A Novel Splice Site Mutation in Anthrax Toxin Receptor 2 (Capillary Morphogenesis Protein 2) Gene Results in Systemic Hyalinosis

Abstract

Infantile Systemic Hyalinosis, now included under the unifying term, “hyaline fibromatosis syndrome” (HFS) is a rare, progressive and fatal autosomal recessive disorder characterized by various dermatological manifestations such as thickened skin, papules and nodules, hyperpigmentation over bony prominences, gingival hypertrophy and systemic manifestations such as joint contractures, osteopenia, recurrent infections, diarrhea, and short stature. Histopathological examination of the affected tissue shows deposition of amorphous hyaline material. Mutations in the gene encoding capillary morphogenesis protein-2/anthrax toxin receptor 2 (ANTXR2) are implicated in the pathogenesis. Here, we report a 16-month-old toddler with HFS whose genetic analysis revealed a novel homozygous frameshift mutation in exon 13 of ANTXR2 gene.

Introduction

Infantile Systemic Hyalinosis (ISH) and its milder counterpart, juvenile hyaline fibromatosis (JHF), together termed HFS, are known to be caused by mutations in the gene encoding capillary morphogenesis protein 2 (CMG2) which is a transmembrane protein that binds the extracellular matrix proteins, laminin, and collagen IV.[¹] Around 150 cases have been reported in the literature.[²] The present case is a 16-month-old male child with classical cutaneous manifestations of ISH who also had intractable diarrhea and recurrent upper respiratory tract infection (URTI). Genetic analysis revealed the presence of a novel pathogenic mutation in anthrax toxin receptor 2 (ANTXR2) gene.

Case Report

A 16-month-old male child born to second-degree consanguineous parents was referred from Pediatrics department for skin lesions over face and trunk since 3 months of age. The child was being managed under pediatrics for URTI and intractable diarrhea with a working diagnosis of arthrogryposis multiplex congenita with recurrent infections. The parents also gave history of excessive cry with minimal handling, stiffness of skin over extremities, and flexion deformity of all four limbs starting from 2 months of age. There was a history of physical developmental delay, while social development was age appropriate. On examination, the child was small for his age and irritable with facial dysmorphism (low set ears and dolichocephaly). Skin appeared tense and shiny over extremities, predominantly over bony prominences. Well-defined skin colored to erythematous papules and plaques were seen over chin, occipital scalp, ear helices, nape of neck, and upper back [Figure 1a and b]. Skin colored to pinkish nodules measuring 1 cm × 1 cm to 6 cm × 5 cm were present over tip of the nose, bilateral elbows, knuckles, mid back, and perianal region [Figure 2a-d]. Gingival hypertrophy was present [Figure 1a]. Hyperpigmentation was seen over knuckles, proximal interphalangeal joints, and over both malleoli. Tender flexion deformities were noted involving all the small and large joints of upper and lower extremities giving a frog leg position. Based on clinical features, differentials were Farber's disease, infantile stiff skin syndrome, Winchester's syndrome, infantile restrictive dermopathy, lipoid proteinosis, and arthrogryposis multiplex congenita. Routine blood investigations were normal. There was radiological evidence of generalized osteopenia with right distal humerus fracture on babygram. Neuroimaging and ultrasonography of abdomen and pelvis showed normal study. Tests for lysosomal storage disorders, metabolic syndromes, and immunodeficiency were negative. Skin biopsy from erythematous papule over nape of neck was suggestive of ISH [Figure 3a and b]. Genetic analysis revealed a pathogenic homozygous NM_058172.6:c. 1069_1069delG frameshift mutation in exon 13 of ANTXR2 gene, confirming the diagnosis of ISH.

Figure 1: (a) Skin-colored nodule over tip of the nose, pearly erythematous papules over chin and gingival hypertrophy. (b) Pinkish papules over occipital scalp, nape of neck, and upper back

Figure 2: (a) Nodule over extensor aspect of left elbow. (b) Nodules over proximal interphalangeal joints of right hand. (c) Pinkish nodule over midback. (d) Papules and nodules in the perianal region

Figure 3: (a) Histopathological examination – low power view (×10) with hematoxylin and eosin stain showing thickened dermis with reduced appendages. (b) Histopathological examination – high power view (×40) with hematoxylin and eosin stain showing coarse collagen with increased number of plump fibrocytes in papillary dermis and thickened collagen bundles in large foci in reticular dermis

Symptomatic treatment was given and the parents were counseled regarding the prognosis of the condition and about the need for prenatal diagnosis in future pregnancies.

Discussion

Landing in 1986 gave a detailed description of ISH.[³] JHF, initially considered as a different entity, is now recognized as an allelic disorder of ISH. Mutations in CMG2/ANTXR2 are responsible for both the disorders.[⁴⁵] Due to significant overlap at molecular, histological, and clinical levels between ISH and JHF, Nofal et al. proposed the unifying term “hyaline fibromatosis syndrome (HFS) or HFS,” signifying that both the entities represent the same disorder but with different degrees of severity.[⁶]

Our patient initially developed painful flexion deformities at 2 months of age involving all the small and large joints of upper and lower extremities. This initial presentation may be mistaken for infantile stiff skin syndrome, infantile restrictive dermopathy, arthrogryposis multiplex congenital, and Winchester's syndrome which are also characterized by multiple joint contractures. However, the development of other characteristic features of ISH such as thickening of skin, fleshy papules and nodules, gingival hypertrophy, pigmentation of bony prominences, and coarse facial features helped in arriving at a clinical diagnosis. Pearly papules on the central face and ears were also seen, features which are considered pathognomonic for ISH.[⁷] Social and language milestones were age appropriate in our case, however, there was physical developmental delay due to virtual immobility. This is consistent with previous reports of patients with ISH whose developmental milestones were normal except for physical development.[¹]

ISH has been classified into 3 grades by Nofal et al. based on the severity, with the 4^th type added by Denadai et al. The grades are as follows G1: Mild type with only skin involvement and gingival hypertrophy, G2: Moderate type with additional joint contractures and bone lesions, G3: Severe type with manifestations resulting from organ involvement, such as persistent diarrhea and recurrent pulmonary infections, G4: Lethal type with organ failure and/or septicemia.[⁸⁹] Our patient comes under Grade 3 ISH with recurrent URTI and diarrhea. He was being treated with multiple antibiotics before the diagnosis of ISH. Al Mubarak et al. presented a similar case of ISH who presented with intractable diarrhea managed with fluids, electrolytes, albumin, and calorie supplementations.[¹⁰] Diarrhea and protein losing enteropathy are attributed to hyaline deposition in the gastrointestinal tract and also intestinal lymphangiectasia.[¹¹¹²] Likewise, hyaline deposits in the lungs perhaps lead to recurrent respiratory problems which is usually seen late in the disease.[¹] Hence, early and accurate diagnosis of this condition is of utmost importance to prevent unnecessary interventions.

Skin biopsy from one of the papules over nape of neck in our case was suggestive of ISH. Characteristic histopathological features include deposits of amorphous, homogeneous, eosinophilic, PAS positive hyaline substance in the papillary, and reticular dermis.[⁹¹³] These can be explained by the extravasation of hyaline material through the basement membrane into the perivascular space due to defect in ANTXR2 gene.[⁴]

The confirmation of the diagnosis in our case was established by genetic analysis where exons 13 and 14 of ANTXR2 gene were amplified using exon specific primers and analyzed for any pathogenic variations. The child was found to have homozygous pathogenic c. 1069-1069delG frameshift variation in exon 13 of the gene. This variant has not been reported previously in homozygous state in population databases like 1000 Genomes, Exome Variant Server, Exome Aggregation Consortium and Genome Aggregation Database or in homozygous/compound heterozygous state with any other disease-causing mutation in databases like ClinVar, Online Mendelian Inheritance in Man and Human Genome Mutation Database which contain information regarding pathogenic variations. Till date, approximately 49 distinct HFS-associated variants have been published with our case report adding another variant.[¹⁴] Treatment is largely symptomatic with no cure to date.

Conclusion

ISH is a rare genetic disorder which can be easily misdiagnosed due to varied clinical manifestations closely resembling other conditions. Diagnosing early with a high index of suspicion helps in appropriate and timely management, thereby improving the quality of life. Our case adds to the existing database of pathogenic genetic variations causing HFS.

Declaration of consent

The authors certify that they have obtained all appropriate consent forms, duly signed by the parent(s)/guardian(s) of the patient. In the form, the parent(s)/guardian(s) has/have given his/her/their consent for the images and other clinical information of their child to be reported in the journal. The parents understand that the names and initials of their child/children will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.