Multigenerational case examples of hypophosphatasia: Challenges in genetic counseling and disease management

Hypophosphatasia (HPP) is an inherited metabolic condition caused by pathogenic mutations in the ALPL gene. This leads to deficiency of tissue non-specific alkaline phosphatase (TNSALP), resulting in decreased mineralization of the bones and/or teeth and multi-systemic complications. Inheritance may be autosomal dominant or recessive, and the phenotypic spectrum, including age of onset, varies widely. We present four families demonstrating both modes of inheritance of HPP and phenotypic variability and discuss the resultant challenges in disease management, genetic counseling, and risk assessment. Failure to consider different modes of inheritance in a family with HPP may lead to an inaccurate risk assessment upon which medical and reproductive decisions may be made. We highlight the essential role of high-quality genetic counseling and meaningful biochemical and molecular testing strategies in the evaluation and management of families with HPP.

Perinatal Hypophosphatasia in a Premature Infant

A premature male infant was delivered at 32 weeks' gestation due to category-2 fetal tracing after preterm labor. The physical exam showed shortened and bowed long bones, with calvarium felt in small area of the head. Serum alkaline phosphatase was very low on admission. Extensive metaphyseal abnormalities, bowing of long bones, and poor ossification of all bones were noted on skeletal survey (radiography). Based on ultrasound evidence of "bowing" and long bone fractures at 26 weeks, amniocentesis was performed that later diagnosed hypophosphatasia by genetic testing while ruling out osteogenesis imperfecta. Although ventilated initially, the infant gradually improved with enzyme replacement therapy (ERT) and was extubated to noninvasive ventilation for 6 weeks. Following clinical deterioration with hypoxic respiratory failure secondary to sepsis at 4 months of age, he succumbed to severe pulmonary hypertension, likely secondary to chronic lung disease and prolonged ventilation. Early diagnosis allowed timely initiation of appropriate therapy. Radiological improvement with the therapy showed promising results in this rare disease. Despite specific novel therapy being available, variability in presentation dictates prognosis in this previously universally fatal condition. The potential unknown effects of ERT on pulmonary vascular remodeling need further investigation.

Association of ALPL variants with serum alkaline phosphatase and bone traits in the general Japanese population: The Nagahama Study

Although alkaline phosphatase (ALP) activity is relatively low in carriers of recessive type hypophosphatasia (HPP), most are asymptomatic and therefore do not undergo medical evaluations. We analyzed the association of ALP-encoding ALPL variants with serum ALP and bone traits in the general Japanese population. Study participants (n = 9671) were from the Nagahama Study, which was a longitudinal cohort study of an apparently healthy general Japanese population. ALPL variants were analyzed by whole-genome sequencing or TaqMan probe assays using DNA extracted from peripheral blood samples. The speed of sound in calcaneal bone was assessed by quantitative ultrasound (QUS) and used as surrogate measures of bone mineral density. We identified 13 ALPL variants. Minor allele frequencies of three variants were higher than expected. Variant c.529G > A has been reported as a possible pathogenic variant for adult type HPP. Variants c.979C > T and c.1559delT are reported as pathogenic variants for perinatal severe HPP or infantile HPP. The allele frequencies of c.529G > A, c.979C > T, and c.1559delT were 0.0107, 0.0040, and 0.0014, respectively. Serum ALP activity was significantly lower and differed among the three variants (P < 0.001), as well as between individuals with and without any of the three variants (P < 0.001). Serum ALP activity was inversely associated with QUS values, although no direct association was observed between the ALPL variants and QUS values. An association between serum ALP activity and QUS was confirmed; however, we failed to detect an association between ALPL variants and bone traits in the general Japanese population.

Alkaline Phosphatase Replacement Therapy for Hypophosphatasia in Development and Practice

Hypophosphatasia (HPP) is an inherited disorder that affects bone and tooth mineralization characterized by low serum alkaline phosphatase. HPP is caused by loss-of-function mutations in the ALPL gene encoding the protein, tissue-nonspecific alkaline phosphatase (TNSALP). TNSALP is expressed by mineralizing cells of the skeleton and dentition and is associated with the mineralization process. Generalized reduction of activity of the TNSALP leads to accumulation of its substrates, including inorganic pyrophosphate (PP_i) that inhibits physiological mineralization. This leads to defective skeletal mineralization, with manifestations including rickets, osteomalacia, fractures, and bone pain, all of which can result in multi-systemic complications with significant morbidity, as well as mortality in severe cases. Dental manifestations are nearly universal among affected individuals and feature most prominently premature loss of deciduous teeth. Management of HPP has been limited to supportive care until the introduction of a TNSALP enzyme replacement therapy (ERT), asfotase alfa (AA). AA ERT has proven to be transformative, improving survival in severely affected infants and increasing overall quality of life in children and adults with HPP. This chapter provides an overview of TNSALP expression and functions, summarizes HPP clinical types and pathologies, discusses early attempts at therapies for HPP, summarizes development of HPP mouse models, reviews design and validation of AA ERT, and provides up-to-date accounts of AA ERT efficacy in clinical trials and case reports, including therapeutic response, adverse effects, limitations, and potential future directions in therapy.

Differential diagnosis of perinatal hypophosphatasia: radiologic perspectives

Perinatal hypophosphatasia (HPP) is a rare, potentially life-threatening, inherited, systemic metabolic bone disease that can be difficult to recognize in utero and postnatally. Diagnosis is challenging because of the large number of skeletal dysplasias with overlapping clinical features. This review focuses on the role of fetal and neonatal imaging modalities in the differential diagnosis of perinatal HPP from other skeletal dysplasias (e.g., osteogenesis imperfecta, campomelic dysplasia, achondrogenesis subtypes, hypochondrogenesis, cleidocranial dysplasia). Perinatal HPP is associated with a broad spectrum of imaging findings that are characteristic of but do not occur in all cases of HPP and are not unique to HPP, such as shortening, bowing and angulation of the long bones, and slender, poorly ossified ribs and metaphyseal lucencies. Conversely, absent ossification of whole bones is characteristic of severe lethal HPP and is associated with very few other conditions. Certain features may help distinguish HPP from other skeletal dysplasias, such as sites of angulation of long bones, patterns of hypomineralization, and metaphyseal characteristics. In utero recognition of HPP allows for the assembly and preparation of a multidisciplinary care team before delivery and provides additional time to devise treatment strategies.

Neonatal lethal hypophosphatasia: A case report and review of literature

RATIONALE

Hypophosphatasia (HPP) is a very rare metabolic bone disease caused by loss-of-function mutations in the ALPL gene encoding the tissue nonspecific alkaline phosphatase. The severe neonatal form is considered lethal but insights into manifestations of the disease can help to increase our knowledge of the natural history for an early initiation of treatment and improvement of survival.

PATIENT CONCERNS

We report the case of a newborn in which his fetal imaging showed findings of skeletal dysplasia disorder, considering initially achondroplasia as a potential diagnosis.

DIAGNOSIS

A definitive diagnosis compatible with perinatal lethal HPP was established in the 1st days due to the presentation at birth with thoracic and pulmonary hypoplasia, bone hypomineralization, and undetectable alkaline phosphatase. The genetic analysis identified a new heterozygous c.413G>C mutation and another 1 c.473-2G>C previously described in the ALPL gene.

OUTCOMES

The patient died on the 4th day by clinical course complicated without having started enzyme replacement therapy (ERT). Retrospectively, previous analyzes of the parents already showed both a decreased alkaline phosphatase.

LESSONS

This report highlights the importance of prenatal differential diagnosis of bone dysplasia with the key biochemical marker of alkaline phosphatase in the parents. Substitutive ERT administered very soon after birth, seems to change the prognosis in these patients with neonatal HPP.

Normal development of sacrococcygeal centrum ossification centers in the fetal spine: a postmortem magnetic resonance imaging study

PURPOSE

To describe the temporal pattern of the appearance of the S1-Co1 centrum ossification centers (COCs) and provide reference data for the S1-S5 COCs and sacral length at various gestational ages (GAs).

METHODS

Postmortem magnetic resonance imaging (MRI) was performed on 71 fetuses (GA, 17-42 weeks) using the 3D dual-echo steady-state with water excitation T2 sequence in the sagittal plane. To confirm the reliability of this sequence, the MRI data were compared with the CT and histologic data obtained from two fetuses (GAs, 21 and 30 weeks). The presence or absence of each sacrococcygeal COC was recorded. Sacral length and S1-S5 COC height, sagittal diameter, transverse diameter, cross-sectional area, and volume were measured.

RESULTS

All fetuses showed S1-S3 COCs by 17 weeks, S4 COCs by 19 weeks, and S5 COCs by 28 weeks. The S4, S5, and Co-1 COCs were visualized in 70 (98.59%), 51 (71.83%), and 21 (29.58%) fetuses, respectively. Sacral length, height, sagittal, and transverse diameters increased linearly, while cross-sectional area and volume increased exponentially with advancing GA. Mean growth rates of the sagittal and transverse diameters, cross-sectional area, and volume, but not of height, significantly differed among the S1-S5 vertebrae.

CONCLUSION

We have presented the timing of appearance of individual sacrococcygeal COCs and the age-specific, normative MRI reference values for sacral length and the morphometric parameters of the sacral COCs, which are of clinical importance in the diagnosis of congenital sacral abnormalities and skeletal dysplasia.

Evaluating skeletal dysplasias on prenatal ultrasound: an emphasis on predicting lethality

Lethal skeletal dysplasias can be diagnosed by prenatal ultrasound (US) using several sonographic parameters. Degree of femoral shortening, lung volumes, femur length to abdominal circumference ratio, and chest circumference to abdominal circumference ratio are the most sensitive and specific predictors. Although there are more than 450 different skeletal dysplasias, only a few are lethal in the perinatal period. We review current fetal US literature and present an updated algorithmic approach to first establish lethality and, second, evaluate for hallmark sonographic features to help determine a specific diagnosis.

Prenatal diagnosis of hypophosphatasia congenita using ultrasonography

Congenital hypophosphatasia is a rare fatal skeletal dysplasia. Antenatal determinants of Epub ahead of print lethality include small thoracic circumference with pulmonary hypoplasia and severe micromelia. These features were present in the fetus of a 25-year-old female who came for an anomaly scan in her second trimester of pregnancy. Additional findings of generalized demineralization and osteochondral spurs led to the diagnosis of hypophosphatasia congenita. The pregnancy was terminated, and the findings were confirmed on autopsy. Common differential diagnoses with clues to diagnose the above mentioned condition have been discussed here. Early and accurate detection of this medical condition is important as no treatment has been established for this condition. Therefore, antenatal ultrasonography helps in diagnosing and decision making with respect to the current pregnancy and lays the foundation for the genetic counseling of the couple.

Osteogenesis Imperfecta: A Review with Clinical Examples

Osteogenesis imperfecta (OI) is characterized by susceptibility to bone fractures, with a severity ranging from subtle increase in fracture frequency to prenatal fractures. The first scientific description of OI dates from 1788. Since then, important milestones in OI research and treatment have, among others, been the classification of OI into 4 types (the 'Sillence classification'), the discovery of defects in collagen type I biosynthesis as a cause of most cases of OI and the use of bisphosphonate therapy. Furthermore, in the past 5 years, it has become clear that OI comprises a group of heterogeneous disorders, with an estimated 90% of cases due to a causative variant in the COL1A1 or COL1A2 genes and with the remaining 10% due to causative recessive variants in the 8 genes known so far, or in other currently unknown genes. This review aims to highlight the current knowledge around the history, epidemiology, pathogenesis, clinical/radiological features, management, and future prospects of OI. The text will be illustrated with clinical descriptions, including radiographs and, where possible, photographs of patients with OI.

Clinical characteristics of perinatal lethal hypophosphatasia: a report of 6 cases

Hypophosphatasia is a rare inherited disorder caused by deficient tissue-nonspecific alkaline phosphatase activity. It is classified into 6 subtypes, and the perinatal lethal form of hypophosphatasia is the most severe. Patients with this form suffer from various symptoms, including respiratory failure, premature craniosynostosis, rachitic changes in the metaphyses, convulsions and hypercalcemia. This report presents 6 cases of the perinatal lethal form of hypophosphatasia. All of the patients showed shortening of the long bones in utero in ultrasonographic examinations. Two of the six patients died at birth because they could not establish spontaneous breathing. Three of the remaining four patients also died before 1 yr of age. The major cause of death was respiratory failure due to hypoplastic lung. All of the patients, except for the two who died at birth, experienced convulsions in their clinical courses. Vitamin B6 therapy effectively reduced the frequency and severity of convulsions. However, it could not always make the patients convulsion free. Three patients underwent a genetic analysis. The 1559delT mutation, which abolishes Alkaline Phosphatase (ALP) activity, was a hot spot. A homozygous 1559delT mutation was observed in two patients. However, they differed in severity of symptoms. Although a good genotype-phenotype correlation has been reported in hypophosphatasia, the genotype alone does not always predict the life span of the patients. These cases therefore suggested the importance of genetic counseling.

Hypophosphatasia: molecular testing of 19 prenatal cases and discussion about genetic counseling

OBJECTIVE

We studied hypophosphatasia (HP) mutations in 19 cases prenatally detected by ultrasonography without familial history of HP. We correlated the mutations with the reported ultrasound signs, and discussed genetic counseling with regard to the particular dominantly inherited prenatal benign form of HP.

METHOD

The coding sequence of the tissue nonspecific alkaline phosphatase (TNSALP) gene was analyzed by DNA sequencing, and 3D modeling was used to locate the mutated amino acids with regard to the functional domains of TNSALP.

RESULTS

Although reported ultrasound signs were heterogeneous, two mutated alleles were found in 18 of the 19 cases studied, indicating recessive transmission of the disease. Functional domains of TNSALP were affected by 74% of missense mutations. In all the cases, including one with only a heterozygous mutation, molecular, biological, and familial data do not corroborate the hypothesis of prenatal benign HP. The mutation c.1133A>T observed in the prenatal benign form of HP and common in USA was not found in this series.

CONCLUSION

The results point out the prenatally detectable allelic heterogeneity of HP. The nature of the detected mutations and the evidence of recessive inheritance do not support these cases being affected with prenatal benign HP.