Phenotype-genotype correlation of patients with congenital cataracts and hair anomalies

• Introduction
• Methods
• Results
• Discussion

Abstract

Purpose: Hair anomalies represent a common associated symptom of congenital cataracts. Early diagnosis is crucial for treatment and predicting prognosis. However, the insidious and nonspecific nature of the symptoms in young children makes diagnosis challenging, often necessitating tools such as whole-exome sequencing (WES) for accurate assessment. This study aims to propose a simple and expedient approach to guide clinical management by analyzing phenotype-genotype correlations.

Methods: A prospective cohort study was conducted among participants who underwent clinical examinations and WES between 2021 and 2023. Bioinformatic analysis was performed. In total, 170 unrelated congenital cataract probands were tested. The suspected pathogenic variants were validated through Sanger sequencing in both the probands and available family members. Correlation analyses were then performed, integrating clinical characteristics, cataract phenotype, and genotype data.

Results: Nine probands presented with both cataracts and hair anomalies. Potential pathogenic variants were detected in all patients with hair anomalies, including a novel variant in LSS. Phenotype-genotype analysis supports the classification of patients into two groups: hypotrichosis 14 and ichthyosis follicularis with atrichia and photophobia syndrome 2, based on the cataract phenotype, severity of the hair anomalies, and the presence of corneal pannus. These patients should be monitored closely for the development and progression of glaucoma and corneal lesions.

Conclusions: We identified nine probands with hair anomalies in our large cohort of congenital cataract probands. Using WES and comprehensive clinical examinations, we established definitive diagnoses, broadened the phenotype and genotype, and proposed phenotype-genotype correlations.

Congenital cataract is a pathological condition in children characterized by opacity or a color alteration of the crystalline lens, leading to a deterioration of optical quality [1,2]. It constitutes a large component of blinding diseases in the pediatric population [3]. The incidence of congenital cataract ranges from 12 to 136 per 100,000 births, with a substantial proportion attributable to genetic variants [4]. A discernible phenotype-genotype correlation exists in congenital cataracts, where the timing, location, and symmetry of lens opacities provide valuable clues to specific causative genes [5]. The co-occurrence of other ocular and systemic anomalies also can guide diagnostic procedures [6,7].

A significant fraction of patients with congenital cataracts present with co-occurring systemic anomalies [8,9]. In a large Danish series of genetic congenital cataracts, approximately 7% had associated ocular abnormalities, 3% had concomitant systemic abnormalities, and another 20% occurred as part of metabolic or chromosomal syndromes [10]. Among the extraocular findings, hair anomalies are common and easily detectable. Currently reported syndromes associated with congenital cataracts and hair anomalies include hypotrichosis 14 (OMIM 618275) [11]; cataract, alopecia, oral mucosal disorder, and psoriasis-like syndrome [12]; ichthyosis follicularis with atrichia and photophobia syndrome 2 (OMIM 619016) [13]; Rothmund-Thomson syndrome, type 1 (OMIM 618625) [14]; trichothiodystrophy 1, photosensitive (OMIM 601675) [15]; and uncombable hair syndrome 2 (OMIM 617251) [16]. However, differentiating among these patient groups poses a challenge, as in early childhood, they manifest only hair and eye anomalies, with other systemic anomalies not being evident. Genetic testing, due to its time-consuming and high-cost nature, has not been widely adopted. Therefore, the diagnosis and treatment of these patients in their early years need refinement. By conducting detailed examinations of systemic and ocular conditions, along with whole-exome sequencing (WES), this study aims to establish a phenotype-genotype correlation for congenital cataracts associated with hair disorders. This correlation enables the genotype prediction through an analysis of cataract type, thereby guiding clinical practice.

In our cohort, variants in the LSS and SREBF1 genes are associated with syndromes involving congenital cataracts and hair anomalies. By observing the deep phenotypes of cataracts, associated ocular anomalies, and the characteristics of the hair anomalies, a definitive diagnosis can be established, even in young patients. In addition to broadening the phenotypic spectrum of LSS and SREBF1 genes, we suggest that vigilant attention is essential for detecting glaucoma and corneal erosion in this patient group to prevent visual impairment.

We conducted WES on 170 congenital cataract probands and identified potential pathogenic variants in 71.2% (121 patients, data not shown). Among them, 51.8% were male, and 48.2% were female. The median age of the enrolled patients was 56 months (range, 2-168 months). Nine patients with hair anomalies carried candidate pathogenic variants in either LSS or SREBF1, which were absent in patients without hair anomalies (Figure 1, Appendix 1). Specifically, seven probands and one affected family member carried compound heterozygous variants in LSS, and two probands and one affected family member carried heterozygous variants in SREBF1 (Figure 2, and Appendix 2). Eight different variants in the LSS gene were identified, including a novel truncation variant (c.1306C>T p.Gln436Ter), six variants previously reported by our group [7], and one known missense variant (c.1025T>G). The LSS c.1025T>G variant was identified as a recurrent variant, while the remaining seven variants were found exclusively in our cohort [7]. One pathogenic variant in the SREBF1 gene was identified in two unrelated probands. Screening of their family members revealed that the SREBF1 variant in family 8 was inherited from the affected father, while the variant in family 9 was a de novo variant. All the identified variants in the LSS and SREBF1 genes occurred in evolutionary regions conserved in mammals through Xenopus and zebrafish (Figure 3). The clinical characteristics of the 11 patients with hair anomalies are listed in Appendix 1. We observed variations in the severity of abnormal hair conditions among the probands: six patients have completely absent hair, while the remainder have sparse and brittle hair. The six patients with alopecia also lack eyelashes and eyebrows. Additionally, patient F9 II:1 has ichthyosis and congenital glaucoma, and affected individuals in family 8 also have disordered teeth. All 11 patients have bilateral cataracts, with 8 presenting with bilateral nuclear cataracts and 3 presenting with bilateral cortical cataracts (the types of cataracts in patients F1 II:1, F2 II:1, F6 II:1, and F7 II:1 were confirmed from their medical records before surgery). Three patients have corneal pannus, two patients have glaucoma, and two patients presented with genitourinary system anomalies. Notably, no intellectual disability or developmental delay was observed in any of the affected individuals.

We examined the phenotype-genotype correlations for all probands and their affected family members, including combined hair anomalies, congenital cataract morphology, and other ocular findings (Figure 4). We found that all patients with nuclear cataracts carry variants in the LSS gene, while those with cortical cataracts carry variants in the SREBF1 gene. Furthermore, patients with complete alopecia and without eyebrows and eyelashes carry variants in the LSS gene. Among patients with brittle hair, normal eyebrows, and normal eyelashes, two carry LSS variants, while three carry SREBF1 variants. Additionally, we observed that all three patients carrying SREBF1 variants also have corneal pannus, while none of the patients with LSS variants do. Glaucoma was diagnosed in two patients: one with LSS compound heterozygous variants presenting as secondary glaucoma in the right eye and another with an SREBF1 variant presenting as bilateral congenital glaucoma.

We conducted comprehensive clinical examinations and WES analysis for 170 probands with congenital cataracts and identified potential pathogenic variants in 71.2% of patients. Among them, nine patients had concurrent hair anomalies, and candidate pathogenic variants were identified in either LSS or SREBF1, including a novel variant in LSS. The establishment of phenotype-genotype correlations makes it possible to predict genotypes based on phenotypes.

The diagnostic yield in this study (121/170, 71.2%) is higher than that in previous size-comparable studies [22,23]. This difference can be attributed to several factors related to our study design and patient selection [2,7]. First, our study exclusively enrolled patients with bilateral congenital cataracts. This specific patient group is more likely to have a genetic etiology, as opposed to unilateral or acquired cataracts, which may have a higher incidence of nongenetic causes. Second, a significant proportion (41.76%) of the patients in our cohort had a family history of congenital cataracts. The inclusion of familial cases increases the likelihood of identifying pathogenic variants, as these families are more prone to monogenic inheritance patterns. Additionally, our cohort had a relatively high median age of 56 months, which allowed for more comprehensive clinical assessments and accurate phenotyping, potentially aiding in the identification of associated genetic variants.

The establishment of phenotype-genotype correlations might contribute to diagnostic precision through the analysis of cataract deep phenotypes and extralenticular and extraocular signs, offering valuable guidance for clinical treatment. Previous studies have suggested phenotypic and genotypic heterogeneity in congenital cataracts [22]. Therefore, genetic testing is often employed for disease diagnosis. However, the widespread adoption of this technology faces challenges due to its high cost, time-consuming nature, and interpretative complexities, particularly in some countries and regions [24]. Our previous study demonstrated that by objectively and meticulously assessing lens, ocular, and extraocular findings, correlations could be established between specific genes and their deep phenotypes [7]. In the present cohort, all patients with nuclear cataracts exhibited variants in the LSS gene, while those with cortical cataracts presented variants in the SREBF1 gene. Furthermore, all three patients carrying SREBF1 variants had photophobia and corneal pannus, while patients carrying LSS variants did not, emphasizing the importance of protecting the cornea in the former group to mitigate the risk of severe complications and their consequences. In this study, all patients with alopecia and absent eyebrows and eyelashes carried variants in the LSS gene, while all children with SREBF1 gene variants presented with sparse hair, although some patients with LSS variants presented similarly, so this finding alone cannot discriminate between the two groups. Based on these findings, we propose categorizing patients with nuclear cataracts, alopecia, and absent eyebrows and eyelashes as having hypotrichosis 14 due to LSS gene variants, while cortical cataracts and corneal vascularization are indicative of ichthyosis follicularis with atrichia and photophobia syndrome 2 caused by SREBF1 gene variants.

Previous studies have suggested a tentative genotype-phenotype correlation for LSS variants, with variants occurring within the N-terminus associated with hair loss and variants toward the C-terminus linked to ocular abnormalities [11]. However, our study found that all patients with LSS gene variants exhibited both cataracts and alopecia, and their variants were not confined to the N-terminus or C-terminus. Additionally, other studies analyzing previously reported LSS patients found that there was no absolute correlation between LSS variants and phenotypes [25,26]. This underscores the complexity of genotype-phenotype correlations in LSS-related disorders and highlights the need for further research to better understand these relationships.

This study broadens the phenotypic spectrum of LSS and SREBF1 genes. Glaucoma, not previously reported in congenital cataracts linked with hair anomalies, occurred in 18.2% of the cases (2/11) in this cohort. One patient developed secondary glaucoma following cataract surgery, a condition that may be attributable to either the surgical intervention or underlying genetic factors. In contrast, another patient with an SREBF1 gene variant developed glaucoma without undergoing cataract surgery, highlighting the potential role of genetic predisposition in glaucoma development within this patient group. Given the serious threat childhood glaucoma poses to vision, characterized by insidious visual impairment and unclear pathogenesis [27], it is crucial to maintain vigilant monitoring for this condition in high-risk patients. This applies to both preoperative assessments and postoperative follow-ups, as early detection can help prevent avoidable visual impairment and ensure timely intervention.

Multiple genes have been reported to be associated with cataracts and hair anomalies in various sterol and cholesterol metabolic pathways [11,13,28]. Among them, the membrane-bound transcription factor protease, site 1 (MBTPS1) and the membrane-bound transcription factor protease, site 2 (MBTPS2) genes encode site 1 protease and site 2 protease, respectively. They collaborate in cholesterol biosynthesis by cleaving sterol regulatory element–binding proteins (SREBPs) and site 1 protease itself [12]. SREBPs, encoded by SREBF1 and SREBF2, are transcription factors that bind to the specific sequence sterol regulatory element in the promoter regions of lipogenes, thereby regulating the biosynthesis of cholesterol and fatty acids [13]. Another gene involved in the cholesterol pathway is LSS (OMIM 600909), which encodes lanosterol synthase, catalyzing the conversion of (S)-2,3-oxidosqualene to lanosterol, thereby participating in the biosynthesis of cholesterol [11,29]. The correlation among these genes in the cholesterol metabolic pathway may elucidate the shared clinical spectrum observed in these syndromes.

This study has some limitations. Specifically, functional analyses of the identified variants were not performed in this study, and the cohort size was limited. Future research should continue to refine the phenotypic and genotypic correlation studies of the LSS and SREBF1 genes and further enhance our understanding of these complex diseases.