap among skeletal syndromes associated with brachydactyly and AHO-like features, and the need for a close interdisciplinary workout in these rare patients. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Whole Genome Sequencing Unravels New Genetic Determinants of Early-Onset Familial Osteoporosis and Low BMD in Malta

Background: Osteoporosis is a skeletal disease with a strong genetic background. The study aimed to identify the genetic determinants of early-onset familial osteoporosis and low bone mineral density (BMD) in a two-generation Maltese family. Methods: Fifteen relatives aged between 28–74 years were recruited. Whole genome sequencing was conducted on 12 relatives and shortlisted variants were genotyped in the Malta Osteoporotic Fracture Study (MOFS) for replication. Results: Sequential variant filtering following a dominant inheritance pattern identified rare missense variants within SELP, TGF-β2 and ADAMTS20, all of which were predicted to be likely pathogenic and participate in osteoimmunology. TGF-β2 c.1136C>T was identified in five individuals from the MOFS in heterozygosity, four of whom had osteopenia/osteoporosis at the lumbar spine and hip, and/or had sustained a low-trauma fracture. Heterozygosity for the ADAMTS20 c.4090A>T was accompanied by lower total hip BMD (p = 0.018) and lower total serum calcium levels in MOFS (p < 0.01), recapitulating the findings from the family. Women carrying at least one copy of the alternative allele (TC/CC) for SELP c.2177T>C exhibited a tendency for lower lumbar spine BMD and/or wrist fracture history relative to women with TT genotype. Conclusions: Our findings suggest that the identified variants, alone or in combination, could be causal factors of familial osteoporosis and low BMD, requiring replication in larger collections.

Physiological and Pharmacological Roles of PTH and PTHrP in Bone Using Their Shared Receptor, PTH1R

Parathyroid hormone (PTH) and the paracrine factor, PTH-related protein (PTHrP), have preserved in evolution sufficient identities in their amino-terminal domains to share equivalent actions upon a common G protein-coupled receptor, PTH1R, that predominantly uses the cyclic adenosine monophosphate-protein kinase A signaling pathway. Such a relationship between a hormone and local factor poses questions about how their common receptor mediates pharmacological and physiological actions of the two. Mouse genetic studies show that PTHrP is essential for endochondral bone lengthening in the fetus and is essential for bone remodeling. In contrast, the main postnatal function of PTH is hormonal control of calcium homeostasis, with no evidence that PTHrP contributes. Pharmacologically, amino-terminal PTH and PTHrP peptides (teriparatide and abaloparatide) promote bone formation when administered by intermittent (daily) injection. This anabolic effect is remodeling-based with a lesser contribution from modeling. The apparent lesser potency of PTHrP than PTH peptides as skeletal anabolic agents could be explained by lesser bioavailability to PTH1R. By contrast, prolongation of PTH1R stimulation by excessive dosing or infusion, converts the response to a predominantly resorptive one by stimulating osteoclast formation. Physiologically, locally generated PTHrP is better equipped than the circulating hormone to regulate bone remodeling, which occurs asynchronously at widely distributed sites throughout the skeleton where it is needed to replace old or damaged bone. While it remains possible that PTH, circulating within a narrow concentration range, could contribute in some way to remodeling and modeling, its main physiological role is in regulating calcium homeostasis.

Shortened Fingers and Toes: GNAS Abnormalities are Not the Only Cause

The PTH/PTHrP receptor (PTHR1) mediates the actions of parathyroid hormone (PTH) and PTH-related peptide (PTHrP) by coupling this G protein-coupled receptor (GPCR) to the alpha-subunit of the heterotrimeric stimulatory G protein (Gsα) and thereby to the formation of cAMP. In growth plates, PTHrP-dependent activation of the cAMP/PKA second messenger pathway prevents the premature differentiation of chondrocytes into hypertrophic cells resulting in delayed growth plate closure. Heterozygous mutations in GNAS, the gene encoding Gsα, lead to a reduction in cAMP levels in growth plate chondrocytes that is sufficient to cause shortening of metacarpals and/or -tarsals, i. e. typical skeletal aspects of Albright's Hereditary Osteodystrophy (AHO). However, heterozygous mutations in other genes, including those encoding PTHrP, PRKAR1A, PDE4D, and PDE3A, can lead to similar or even more pronounced acceleration of skeletal maturation that is particularly obvious in hands and feet, and reduces final adult height. Genetic mutations other than those resulting in Gsα haploinsufficiency thus reduce intracellular cAMP levels in growth plate chondrocytes to a similar extent and thereby accelerate skeletal maturation.

A 3.06-Mb interstitial deletion on 12p11.22-12.1 caused brachydactyly type E combined with pectus carinatum

BACKGROUND

Brachydactyly, a developmental disorder, refers to shortening of hands/feet due to small or missing metacarpals/metatarsals and/or phalanges. Isolated brachydactyly type E (BDE), characterized by shortened metacarpals and/or metatarsals, consists in a small proportion of patients with Homeobox D13 (HOXD13) or parathyroid-hormone-like hormone (PTHLH) mutations. BDE is often accompanied by other anomalies that are parts of many congenital syndromes. In this study, we investigated a Chinese family presented with BDE combined with pectus carinatum and short stature.

METHODS

A four-generation Chinese family was recruited in June 2016. After informed consent was obtained, venous blood was collected, and genomic DNA was extracted by standard procedures. Whole-exome sequencing was performed to screen pathogenic mutation, array comparative genomic hybridization (Array-CGH) analysis was used to analyze copy number variations, and quantitative real-time polymerase chain reaction (PCR), stride over breakpoint PCR (gap-PCR), and Sanger sequencing were performed to confirm the candidate variation.

RESULTS

A 3.06-Mb deletion (chr12:25473650-28536747) was identified and segregated with the phenotype in this family. The deletion region encompasses 23 annotated genes, one of which is PTHLH which has been reported to be causative to the BDE. PTHLH is an important regulator of endochondral bone development. The affected individuals showed bilateral, severe, and generalized brachydactyly with short stature, pectus carinatum, and prematurely fusion of epiphyses. The feature of pectus carinatum has not been described in the PTHLH-related BDE patients previously.

CONCLUSIONS

The haploinsufficiency of PTHLH might be responsible for the disease in this family. This study has expanded the knowledge on the phenotypic presentation of PTHLH variation.

A Heterozygous Splice-Site Mutation in PTHLH Causes Autosomal Dominant Shortening of Metacarpals and Metatarsals

Short metacarpals and/or metatarsals are typically observed in pseudohypoparathyroidism (PHP) type Ia (PHP1A) or pseudo-PHP (PPHP), disorders caused by inactivating GNAS mutations involving exons encoding the alpha-subunit of the stimulatory G protein (Gsα). Skeletal abnormalities similar to those in PHP1A/PPHP were present in several members of an extended Belgian family without evidence for abnormal calcium and phosphate regulation. Direct nucleotide sequencing of genomic DNA from an affected individual (190/III-1) excluded GNAS mutations. Instead, whole exome analysis revealed a novel heterozygous A>G change at nucleotide -3 upstream of PTHLH exon 3 that encodes the last two amino acids of the prosequence and the mature PTHrP. The same nucleotide change was also found in her affected mother and maternal aunt (190/II-2, 190/II-1), and her affected twin sons (190/IV-1, 190/IV-2), but not in her unaffected daughter (190/IV-3) and sister (190/III-2). Complementary DNA derived from immortalized lymphoblastoid cells from 190/IV-2 (affected) and 190/IV-3 (unaffected) was PCR-amplified using forward primers located either in PTHLH exon 1 (noncoding) or exon 2 (presequence and most of the prosequence), and reverse primers located in the 3'-noncoding regions of exons 3 or 4. Nucleotide sequence analysis of these amplicons revealed for the affected son 190/IV-2, but not for the unaffected daughter 190/IV-3, a heterozygous insertion of genomic nucleotides -2 and -1 causing a frameshift after residue 34 of the pre/prosequence and thus 29 novel residues without homology to PTHrP or any other protein. Our findings extend previous reports indicating that PTHrP haploinsufficiency causes skeletal abnormalities similar to those observed with heterozygous GNAS mutations. © 2018 American Society for Bone and Mineral Research.

Diagnosis and management of pseudohypoparathyroidism and related disorders: first international Consensus Statement

This Consensus Statement covers recommendations for the diagnosis and management of patients with pseudohypoparathyroidism (PHP) and related disorders, which comprise metabolic disorders characterized by physical findings that variably include short bones, short stature, a stocky build, early-onset obesity and ectopic ossifications, as well as endocrine defects that often include resistance to parathyroid hormone (PTH) and TSH. The presentation and severity of PHP and its related disorders vary between affected individuals with considerable clinical and molecular overlap between the different types. A specific diagnosis is often delayed owing to lack of recognition of the syndrome and associated features. The participants in this Consensus Statement agreed that the diagnosis of PHP should be based on major criteria, including resistance to PTH, ectopic ossifications, brachydactyly and early-onset obesity. The clinical and laboratory diagnosis should be confirmed by a molecular genetic analysis. Patients should be screened at diagnosis and during follow-up for specific features, such as PTH resistance, TSH resistance, growth hormone deficiency, hypogonadism, skeletal deformities, oral health, weight gain, glucose intolerance or type 2 diabetes mellitus, and hypertension, as well as subcutaneous and/or deeper ectopic ossifications and neurocognitive impairment. Overall, a coordinated and multidisciplinary approach from infancy through adulthood, including a transition programme, should help us to improve the care of patients affected by these disorders.

Novel Mutation in PTHLH Related to Brachydactyly Type E2 Initially Confused with Unclassical Pseudopseudohypoparathyroidism

BACKGROUND

Autosomal-dominant brachydactyly type E is a congenital abnormality characterized by small hands and feet, which is a consequence of shortened metacarpals and metatarsals. We recently encountered a young gentleman exhibiting shortening of 4th and 5th fingers and toes. Initially, we suspected him having pseudopseudohypoparathyroidism (PPHP) because of normal biochemical parameters, including electrolyte, Ca, P, and parathyroid hormone (PTH) levels; however, his mother and maternal grandmother had the same conditions in their hands and feet. Furthermore, his mother showed normal biochemical parameters. To the best of our knowledge, PPHP is inherited via a mutated paternal allele, owing to the paternal imprinting of GNAS (guanine nucleotide binding protein, alpha stimulating) in the renal proximal tubule. Therefore, we decided to further analyze the genetic background in this family.

METHODS

Whole exome sequencing was performed using genomic DNA from the affected mother, son, and the unaffected father as a negative control.

RESULTS

We selected the intersection between 45,490 variants from the mother and 45,646 variants from the son and excluded 27,512 overlapping variants identified from the father. By excluding homogenous and compound heterozygous variants and removing all previously reported variants, 147 variants were identified to be shared by the mother and son. Variants that had least proximities among species were excluded and finally 23 variants remained.

CONCLUSION

Among them, we identified a defect in parathyroid hormone like hormone (PTHLH), encoding the PTH-related protein, to be disease-causative. Herein, we report a family affected with brachydactyly type E2 caused by a novel PTHLH mutation, which was confused with PPHP with unclassical genetic penetrance.

What to consider when pseudohypoparathyroidism is ruled out: iPPSD and differential diagnosis

BACKGROUND

Pseudohypoparathyroidism (PHP) is a rare disease whose phenotypic features are rather difficult to identify in some cases. Thus, although these patients may present with the Albright's hereditary osteodystrophy (AHO) phenotype, which is characterized by small stature, obesity with a rounded face, subcutaneous ossifications, mental retardation and brachydactyly, its manifestations are somewhat variable. Indeed, some of them present with a complete phenotype, whereas others show only subtle manifestations. In addition, the features of the AHO phenotype are not specific to it and a similar phenotype is also commonly observed in other syndromes. Brachydactyly type E (BDE) is the most specific and objective feature of the AHO phenotype, and several genes have been associated with syndromic BDE in the past few years. Moreover, these syndromes have a skeletal and endocrinological phenotype that overlaps with AHO/PHP. In light of the above, we have developed an algorithm to aid in genetic testing of patients with clinical features of AHO but with no causative molecular defect at the GNAS locus. Starting with the feature of brachydactyly, this algorithm allows the differential diagnosis to be broadened and, with the addition of other clinical features, can guide genetic testing.

METHODS

We reviewed our series of patients (n = 23) with a clinical diagnosis of AHO and with brachydactyly type E or similar pattern, who were negative for GNAS anomalies, and classify them according to the diagnosis algorithm to finally propose and analyse the most probable gene(s) in each case.

RESULTS

A review of the clinical data for our series of patients, and subsequent analysis of the candidate gene(s), allowed detection of the underlying molecular defect in 12 out of 23 patients: five patients harboured a mutation in PRKAR1A, one in PDE4D, four in TRPS1 and two in PTHLH.

CONCLUSIONS

This study confirmed that the screening of other genes implicated in syndromes with BDE and AHO or a similar phenotype is very helpful for establishing a correct genetic diagnosis for those patients who have been misdiagnosed with "AHO-like phenotype" with an unknown genetic cause, and also for better describing the characteristic and differential features of these less common syndromes.