We studied a total of 229 (114 male and 115 female patients) unrelated Puerto Rican patients presenting with albinism. Patients’ ages ranged from 1 month to 58 years (mean 8.24 ± 10.34 years). We studied a total 585 individuals in the 229 families. In families with multiple affected members, only one affected individual was included in the study group. All cases appeared consistent with autosomal recessive inheritance, although two families exhibited apparent pseudo-dominance due to an affected parent. Indeed, 80% of families reported albinism in the extended family, such as probands’ cousins, aunts, uncles, or grandparents, suggesting that the frequency of disease alleles is relatively high on the island.
We observed considerable phenotypic heterogeneity among the study patients. All patients had low vision with reduced visual acuity, nystagmus, and photophobia. Apparent skin pigmentation was greatly reduced or absent among patients subsequently found to have OCA1 or OCA2. In contrast, skin hypopigmentation was quite variable and often subtle among patients subsequently found to have HPS3, although patients were invariably less pigmented than their parents or unaffected sibs. Three family members of HPS3 patients were found to have undiagnosed HPS3 when genetic testing was carried out and homozygosity for the HPS3 gene 3,904-bp deletion was established. These three cases had nystagmus, very mild hypopigmentation, and ocular albinism as well as easy bruising.
As most of the HPS patients studied here were children, the severe clinical manifestations of HPS were not found as often as reported in older patients. The most common respiratory symptoms among the pediatric HPS patients included rhinitis, cough, and bronchial wheezing; some patients had been diagnosed with bronchial asthma. Four patients with HPS3 had colitis or gastrointestinal bleeding of unknown etiology at very early ages, including one patient at birth and one prior to 2 years of age. Two out of these four patients who presented lower gastrointestinal bleeding and/or colitis were considered to have mild bleeding associated to the bleeding diathesis. Their signs and symptoms disappeared promptly; thus, they were not evaluated further. However, in the other two patients, the signs and symptoms were significant enough to get gastrointestinal bleeding consultation and biopsy. In these two cases (ages 5 and 20 years, respectively), the gastrointestinal bleeding biopsy was reported by an experienced pathologist as colitis. These patients are being followed by pediatric and adult gastroenterologists.
Approximately half of the 229 patients with OCA included in this report were cases referred directly to the genetics laboratory in our institution by ophthalmologists who asked for HPS genetic testing. Most of these patients were not available for evaluation in our hematology service. Out of the 229 albino patients reported in our article, 109 (47%), most of them with a variable bleeding diathesis, were tested for bleeding time (BT) and/or platelet aggregation tests. Forty-nine of these 109 (45%) were homozygous for the HPS1 gene 16-bp duplication and 16 (32.7%) for the HPS3 gene 3,904-bp deletion. Thirty-nine out of these 49 (79.6%) HPS type 1 patients had an abnormal BT, and 13 of the 16 patients with HPS3 (81.2%) had an abnormal result.
Platelet aggregation tests were performed in 33 of the 49 HPS type 1 cases, out of which 26 (79%) had an abnormal aggregation test with two or more platelet aggregation agents. Similarly, among the 16 HPS3 patients, 13 (77%) showed abnormal aggregation with two or more platelet agonists.
These hematological tests were also performed in a group of 16 albino patients who did not have homozygous HPS1 or HPS3 gene mutations, including five OCA1 and two OCA2 patients, and nine cases who were heterozygous for the HPS1 or HPS3 gene mutations frequent among Puerto Rican patients. The BT was normal in all five homozygous OCA1 and two OCA2 patients tested. Similarly, the platelet aggregation tests were normal in two out of three OCA1 and in both OCA2 patients tested. Corresponding tests performed in the nine patients who were heterozygous for the HPS1 gene 16-bp duplication showed that six had abnormal BTs, and two out of four (who reported mild to moderate bleeding diathesis) also had an abnormal platelet aggregation.
We tested patient DNA samples for the most common Puerto Rican HPS mutations and, in some cases, screened for additional mutations in the HPS1
, and OCA2
genes. Patients who were heterozygous for the HPS1
gene 16-bp duplication or the HPS3 3,904-bp deletion were screened for mutations in the HPS1
, and OCA2
genes by exon screening. Those who were heterozygous for the HPS1
gene 16-bp duplication were also screened for the HPS3
gene 3,904-bp deletion. No exon screening of the HPS3
gene was performed, since we had only two cases of patients heterozygous for the HPS3
gene 3,904-bp deletion. Among the 229 unrelated albinism probands, 98 (42.8%) were homozygous for the common Puerto Rican HPS1
gene 16-bp duplication (Oh et al., 1996
) (). Another 11 patients (4.8%) were heterozygous for this mutation (); exon screening and sequencing detected no other pathologic mutations of the HPS1
gene in these patients. In toto
, 47.5% of patients were either homozygous or heterozygous for the common HPS1
duplication. HPS type 1 thus comprised the major apparent OCA disorder in this study, principally (>90%) involving patients derived from northwest PR.
Results of mutation analysis of the HPS1, HPS3, TYR, and OCA2/P genes in 229 Puerto Ricans with albinism
An additional 39 (17%) patients were homozygous for the common HPS3
gene deletion (Anikster et al., 2001
); two (0.9%) were heterozygous for this mutation. Nineteen (~49%) of these cases were from central PR and the rest were from various townships, including Ponce, Salinas, Patillas, Cayey, Cidra, Bayamón, Caguas, San Lorenzo, Toa Alta, and San Juan. These findings indicate that this mutation probably arose in central PR, but is now distributed widely across the island.
Analysis of the principal OCA genes (TYR
) by exon screening and/or mutation-specific multiplex PCR detected seven patients (3.0%) homozygous for the TYR
G47D mutation. This mutation, originally observed in Caucasian patients, was subsequently observed in 11 Puerto Rican OCA1 patients (Oetting et al., 1993
) and was attributed to migration to PR from the Canary Islands. However, the G47D mutation is also extremely common among Moroccan Jews (Gershoni-Baruch et al., 1994
), and it may alternatively be that this mutation was introduced to PR from the Canary Islands, and to North Africa directly from Spain or via some other route. Whatever its origin, the G47D mutation appears to be the most common TYR
gene mutation among Puerto Ricans with OCA1.
Three patients (1.3%) were found to be homozygous, and 14 (6.1%) heterozygous, for the OCA2
2.7-kb deletion commonly found in African (Spritz et al., 1995
; Stevens et al., 1997
) and African-American (Lee et al., 1994
; Durham-Pierre et al., 1996
) patients with OCA. This is the first report of the 2.7-kb P
gene deletion in PR, although the finding of this mutation in PR is not surprising given the historical admixture of Africans into the Puerto Rican population (the slave trade in PR began in the early 16th century (Izquierdo et al., 1993
), principally from central west Africa and Bantu-speaking Africa (Nagel and Ranney, 1990
)). The OCA2
gene deletion is remarkably frequent among the Bantu-speaking peoples of Tanzania (Spritz et al., 1995
) and South Africa (Stevens et al., 1995
To determine the frequency of the common HPS1
mutant alleles in PR, we tested 2000 dried blood samples of newborns from the indigenous island population (indigenous meaning children born from Puerto Rican parents living in the island of PR and not in the mainland United States). As shown in , the frequency of the common HPS1
gene 16-bp duplication allele is 0.0476 (1/21), essentially as estimated by Witkop et al. (1990a
from epidemiologic surveys of patients from northwest PR (). The frequency of the HPS1
mutant allele is lower among individuals from east and southeast PR. As expected, the frequency of the HPS3
deletion allele is higher in the central region of PR (1/32) than elsewhere on the island, and is least prevalent (1/187) in northwest PR (). Interestingly, the overall summary frequencies of the HPS1
mutant alleles for the island in toto
are essentially identical – 1/59. The HPS1
16-bp duplication allele appears to be in Hardy–Weinberg equilibrium in the populations of northwest and central PR (P
< 0.05), but not in the total population and that of the main region of the island (Supplementary material
). We do not know the reason for the discrepancies between the observed and expected frequencies, which could be due to one or more of the Hardy–Weinberg conditions not having been met.
Results of screening Puerto Rican newborns for the HPS1 gene 16-bp duplication and the HPS3 gene 3,904-bp deletion
The genotype frequencies for the HPS1
gene mutation are significantly different in the three principal geographic regions of the island (χ2
= 28.8; P
= 0.000; see Supplementary material
). These differences may be due to the founder effect already demonstrated to be present in northwest PR for HPS, which results in a high genotype frequency and an expected difference in the genotype frequency in other regions of the island. Alternatively, these differences may be due to recent major changes in the population structure of the main region and, thus, in the island as a whole. In contrast, the HPS3
deletion allele appears to be in Hardy–Weinberg equilibrium in the three geographic regions of the island (χ2
< 0.2, which is
3.84, for P
< 0.05; see Supplementary material
), which suggests that conditions for Hardy–Weinberg equilibrium were met.
Our findings provide insights into the genetic epidemiology of albinism in PR. In the island of PR, HPS1 is the most frequent cause of albinism, accounting for at least 43% of cases, particularly among patients from the northwest part of the island. HPS3 is next in prevalence, accounting for 17% of the total, particularly among patients from central PR. Island-wide, however, the carrier frequencies of the common HPS1
gene mutations were essentially equivalent (1/59). The scientific literature contains many reports of the high prevalence of HPS in northwest PR (Witkop et al., 1990a
; Oh et al., 1996
; Anikster et al., 2001
and many others); hence, the increased awareness of the high incidence of HPS among Puerto Ricans in the island’s medical community has increased the reporting of HPS in that region of the island, where ~17% of the island population resides. In contrast, awareness of the existence of a second type of HPS in central PR is low, and was not reported in the scientific literature until 2001. The carrier frequency of the HPS1
gene 16-bp duplication among newborns was higher in northwest PR (1:21) than the carrier frequency of the HPS3
gene 3,904-bp deletion in central PR (1:32), which can cause a higher number of cases in this more densely populated region of the island, in combination with occult or known inbreeding, which has been reported in both types of HPS in Puerto Rican patients. These facts, combined with the milder phenotype of HPS type 3 in Puerto Rican patients, has contributed to HPS cases not being properly diagnosed early in childhood, as evidenced by the undiagnosed cases mentioned above and may account for the difference in the number of HPS type 1 versus
HPS type 3 cases referred to the genetics laboratory reported here.
At least 3% of Puerto Rican albinism patients have OCA1, principally involving the G47D substitution. At least 1.3% of patients have OCA2, principally involving the 2.7-kb gene deletion. Altogether, we were able to establish molecular diagnoses for ~64% of the albinism patients in our Puerto Rican cohort. Another 12% had heterozygous mutations in the HPS1, HPS3, or OCA2/P genes, but no second mutation was found. These patients either had cryptic mutations that were not detected by the PCR-based exon screening methods used or are chance carriers and have mutations in other genes not studied here. A total of 24% of patients had no detectable mutations in any of the genes tested, and again either have mutations not detected by the methods used here or have mutations in other genes not studied.
Our data show that the HPS1
16-bp duplication mutation is carried by 1/21 newborns in northwest PR, similar to the previous estimate of 1/22 based on clinical survey (Witkop et al., 1990b
) and 1/59 on the island overall. Based on 2000 US Census data, we thus estimate that there may be 308–464 cases of HPS1 on the island. We also provide the first estimate of the carrier frequency of the HPS3
gene deletion in central PR – 1/32. Surprisingly, the carrier frequency of the HPS3
deletion allele is also high in the main region – 1/52. The majority of HPS3 cases may thus derive from areas of PR other than the northwest or central regions. The results of our mutation testing among newborns can be extrapolated to the number of children born in the island each year (~50,000/year). Assuming Hardy–Weinberg equilibrium, we estimate that ~7–8 children with HPS (HPS1 and HPS3) will be born on the island each year.
Finally, because >90% of the patients in our study were <18 years of age, some of the severe long-term complications of HPS, such as pulmonary fibrosis and granulomatous colitis, were uncommon in our study population. In several instances, patients’ relatives were identified as having OCA only after the proband had been investigated due to clinical hypopigmentation, bruising, low vision, or nystagmus. Thus, some cases, particularly of the clinically milder HPS3, may not be recognized until relatively late, even in adulthood. We strongly recommend that all Puerto Ricans presenting with even mild hypopigmentation, reduced visual acuity and/or nystagmus, and easy bruising undergo molecular testing for the common HPS1 and HPS3 gene mutations. These evaluations will permit correct diagnosis, appropriate management, and genetic counseling of patients from this specific Hispanic population at high risk for these life-threatening disorders.